Subject: Sci.chem FAQ - Part 4 of 7
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Date: Sun, 17 Nov 1996 08:05:37 GMT
Archive-name: sci/chem-faq/part4
Posting-Frequency: monthly
Last-modified: 17 November 1996
Version: 1.08
Subject: 15. Chemical Demonstrations
15.1 Are there any good compilations of demonstrations?
Yes. Good places to start are the four volume "Chemical Demonstrations"
by B.Z.Shakhashirir [1], or the two volume "Chemical Demonstrations - A
Sourcebook for teachers" by Summerlin and Ealy [2]. The Journal of Chemical
Education is also an excellent on-going source of demonstrations.
15.2 What are good outdoor demonstrations for under 12s?
15.3 What are good outdoor demonstrations for over 12s?
15.4 What are good indoor demonstrations for under 12s?
15.5 What are good indoor demonstrations for over 12s?
While waiting for a promised contribution, here is my only contribution,
and some from my sci.chem archives. Unfortunately, enthusiastic editing
by others allows some of the culprits to go uncredited :-).
The ability of water-miscible solvents to mask the hydrophobic nature of
Goretex can be demonstrated. Goretex is just a porous PTFE, the same
material as PTFE filters - such as Millipore HF. You can easily filter
liquid water through porous PTFE, provided the filter is previously wetted
with a water-miscible solvent ( usually ethanol ). If a filter is set up on
a vacuum flask, ensure the filter is completely wetted with ethanol, turn on
the vacuum, and immediately add water - it rapidly filters through. Once it
has stopped, it only takes about 15 seconds for the air to dry the filter,
then ask a student to filter more water from the same flask. No chance.
Pour off the water, surreptitiously add a few mls of ethanol, immediately
followed by the same water - and watch it filter through again :-).
This is the nearest equivalent our laboratory has to the workshop practice
of sending an apprentice out to purchase a spark plug for a diesel engine.
It does relate slightly to the real world - indicating why "breathing"
fabrics like Goretex should not be used with solvents.
From: brom@yoyo.cc.monash.edu.au (David Bromage) Date: Tue, 14 Sep 1993
Subj: Re: Need: A safe chemical display
The so-called "Blue Bottle Reaction" might be useful.
Half fill a 1 litre flask with water and add 10g og NaOH, then add 10g
of glucose and up to 1ml of 1% methylene blue. Stopper the flask and
swirl gently to dissolve the contents. On standing for a few minutes the
solution should turn colourless. When the flask is shaken the solution
will turn blue then decolorise on standing.
Methylene blue exists in solution as a reduced colourless form and an
oxidised blur form. The initially blue dye is reduced by the alkaline
form of glucose and re-oxidised by dissolved oxygen. When the solution
is shaken, atmospheric oxygen enters into solution at a more rapid rate
than when left standing. The dye acts here as a catalyst whose colour
indicates the redox state.
How about a chemical garden?
Make up (or dilute a commercial preparation) of sodium silicate to
1.1g/ml. Place this solution in a large glass container then add 'lumps'
or large crystals of salts to be grown. Lumps should not be more than
0.5cm in diameter. As a salt dissolves it forms an insoluble silicate
which forms a membrane around the lump of salt. The membrane is
permeable to water which enters and dissolves more salt. The resulting
pressure bursts the membrane releasing more salt solution to form more
membrane. As the salt solution is less dense than the silicate solution,
the membrane grows as a convoluted vertical tube.
Salt Colour Growth time
Ferric chloride brown 1 hour
Ferrous sulphate grey-green 3 hours
Cobalt chloride purple 5 hours
Chromium chloride grey-green 6 hours
Nickel sulphate yellow-green ~24 hours
Cupric sulphate blue ~24 hours
Potassium aluminium sulphate white ~1 day
To produce a "garden" which is not completely overgrown with the faster
species it is necessary to take growth rates into account. Distilled
water should be used as Ca and Fe in tap water can cause cloudiness.
If you really want oscillating reactions, I know of two.
A. Iodate reaction.
Make up 3 solutions
1) Dilute 200ml of 100 vol hydrogen peroxide to 500ml
2) dissolve 21g of potassium iodate (KIO3) and 1.5ml of conc sulphuric
acid in 500ml of water.
3) Dissolve 7.8g of malonic acid and 1.4g of manganese sulphate in 400ml
of water and add 1.5g of starch in 100ml of water.
Add equal volumes (50-100ml) of each solution to a flask in any order.
Colourless-blue oscillations should start within 2 minutes. If not, try
10-20% variations in relative volumes. (try increasing 2 first).
Oscillations should last up to 10 minutes but I my experience have lasted
up to 3 hours.
B. The Belusov reaction
Prepare 5 solutions.
1) 58g of malonic acid on 500ml of water
2) 6M sulphuric acid
3) 21g of potassium bromate (KBrO3) in 500ml of water
4) Dilute 5ml of solution 2 to 500ml then add 1.75g of cerous sulphate.
5) 1.6g of 1,10-phenanthroline and 0.7g of ferrous sulphate in 100ml of
water (or commercial ferroin solution to 0.025M)
Mix together 50ml of 1 to 4 and 5ml of solution 5. Blue-pink oscillations
should start within a few minutes.
For either oscillating reaction the choice exists of complete mixing with
uniform oscillations or waves of colour (eg in a measuring cylinder).
Some interchange of reagents is possible. The Bray reaction omits
malonic acid from the Iodate reaction. Malonic acid can be replaced by
citric or succinic acids.
A particularly dramatic 'trick' is not to burn paper. Make up a solution
containing 57% v/v ethanol and 43% v/v water with 5% w/w sodium
chloride. Soak a filter circle in the solution and hold it near a flame
(with tongs) just long enough to ignite. After the flames die down the
filter circle will still be intact. The ethanol burns but just enough
water remains in the paper to prevent ignition. NaCl is added to provide
a more convincing flame. To add drama, 'burn' a banknote - but ensure
that all of the note, especially the corners, is soaked.
From: lmartin@uclink.berkeley.edu (Lonnie C Martin) Date: 17 Feb 1993
Subj: Re: Growing a Silver Tree in Beaker?
In article <...> xslkkk@oryx.com (kenneth k konvicka) writes:
>Am trying to do a demo for elementary school kids. How do you grow a tree
>of silver using copper wire(?) submerged in a solution of AgNO3? Saw one
>in high school physics class about a thousand years ago at good ol' Reagan
>HS, Austin, Tx. Was really beautiful. The silver formed nice large
>plates. Any demonstration books you could steer me toward?
What you have described is about all there is to it. I do this
demonstration for the chemistry classes here at Berkeley about twice a
year, or so. Just make a "tree" out of copper wire (you might clean it
with sandpaper or steel wool) so that it will fit into a beaker of your
choice (we use 4 litre here), and pour in the silver nitrate solution.
I think we use 0.1 molar, but as long as the concentration is fairly close
to that, it will work just fine.
It is not necessary to make the tree very "bushy". The silver will fill it
out nicely with fuzzy thick hanging globs of crystals. The solution will
change from colourless to blue, as copper nitrate is formed. A very nice
experiment. You can expect this to take on the order of an hour to get
fully developed.
From: flatter@rose-hulman.edu (Neil Flatter) Date: Tue, 14 Sep 1993
Subj: Re: Need: A safe chemical display
We use cobalt (II) chloride in a saturated sodium chloride solution to
demonstrate cooling coils. It changes from red/pink to a blue/purple when
heated and reverses as it is cooled. We cycle it through a condenser from
a distillation to illustrate that portion of a simple set-up.
Subject: Stupid lab tricks -Compiled- VERY LONG Date: 7 Jul 92
From: Nazman
Ever try taking an empty ditto fluid can, put some water in it, heat it
until steam is coming out, cap it back up and let it cool off?. You would
be surprised what a little air pressure can do. That one amazed me when I
was young.
I was amazed again when I saw a brief description on TV of how science
teachers are trying to make science fun again. Four teachers on stage, set
up a few ring stands and a few bunsen burners, and placed a 55 gallon oil
drum on top. Boiled the water, capped it. Put a hell of a dent in the drum
when it collapsed.
A favourite of mine requires a little preparation, but is great fun. Try
tearing an aluminum can in half. Kinda difficult. Now, if you take an empty
can, gently score around the circumference on the inside, (the inside is
coated to prevent a reaction between the soda and the can) and fill the can
with a solution of warm water and Copper(II)Chloride (CuCl2) so that the
solution is just above the score mark. Let this sit for a few minutes.
You are done with the solution when the outside of the can appears brownish
(blackish) where the score mark is. Gently pour out the solution (keep it)
and let the can sit. When ready, hold the top of the can in one hand and
the bottom in the other, and break like you are breaking a stick in half.
Two bits of advice :
BE CAREFUl!. You will end up with the sharp edge of the can, which can cut
severely!
Try this ahead of time just to make sure you get it right. Wait too long,
and when you pick up the can, it will split due to the weight of the
solution. Don't wait long enough and it won't work. My guess is about
5 minutes
P.S. This is more of a demonstration of structure of an aluminum can, but
if you want to demonstrate the "strength" before you rip it in half, place
the can on the floor, so it is sitting like it normally would, and balance
on one foot off the top of the can. It helps to have something nearby to
hold on to, and the can cannot have any dents. You would be surprised how
strong an empty can is. I weigh about 190 lbs, and have stood on an empty
soda can for 30 seconds, get off the can, and not have it collapse. This
takes some practice, so give it a try.
From: A_ROSATI@GUVAX.GEORGETOWN.EDU
You can followup the bromophenol blue trick by brewing a cup of tea
and, while they watch, add some lemon juice. The color will lighten.
There is an indicator in tea that changes with the acidity of
ascorbic acid.
Another neat trick is to demonstrate the dehydration capacity of
concentrated sulfuric acid. Take a 500 mL beaker about one third full
of white table-sugar. Then add about a half-inch to one inches worth
of concentrated sulfuric acid. (This demonstration _MUST_ be conducted
in a hood) Let it sit for about five minutes. Within that time, the
sulfuric acid will seep in, start turning the color of the sugar
brown, and then black, followed by an intense, hot dehydration. The
sugar will start to form a jet black, smelly, sticky column that rises
out of the beaker. It is really impressive.....
You might want to also look up "oscillating reactions" in your
chemistry library. Many of these are simple to set up and generate
neat color cycles that would impress the kids!
From: mfrancis@ucsd.edu < Lyn Francisco >
1. Take a balloon, blow it up, tie it, then stick it in a vat of
liquid nitrogen. Wait until it shrinks (around 3 s or so), take it
out, and then watch it inflate in your hands. This will very nicely
illustrate the relation between temperature and pressure.
2. We did this during a demonstration to let the world know about
ACS on campus. Take a large container (like one of those 10-gallon
water containers, cut in half or something), fill it up with water,
then put in one can of the original Coke and one can of diet Coke.
Make sure that both cans are unopened. Now, drop a few pieces of
dry ice in the container. The original Coke should drop to the
bottom, and the diet Coke stay up toward the top. It was cool, and
attracted all the frat-types and non-science people to our table.
From: dfield@nike.calpoly.edu < Dan Field >
If you really want to fire them up, my favorite has always been the
hydrogen balloons. Just fill up several balloons, one color with air or
He, another with H2, and another with 2H2 + O2. You can fill them ahead of
time, or better yet if demonstration time allows, use the products of one
of your demonstration reactions to fill the balloons. Light a candle on
a L O N G stick, dim the lights, and pop!, boom!, B O O O M !!.
You'll have instantly created little monsters, young pyromaniacs virtually
guaranteed to associate some excitement with chemistry.
From: edremy@d31ha0.Stanford.edu < Eric R.>
There are lots of things you can do with liquid N2. Try freezing a
banana and using it as a hammer. (Follow by using an unfrozen banana:
kids love it!) Simply adding some to a test tube and (lightly!) corking
it is fun, provided you're careful with the cork. Shattering a
superball is also good.
However, my personal favorite for spectacular demos is the HCl fountain.
Ascii graphics follow
---------
\ / Top flask is filled with HCl gas
\ S S=rubber stopper w/ hole for needle
\ /
-|-
/ | \ Run tube from top into bottom
/--|--\ Bottom flask filled with water and
/ | \ acid/base indicator.
-----------
MAKE SURE THAT THESE FLASKS ARE VACUUM SAFE!!!
To start this whole extravaganza, inject 20-30 cc of water into the top
flask. The HCl gas goes into solution, creating a partial vacuum, sucking
the water up from the bottom. As the water spurts out of the tube, it
collects more HCl (And changes color as it becomes acid) and accelerates
the reaction... Quite impressive.
We used to do this for our chemistry magic show every year. The only
problem is that the failure mode is somewhat dangerous: One year the
top flask had a flaw and imploded, sending glass and HCl everywhere.
Best to do behind a shield
From: Bill
I believe that the same thing can be done with ammonia.
The same precautions apply.
From: ?
Bubble H2 through a soap solution and you get bubbles that float up.
Have them float through a bunsen burner flame suspended over the table and
they explode. VERY NEAT effect.
From: joec@morgan.com
USUAL WARNINGS: many chemicals are poisonous and some reactions
may be difficult to control. Use your head.
Best done indoors
-----------------
Dissolve silver nitrate in warm water. Get some copper wire
and clean it with steel wool. Insert copper wire (preferably coiled
at one end) in the solution and it will immediately dull. Some time later,
silver crystals will be CLEARLY visibly growing on the copper.
The best effect is to let it sit overnight. The resulting
effect is downright beautiful
Dissolve Cobalt Chloride in warm water. Put some Aluminum foil
in it and watch it tarnish. Clean, polished Aluminum works
best but household aluminum foil also works (just slower).
The Aluminum slowly disappears and Cobalt metal shows up at the bottom.
This is a slow one but it does work.
Light an alcohol lamp, i.e. denatured alcohol and bring a magnet near
the flame but not above it- to the side. Watch the flame get pulled in
the direction of the magnet.
Sprinkle iron filings over the same alcohol lamp and watch sparks fly!
Ignite some Magnesium ribbon and drop into an atmosphere of CO2. It
will continue to burn with lots of noise and sparks. Carbon dust will
rain down as a byproduct.
Mix water and household (3 in 1) oil. Note the phase boundary.
Add soap and shake. Watch the phase boundary disappear.
Heat up a piece of blackboard chalk with a propane torch. Chalk
is CaCO3 - heating it up will drive off CO2, leaving CaO (also known
as lime). Heating up lime will cause the it to emit a whitish light,
which is where the phrase 'limelight' comes from.
[ Note - not all blackboard chalk is CaCO3 - test carefully first - BH]
Do these outdoors:
------------------
Get some KMNO4 and pour into a small pile. Depress the center of
the pile slightly and add a drop or two of Glycerine and stand back.
Something between 1-5 minutes later, it will burst into flame.
When it dies down, drop some more glycerine on it to have it flare up
again. Be careful disposing of the KMNO4 left over - its a powerful
oxidizer.
We also do THERMITE periodically (Aluminum powder and rust). Details for
those who ask - it burns *BRIGHT* and *HOT*.
Drop some dry ice chunks into a 2 liter PLASTIC soda bottle 1/2 full with
warm water which is then quickly sealed. Get at least 50' ft back rather
quickly. The pressure will build up and detonate with a LOUD *BOOM* after
a brief and unpredictable time. The bottle will break into many hundreds
of parts (don't use GLASS!) and you will get a mist cloud some 20-30'
across. Note: It is quite LOUD and may scare a younger audience.
Make Hydrogen soap bubbles and set them off. Get an erlenmeyer flask and
fit a cork into the top and route a glass tube through the tube and
have it bend down and into a jay of soapy water. Remove the cork and
drop in Zinc metal and pour in somewhat dilute HCL. Put the
cork back in and let the H2 bubble into the soapy water. This will
make H2 soap bubbles. Let them break free and ignite them with a light
match on long pole.
Thermite reaction
First of all....this is a fairly vigorous reaction so take the usual
precautions:
1-Do it outside, preferably on sand or dirt. Since it burns at 4000 degrees
fahrenheit, it will melt most anything. By the way, a nuclear explosion
burns at 8000 and the surface of our Sun burns at 10000. It will readily
melt rock salt, beach sand, etc. You get the idea.
2-It can spray sparks around. Keep it away from burnable materials. The
burning sparks are either molten Aluminum or molten Iron.
3-It is VERY bright so you shouldn't stare at it.
4-It puts out lots of smoke.
Here is how I do it.
Ingredients:
1-Aluminum powder
2-Iron Rust (Red-Fe2O3).
Grind carefully and separately into a powder-like consistency.
Mix in roughly equal proportions, by volume with an excess of rust.
Mix thoroughly to get an even color.
Pour the powder mixture on the ground in a pile.
Get magnesium ribbon and lay it on top of the pile, and press partially
into the pile. Do not smother the Mg ribbon. Ignite the ribbon with a
propane torch and get back quickly.
When done, be careful...it will leave molten, glowing red iron as a
byproduct.
You can make rust by mixing household clorox with steel wool pads and let
sit overnight and then filtering out the rust.
Have fun and be careful.
Usual disclaimers apply
From: gallivan@after.math.uiuc.edu < Justin Gallivan >
This works nicely with soap bubbles in a dish. If you have the H2 and O2
tanks available, Try a few with the H2 only which makes a nice quiet
flame and add the O2 later for a little shock value. You may want to
try this first for safety's sake but it always went off without a hitch in
my general chemistry days.
From: Rob
I hope I'm not too late. An extremely simple trick is done with a chunk
of styrofoam (larger the better) and some acetone, which is an excellent
theta-solvent for styrofoam.
Simply spray the acetone out of a bottle onto the styrofoam, and the
styrofoam rapidly decomposes, losing its structure, and appears to
actually be melting. It is quite a "dramatic" demonstration, and can be
offset against how nicely styrofoam coffee cups hold water/coffee, but not
acetone.
From: ?
I thought this one was neat...
Take a bottle (should be reasonable size, like a ketchup bottle)
fill it to within 2" of the top, color light blue (not opaque!)
with methylene blue. Drop in a NaOH pellet and a few drops of
Karo clear syrup. Other reducing sugars might work; I just know
it works with this syrup. (Or did; the last time I tried it was
almost 20 years ago, and they may have changed the formula since
then.)
Over a period of a few minutes, the blue color will fade. Shake
the bottle, and suddenly it's blue again. Leave it, and it will
slowly fade. It'll last for a couple of days, until random
microbes do in the sugar I suppose.
From: ?
A "Bottle of fire" for lighting bunsen burners and such:
Get a dark, heat-resistant glass bottle, and put just enough pentane
in it to wet the sides. (i.e., rinse it with pentane and dump out
the excess.) Light the top of the bottle. The flame will burn down
into the neck of the bottle a little, but be almost invisible to the
audience. Pick up the bottle, turn it over, and flames will pour
out. Set it down, and the flames seem to go out.
When Dr. Toffel did this, someone said "There's something in the
bottle!" He said "Nope," poured some water from the faucet into the
bottle, dumped fire and water into the sink, then showed that the
bottle would still "pour fire". (This probably takes some practice.)
From: mvp@hsv3.lsil.com < Mike Van Pelt >
Portable bunsen burner:
Bubble air through a test tube of pentane, and run this to
your bunsen burner. You can use a large balloon as your air source,
or have a vict... I mean, volunteer, blow through the tube.
From: Howard Clase.
One experiment that I like was you make a solution of lead nitrate,
which is clear, and a solution of some iodide salt (potassium iodide),
which is also clear. When you mix the two of them together you form
a yellow solid - lead iodide.
This is only half of it! If you don't use too much of the chemicals
to produce your "instant orange juice" - but DON'T let anyone drink it.
You will find that the lead iodide will dissolve if you heat the solution.
On
Cooling it re-precipitates as beautiful golden spangles.
From: mgray1@metz.une.oz.au < Matthew Gray >
Another exciting and easy impress all trick is to get two solutions, one
of Ag(I) and another of Cu(I), usually both hexamine complexes. When
these two are mixed, a redox reaction takes place, producing a silver
mirror effect. Other reducing metals can be used, such as iron, but I
haven't tried these myself.
From: ?
Grind some potassium permanganate to a fine powder (to speed up the
reaction). Put it in a small heap (1 teaspoon) on a tile, make a dent in
the top and pour one drop of glycerine in the hole. After about 10-15
seconds the heap will catch fire.
From: torin.walker@rose.com
Here are some that are rather interesting. All of these tests have been
performed in my workshop and are all safe (with the exception of the
handling of HCl and the irritating effect of experiment #2). Experiment #3
is by far the most fascinating.
1 Copper Sulfate couple grams in a test tube.
Sodium Bicarbonate - same as above.
These two liquids are transparent but when mixed, turn into a soft blue
opaque suspension.
2 Glycerin and HCL
Takes a long time (couple of hours) to complete but when these two clear
liquids are mixed together, it turns from clear to a deep transparent red
and slowly goes brown. Warning - this is extremely irritating to the eyes
if you are exposed to it for a while - usually, an hour is enough to
really get you annoyed.
3 (My favorite) Acetone (you can buy large tins of this stuff (1L) at a
hardware store in the automotive section (usually with the bondo and
other body repair supplies) and styrofoam (a large bag of popcorn type
packaging filler will be needed.)
When styrofoam is placed in acetone, a reaction called polymerization
takes place. [ Don't blame me, I'm only reposting these - BH ]. The
styrofoam (large volume styrofoam for a small volume of acetone)
dissolves and becomes a wet, play-dough like substance that feels cold
to the touch.
This experiment is harmless unless swallowed :-) and should prove to be
quite interesting to the students.
The coldness is due to the evaporation of the acetone from your skin
(ever use nail polish remover? That's acetone.) The acetone will
eventually all evaporate (a 2 inch sphere of this will take a day or two)
and the result will be a porous (trapped acetone bubbles) material that
can be molded to any shape you wish.
From; David O'Driscoll. University of Central Queensland...
Hope someone hasn't already done this one, I have been studying for
exams so have not been reading all of them.
The one we use at our high school demos are pH clocks....
quite good as they are not static displays.
First, take three or four large (1L) beakers and 3/4 fill them then
take your favourite pH indicators (ones with good colours), and add a
few drops to them, then add some dilute sodium hydroxide or something
to make them slightly basic. Next add a handful of dry ice to each beaker.
This creates a nice bubbling mixture with good visual effects, what happens
is obvious (I hope!!!). Some of the CO2 is dissolved in the water, turning
the mixture acidic and when the end-point of the indicator is reached the
colour changes - sometimes quite dramatically. The kids seem to like it and
the chemistry is not too involved.
From:webbb@mbf.UUCP ( Bryan Webb )
I didn't see the originating message of this thread, but from the
responses that have made it here, I think this is the kind of stuff
you might be looking for. In earlier times, I've done these:
1) Place a small pile (several grams) of powdered magnesium on a surface
you don't care about in an environment provided with plenty of
ventilation. On top of this, place a couple of grams of powdered
iodine (well, as close as you can get to it, though that might not
be crucial). Now, put a couple drops of water on the iodine ...
enough to also contact the magnesium ... and stand back. The heat
of the reaction vaporizes some of the remaining iodine into a purple
vapor.
2) This is pretty dangerous, so be very careful. Take a couple of grams
of red phosphorous and place on top of a couple of grams of potassium
iodate. Rapidly stand back... spontaneous combustion. My experience
was a time delay of a couple of seconds, but I wouldn't want to count
on it... I discovered this accidentally... boy was I surprised. The
speed of the reaction may be related to the humidity.
3) Potassium dichromate is normally bright orange at room temperatures. If
it is cooled to liquid nitrogen temperatures, it becomes yellow. If
heated, it becomes a deeper red color. I'm not aware of any other
inorganics that have this range of color change when the temperature
is varied.
4) Ahhh, my favorite... When I was in high school, I took the 2nd year
chemistry class that was offered. We had the resources of the
school at our disposal, so long as the experiment we wanted to do
was "in a book". The book I had was "Chemistry Magic", and described
an "experiment" where some cotton balls were placed on a fireproof
surface, a few grams of Sodium Peroxide was placed on top, and then
you put a drop or two of water that will wet at least a little bit
of both the peroxide and the cotton.
It's a long story, but I this experiment worked, at least on other
cellulose objects like paper towels. In fact, the fire in the
metal trash basket was hot enough to melt/burn away the bottom,
the linoleum underneath, and some of the concrete in the floor.
The flames formed a "solid" yellow flame and lots of thick white
smoke (containing NaOH dust). You really don't want to breathe
this stuff. We didn't, anyway :-)
5) Oh, another thing we did in that class was take the gas outlet used
for the bunsen burners and direct it into a test tube that was
partially submerged in liquid nitrogen. (The whole system was
sealed.) The gas condenses into a liquid... the only problem
was safe disposal. It helps to plan ahead! :-)
[ Note that nuke@reed.edu subsequently supplied the following warning ]
" If you decide to try this be aware that liquid nitrogen will condense
liquid oxygen in a vessel open to the air immersed in it. Liquid O2
forms explosive mixtures with many organics. IF you still want to try
it, immerse the tube in the nitrogen and then immediately run the gas
in. only do a little bit. How much you get depends on what proportion
of weights of low hydrocarbons the gas contains ( I think methane
condenses at this temp, but not quantitatively like some stuff, unless
there'e a large surface area)."
6) One of my classmates made luciferin [sic]. It's a liquid that
glows in the dark for about 12 hours. That was fun too!
Happy researching!
Standard disclaimers apply; I'm not sure my company would have hired
me if they had the foregoing admissions before them.
Non-standard disclaimers too: I don't recommend you do any of these things
either.
From: fred@theory.chem.pitt.edu < fred >
If you would like to condense out methane gas in a relatively safe way,
fill a balloon with the gas and THEN condense the gas with liquid N2.
You can use scissors to cut the balloon, and pour the liquid CH4 into
a beaker with water in it (notice that it floats, forms ice, etc.) and
light it. Only the fumes burn as they mix with atmospheric oxygen.
This makes a fair "olympic torch." Wear goggles etc.
From: Larry (Call me "Lefty") C
One that can be safely performed with a long enough spatula.
Mix Calcium Carbide with any strong oxidizer (KMnO4, NaNO3, even MnO2
works). Proportions aren't real important here.
Using face shield, gloves, lab coat and long spatula, drop a SMALL
amount (say, 1 gram or so) of this into common household bleach.
Acetylene and chlorine are evolved, which immediately, uh... exploded
Delightful chlorinated hydrocarbons result, unfortunately :(
------------------------------
Subject: 16. Laboratory Procedures
16.1 What are the best drying agents for liquids and gases?
The Rubber Handbook lists the traditional information on drying agents
that involve on chemical action. This lists phosphorus pentoxide and
magnesium perchlorate as the most effective desiccants. However, later
work by Burfield [1-9] has demonstrated that much of the traditional
information is misleading. He found that the efficiency of the desiccant
is strongly dependent upon the solvent. He also found that Drierite
( anhydrous calcium sulphate ) is only a moderately efficient desiccant for
organic solvents [9], and that correctly prepared molecular sieves are
often the preferred desiccant [2]. His publications are highly recommended.
16.2 What is the effect of oven drying on volumetric glassware?
Many older laboratory texts insist that volumetric glassware should not
be oven dried because of the danger of irreversible and unpredictable
volume changes. However most modern laboratory glassware is now made of
Pyrex, and work by D.R.Burfield has demonstrated that low temperature
drying does not significantly affect the calibration of volumetric
glassware [10]. He demonstrated that exposing volumetric flasks and
pipettes to 320C, either continuously or thermally cycled, resulted in no
significant detectable change to the calibration. He concluded that
"oven temperatures in the range of 110-150C should provide efficient drying
of glassware with no risk of discernable volume changes, even after
prolonged use, providing that Pyrex glass is the material of construction".
16.3 What does the Karl Fischer titration measure?
In 1935 Karl Fischer used the reaction between iodine, sulfur dioxide, and
water to produce a technique for quantifying water [11]. In aqueous solution,
the reaction can be presented as I2 + SO2 + 2H2O <=> 2HI + H2SO4.
He used anhydrous methanol to dissolve the I2 and SO2, and added pyridine
to move the equilibrium to the right by reacting the acidic products.
Fischer assumed his modifications did not change the reaction and one mole
of iodine was equivalent to two moles of water. Smith et al. [12], demonstrated
that both the methanol and pyridine participate in the reaction and one mole
of iodine is equivalent to one mole of water. They suggested two steps:-
(1) SO2 + I2 + H2O + 3RN -> 2RN.HI + RN(SO2)O
(2) RN(SO2)0 + CH3OH -> RN(SO4CH3)H (where R = base = C5H5 for pyridine)
This was further investigated by E.Scholz [13], who proposed:
(1) CH3OH + SO2 + RN -> (RNH)SO3CH3
(2) H20 + I2 + (RNH)SO3CH3 + 2RN -> (RNH)SO4CH3 + 2(RNH)I (where R = Base)
The advantage of the Karl Fischer titration is that it has few interferences
and can quantify water from < 1ppm to 100% in diverse samples, ranging from
gases to polymers. It will measure all water that is made available to the
reagent. the endpoint is usually ascertained using a dead-stop endpoint,
and for low water levels coulometric techniques are used to quantitatively
produce the iodine by anodic oxidation of iodide. The procedures are
described in detail in ASTM, AOAC etc.
16.4 What does the Dean and Stark distillation measure?
The Dean and Stark procedure can be used to measure the water content of
a diverse range of samples, and has been extensively used in industrial
laboratories to measure water in petroleum oils. The technique can measure
% levels of water, but is not as accurate as the Karl Fischer titration,
and is not applicable to samples where the water is not liberated by the
solvent. The sample is mixed with a solvent ( usually a toluene/xylene mix )
and refluxed under a condenser using a special receiver. There are two common
designs of receivers, one for solvents that are heavier than water, and the
more common one for solvents that are lighter than water - examples will be
in most laboratory glassware supplier catalogues. The water and solvent are
distilled, and as they condense the two phases separate as they run into
the receiver. The water remains in the receiver while the solvent returns
to the flask. The Dean and Stark receiver is also useful for removing
unwanted water from reactions, eg the synthesis of dibutyl ether by the
elimination of water from two molecules of n-butanol using acidic conditions.
An example of this is provided in the preparation of dibutyl ether described
in Vogel, and detailed procedures for the determination of water are
provided in ASTM and AOAC.
16.5 What does Kjeldahl nitrogen measure?
The Kjeldahl procedure is routinely used to measure the nitrogen content
of organic compounds such as proteins. Contrary to popular belief, the
procedure does not determine total nitrogen on all organic compounds,
as it is not applicable to materials containing N-O or N-N linkages.
This oversight often creates confusion if the actual analytical procedure
is not reported. Some organics compounds require aggressive digestion
conditions to make all the organic nitrogen available, consequently
Kjeldahl procedures should not be used on samples of unknown origin.
Details of procedures for foods are in the AOAC handbooks, and general
procedures are in ASTM.
16.6 What does a Soxhlet extractor do?
The soxhlet extractor enables solids to be extracted with fresh warm solvent
that does not contain the extract. This can dramatically increase the
extraction rate, as the sample is contacting fresh warm solvent. The sample
is placed inside a cellulose or ceramic thimble and placed in the extractor.
The extractor is connected to a flask containing the extraction solvent, and
a condenser is connected above the extractor. The solvent is boiled, and the
standard extractor has a bypass arm that the vapour passes through to reach
the condenser, where it condenses and drips onto the sample in the thimble.
Once the solvent reaches the top of the siphon arm, the solvent and extract
are siphoned back into the lower flask. There is an alternative design where
the hot solvent vapour passes around the thimble, thus boiling the solvent
in the thimble - this can be a problem if low-boiling azeotropes form.
Procedures for using soxhlet extractors are described ( along with
illustrations which might make the above description comprehensible :-) ),
in Vogel and many other introductory organic laboratory texts.
------------------------------
Subject: 17. Preparation of chemicals
17.1 Where do I find laboratory-scale procedures for organics?
The best introductory handbooks are practical textbooks, eg "Organic" Vogel
and "EPOC" Vogel. They provide a diverse range of experiments that soon help
develop synthetic skills. If you master the preparations in Vogel you are
at the stage where you can start to obtain papers from organic chemistry
journals and reproduce their syntheses. There are also several texts that
discuss techniques for purifying laboratory chemicals, eg [1] The parameters
of common specialist synthetic procedures usually are fully described in
specialist texts that will only normally be available in chemistry department
libraries ( eg Palladium Reagents in Organic Syntheses [2]). Most educational
nstitutions will have a structured laboratory programme to develop skills.
17.2 Where do I find laboratory-scale procedures for inorganics?
Most synthetic chemistry of inorganics appears to be devoted more to complex
organometallics, superacids and superconductors than common inorganics, but
it is worth considering that, of the top fifteen industrial chemicals
produced, the only organic compounds are ethylene, propylene, ethylene
dichloride and urea. There are specialist texts available that describe how
to purify inorganic laboratory reagents, eg [1]. I expect some inorganic
chemists to berate me for not knowing the standard inorganic synthesis
textbooks. ;-)
17.3 Where do I find industrial chemical process details?
The standard text for common processes remains Shreve, and I must admit that
I enjoy reading the 1945 first edition to obtain a good overview of an
industry. McKetta provides excellent process design details, along with
comparisons of various processes. Kirk Othmer provides an excellent update
on the various processes and chemicals used extensively today. Kirk Othmer
remains the first port of call, but Ullmann is a close second. Both of these
provide extensive references to more specific texts.
Industry journals, eg Hydrocarbon Processing, offer annual reviews of the
processes used in their industry. Patent literature has to be treated
cautiously, as it is not always immediately obvious which patents detail
actual viable processes. Chemical engineering texts, eg Perry, provide
comprehensive detail of the equipment and operational parameters.
------------------------------
Subject: 18. Sensory properties of chemicals
18.1 How do light sticks work?, and how can I make one?
From: perks@umbc.edu (Mark Perks) Date: 15 Sep 1994
Subject: Re: Chemiluminescence Sticks
Chemical Demonstrations [1] ( v.1 p.146), by Bassam Shakhashiri, offers a
thorough discussion of CYALUME lightsticks. Professor Shakhashiri is at
the University of Wisconsin, Madison, I believe.
"The CYALUME lightstick contains dilute hydrogen peroxide in a
phthalic ester solvent contained in a thin glass ampule, which is
surrounded by a solution containing a phenyl oxalate ester and the
fluorescent dye 9,10-bis(phenylethynyl)anthracene...When the ampule is
broken, the H2O2 and oxalate ester react.."
From: chideste@pt.Cyanamid.COM (Dale Chidester) Date: Mon, 13 Mar 1995
Subject: Re: How to make chemical light ?
The following produce rather spectacular results. Chemicals are
available through FLUKA or ALDRICH. The dyes are expensive.
9,10-bis(phenylethynyl)anthracene (BPEA) (yellow) FLUKA 15146
9,10-diphenylanthracene (DPA) (blue) FLUKA 42785
5,6,11,12-tetraphenylnaphthacene (rubrene) (red) FLUKA 84027
bis(2-carbopentyloxy-3,5,6-trichlorophenyl)oxalate (CPPO) ALDRICH 39,325-8
bis(2-ethylhexyl)phthalate (solvent) FLUKA 80032
sodium salicylate (catalyst) FLUKA 71945
35% hydrogen peroxide FLUKA 95299
Saturate solvent with dye and CPPO. Sonicate to help solvation. Start with
about 50 mg dye (BPEA, DPA or rubrene) in 10 g solvent with 50 mg CPPO and
5 mg sodium salicylate. CPPO is limiting reagent.
Put small quantity (20 drops) in a small vial and add equal volume of
hydrogen peroxide. Mix vigorously. There will be two phases. Avoid skin
contact! Don't cap tightly!
The following explanation of the chemistry is
From: sbonds@jarthur.claremont.edu (007)
>>>>>>>>>>>>>>>>>>>>>>
All of the material below is taken from a chemical demonstrations book
for which, unfortunately, I do not have the bibliographic information. It
was titled something like "Chemical Demonstrations for Instructors", and
was a four-volume set. Anyway, on to the meat of the matter.
[ perks@umbc.edu (Mark Perks) subsequently resupplied the information ;-
Shakhashirir, B. Z. "Chemical Demonstrations" ; University of Wisconsin
Press: Madison, Wisconsin, 1992.]
The oxidant is hydrogen peroxide contained in a phthalic ester solvent.
The concentration is very low, less than 0.5%. The fluorescing solution
consists of a phenyl oxalate ester and a fluorescent dye. The dye used is
9,10-bis-(phenylethynyl)anthracene (for green) or 9,10-diphyenylanthracene
(for blue).
Here is the reaction sequence:
1) (Ph)-O-CO-CO-O-(Ph) + H2O2 --> (Ph)-O-CO-CO-O-OH + (Ph)-OH
2) (Ph)-O-CO-CO-O-OH --> O-O
| | + (Ph)-OH
OC-CO
3) C2O4 + Dye --> Dye* + 2CO2
4) Dye* --> Dye + hv
In 1) The hydrogen peroxide oxidizes the phenyl oxalate ester to a
peroxyacid ester and phenol. The unstable peroxyacid ester decomposes to
the cyclic peroxy compound and more phenol in step 2). The cyclic peroxy
compound is again unstable and gives off energy to the dye as it decomposes
to the very stable carbon dioxide. The dye then radiates this energy as
light.
>>>>>>>>>>>>>>>>>>>>>
18.2 How do hand warmers work?, and how can I make one?
They consist of an aqueous solution of sodium acetate with a small "clicker"
disk to impart a physical shock. The solute is dissolved into solution by
prior warming. when the heat is required, the disk is "clicked" to shock the
solution, and this causes the sodium acetate to crystallise from the now
supersaturated solution. The heat of crystallisation is slowly released.
18.3 What are the chemicals that give fruity aromas?
Most of the desirable food aromas come from low to medium molecular weight
organic compounds. Some examples of chemicals, and their use for both
fragrances and flavours.
Chemical Application
butyl acetate apple
isoamyl acetate banana
hexyl acetate pear
ethyl butyrate pineapple
ethyl isovalerate blueberry
ethyl 2-methylbutrate apple
ethyl hexanoate pineapple
2-propenyl hexanoate pineapple
ethyl 2t-4c-decadienoate pear
1-octen-3-ol mushroom
3-octanol mushroom
2,6-dimethyl-2-heptanol freesia
2t-6c-nonadien-1-ol violet
decanal citrus
acetoin butter
2,3-butadione butter
geraniol flowery, roselike
linalool lily of the valley
myrcenol lime
dihydromyrcenol lavender
citral lemon
citronellal balm mint
linalyl acetate bergamot
limonene lemon
alpha-terpineal lilac
8-mercapto-p-menthan-3-one blackcurrant
1-p-methene-8-thiol grapefruit
3-methyl-2-cyclopenten-2-ol-1-one caramel
phenethyl alcohol rose
phenethyl isoamyl ether chamomile
phenethyl acetate rose
alpha-trichloromethylbenzyl acetate rose
1-(4-hydroxyphenyl)-3-butanone raspberry
4 methyl-2(2-methyl-1-propenyl)tetrahydropyran rose
hexyl salicylate azalea
benzyl acetate jasmine
acetophenone orange blossom
18.4 What is the most obnoxious smelling compound?
Many low molecular weight sulfur-containing compounds tend to induce adverse
reactions in people, even if they have not encountered them before, eg the
glandular emissions of skunk (n-butyl mercaptan, dicrotyl sulfide).
Butyric acid reminds people of vomit, and cadaverine ( 1,5 Pentadiamine )
reminds people of rotten tissue, but without an earlier association they may
not regard them as unusually obnoxious.
18.5 What is the nicest smelling compound?
Aside from thinking about your stomach, then most people like the smell
of flowers and citrus fruits. Their aromas usually consist of medium
volatility compounds, often terpenes ( geraniol = (E)-3,7-dimethyl-2,6-
octadiene-1-ol = rose aroma; linalool = 3,7-dimethyl-1,6 -octadiene-3-ol
= bergamot or french lavender ). Many aromatic oils are mixtures of terpene
esters ( oil of geranium = geraniol esters ) or aldehydes ( oil of lemon
grass = citral = 3,7-dimethyl-2,6-octadienal ) The genuine flower smell is
usually a blend of compounds, and detailed compositions of your favourite
smell are often available in the journal " Perfumer and Flavorist "
18.6 What is the most bitter compound?
Denatonium Benzoate = Bitrex, or even in some strange chemistry circles,
N-[(2-[2,6-Dimethylphenyl)amino]-2-oxoethyl]-N,N-diethylbenzenemethan-
aminium benzoate. It is added to toxic chemicals as a deterrent to
accidental ingestion :-)
18.7 What is the sweetest compound?
According to Kirk-Othmer, an aspartic acid derivative ( RN = 61091-21-2 =
DL-Serine (9CI), N-L-.alpha.-aspartyl-O-methyl-3-oxo-, 1-(1,3,3-trimethyl
bicyclo(2.2.1)hept-2-yl) ester ), is 60X sweeter than saccharin, which is
itself about 500X sweeter than sucrose. There could be something even more
sweet?.
------------------------------
Subject: Sci.chem FAQ - Part 5 of 7
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Date: Sun, 17 Nov 1996 08:05:54 GMT
Archive-name: sci/chem-faq/part5
Posting-Frequency: monthly
Last-modified: 17 November 1996
Version: 1.08
Subject: 19. Physical properties of chemicals
19.1 Rheological properties and terminology
Contributed by Jim Oliver
RHEOLOGY
What is RHEOLOGY ?
RHEOLOGY describes the deformation of a material under the influence of
stresses. Materials in this context can be solids, liquids or gases. In this
FAQ we will be concerned only with the rheological properties of liquids.[1]
Perry discusses the some aspects of the behaviour of gases, and Ullmann
discusses elastic solids.
When liquids are subjected to stress they will deform irreversibly and flow.
The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids
are very few, whereas non-ideal examples abound. Ideal liquids are : water
and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour
mixed with a little water. [2]
What is VISCOSITY ?
VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the
conditions used to measure the VISCOSITY must be given. This is due to the
fact that non-ideal liquids have different values of VISCOSITY for different
test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4]
A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a
particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this
curve reveals the particular type of VISCOSITY for the liquid being studied.
[3]
What is a NEWTONIAN LIQUID ?
NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the
origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids
are mineral oil, water and molasses. (Issac NEWTON first described the laws
of viscosity) [1] All the other types are NON NEWTONIAN.
What does NON NEWTONIAN mean ?
a. PSEUDOPLASTIC liquids are very common. These display a curve starting at
the origin again and curving up and along but falling under the straight
line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results
in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with
increasing shear. Examples are toothpaste and whipped cream.
b. DILATANT liquids give a curve which curves under then upward and higher
than the straight line NEWTONIAN curve. (Like a square law curve) Such
liquids display increasing viscosity with increasing shear. Examples are
wet sand, and mixtures of starch powder with small amounts of water. A car
may be driven at speed over wet sand, but don't park on it, as the car may
sink out of sight due to the lower shear forces (compared to driving over)
the wet sand.
There are other terms used which include :
THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this
case the liquid shows a YIELD or PLASTIC POINT before starting to thin out.
What this means is the curve runs straight up the y axis for a short way then
curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve.
This YIELD POINT is time dependant. Some water based paints left overnight
develop a FALSE BODY which only breaks down to become usable after rapid
stirring. Also: the curve describing a THIXOTROPIC liquid will be different
on the way up (increasing shear rate) to the way down (decreasing shear rate).
The area inside these two lines is a measure of it's degree of THIXOTROPY.
This property is extremely important in industrial products, e.g to prevent
settling of dispersed solids on storage. [3]
A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing
viscosity with a constant shear rate over time. Again, time dependant but in
this case _increasing_ viscosity.
Why do some liquids become solid ?
A few special liquids (dispersions usually) display extraordinary DILATANT
properties. A stiff paste slurry of maize or cornflour in water can appear to
be quite liquid when swirled around in a cup. However on pouring some out
onto a hard surface and applying extreme shear forces (hitting with a hammer)
can cause a sudden increase in VISCOSITY due to it's DILATANCY. The
VISCOSITY can become so high as to make it appear solid. The "liquid" then
becomes very stiff for an instant and can shatter just like a solid material.
It should be noted that the study of viscosity and flow behaviour is
extremely complex. Some liquids can display more than one of the above
properties dependant on temperature, time and heat history.
19.2 Flammability properties and terminology
There are several properties of flammable materials that are frequently
reported. It should be remembered that most discussions concerning
flammable liquids usually consider air as the oxidant, but oxygen and
fluorine can also be used as oxidants for combustion, and they will result
in very different values.
The Flammability Limits in air, are usually reported as the Upper and Lower
limits in volume percent at a certain temperature ( usually 25C ), and
represent the concentration region that the vapour ( liquid HCs can not burn )
must be within to support combustion. Hydrocarbons have a fairly narrow range,
( n-hexane = 1.2 to 7.4 ) whereas hydrogen has a wide range ( 4.0 to 75 ).
The minimum ignition energy is the amount of energy ( usually electrical )
required to ignite the flammable mixture. Some mixtures only require a very
small amount of energy (eg hydrogen = 0.017mJ, acetylene = 0.017mJ ),
whereas others require more (eg n-hexane = 0.29mJ, diethyl ether = 0.20mJ,
ammonia = >1000mJ ).
The Flash Point is the most common measure of flammability today, especially
in transportation of chemicals, mainly because most regulations use the Flash
Point to define different classes of flammable liquids. The Flash Point of a
liquid is the temperature at which the liquid will emit sufficient vapours
to ignite when a flame is applied. The test consists of placing the liquid
in a cup and warming it at a prescribed rate, and every few degrees applying
a small flame to the air above the liquid until a "flash" is seen as the
vapours burn. Note that the flame is not applied continuously, but is
provided at prescribed intervals - thus allowing the vapour to accumulate.
There are a range of procedures outlined in the standard methods for
measuring Flash Point ( ASTM, ISO, IP ) and they have differing cup
dimensions, liquid quantity, headspace volume, rate of heating, stirring
speed, etc., but the most significant distinction is whether the space above
the liquid is enclosed or open. If the space is enclosed, the vapours will be
contained, and so the Flash Point is several degrees lower than if it is
open. Most regulations specify closed-cup methods, either Pensky-Martens
Closed Cup or Abel Closed Cup. It is important to remember that these methods
are only intended for pure chemicals, if there is water or any other volatile
non-flammable compounds present, their vapours can extinguish or mask the
flash. For used lubricants, this may be partially overcome by using the TAG
open cup procedure - which is slightly more tolerant of non-flammable
vapours. A material can be flammable, but may not have a flash point if other
non-flammable volatile compounds are present. For alkane hydrocarbons, Flash
Point increases with molecular weight.
There an older measure, called the Fire Point, which is the temperature at
which the liquid emits sufficient vapours to sustain combustion. The Fire
Point is usually several degrees above the Flash Point for hydrocarbons.
The minimum Autoignition Temperature is the temperature at which a material
will autoignite when it contacts a surface at that temperature. The procedure
consists of heating a glass flask and squirting small quantities of sample
into it at various temperatures until the vapours autoignite. The only
source of ignition is the heat of the surface. For the smaller hydrocarbons
the autoignition temperature is inversely related to molecular weight, but
also increases with carbon chain branching. Autoignition temperature also
correlates with gasoline octane ratings ( refer to Gasoline FAQ available in
rec.autos.tech, which lists octane ratings and autoignition temperatures for
a range of hydrocarbons.)
Flash Point Autoignition Flammable Limits
Temperature Lower Upper
( C ) ( C ) ( vol % at 25C)
methane -188 630 5.0 15.0
ethane -135 515 3.0 12.4
propane -104 450 2.1 9.5
n-butane -74 370 1.8 8.4
n-pentane -49 260 1.4 7.8
n-hexane -23 225 1.2 7.4
n-heptane -3 225 1.1 6.7
n-octane 14 220 0.95 6.5
n-nonane 31 205 0.85 -
n-decane 46 210 0.75 5.6
n-dodecane 74 204 0.60 -
19.3 Supercritical properties and terminology?
Supercritical fluids have some very unusual properties. When a compound is
subjected to conditions around the critical point ( which is defined as
the temperature at which the gas will not revert to a liquid regardless how
much pressure is applied ), the properties of the supercritical fluid become
very different to the liquid or the gas phases. In particular, the solubility
behaviour changes. The behaviour is neither that of the liquid or that of the
gas. The transition between liquid and gas can be completely smooth.
The pressure-dependant densities and corresponding Hildebrand solubility
parameters show no break on continuity as the supercritical boundary is
crossed. Physical properties fall between those of a liquid and a gas.
Diffusivities are approximately an order of magnitude higher than the
corresponding liquid, while viscosities are an order of magnitude lower.
These ( along with the lack of surface tension ) allow SCFs to have
liquid-like solvating power with the mass transport characteristics of a gas.
Potential Supercritical Fluids
Compound Critical Critical
Temperature Pressure
( C ) ( atm )
Ammonia 132.5 112.5
Carbon dioxide 31.3 72.9
Methanol 240.1 82.0
Nitrous oxide 36.5 72.5
Propane 96.8 43.1
Water 374.4 224.1
Xenon 16.6 58.4
Note that using liquid CO2 at pressure ( as for the commercial extraction
of hops ) is still just liquid CO2 extraction, not supercritical CO2
extraction. There are several good general introductions to supercritical
fluids [5,6]
------------------------------
Subject: 20. Optical properties of chemicals
20.1 Refractive Index properties and terminology
When light passes between media of different density, the direction of the
beam is changed as it passes through the surface, and this is called
refraction. In the first medium, the angle between the light ray and the
perpendicular is called the angle of incidence (i), and the corresponding
angle in the second medium is called the angle of refraction (r). The
ratio sine i / sine r is called the index of refraction, and usually the
assumption is that the light is travelling from the less dense (air) to more
dense, giving an index of refraction that is greater than 1. Although the
theoretical reference is a vacuum, air ( 0.03% different ) is usually used.
The refractive index of a compound decreases with increasing wavelength
( dispersion ), except where absorption occurs, thus the wavelength should
be reported. The D lines of sodium are commonly used.
The refractive index of a liquid varies with temperature and pressure, but
the specific refraction ( Lorentz and Lorentz equation ) does not. The molar
refraction is the specific refraction multiplied by the molecular weight,
and is approximately and additive property of the groups or elements
comprising the compound. Table of atomic refractions are available in the
literature, as are descriptions of the common types of refractometers [1].
20.2 Polarimetry properties and terminology
Supplied by: Vince Hamner
Polarimetry is a method of chemical analysis that is concerned
with the extent to which a beam of linearly polarized light is rotated
during its transmission through a medium containing an optically active
species.[2] Helpful discussions regarding polarized light may be found
elsewhere.[3,4] In general, a compound is optically active if it has
no plane of symmetry and is not superimposable on its mirror image.
Such compounds are referred to as being "chiral". Sucrose, nicotine,
and the amino acids are only a few of these substances that exhibit
an optical rotary power.
A simple polarimeter instrument would consist of:
1). a light source -- typically set to 589 nm (the sodium "D" line)
2). a primary fixed linear polarizing lens (customarily called the
"polarizer")
3). a glass sample cell (in the form of a long tube)
4). a secondary linear polarizing lens (customarily called the
"analyzer") and
5). a photodetector.[5]
Biot is credited with the determination of the basic equation
of polarimetry.[6,7] The specific rotation of a substance (at a given
wavelength and temperature) is equivalent to the observed rotation (in
degrees) divided by the pathlength of the sample cell (in decimeters)
multiplied by the concentration of the sample (for a pure liquid,
-density- replaces concentration). Influences of temperature,
concentration, and wavelength must always be taken into consideration.
If necessary, it is possible to apply corrections for each of these
variables.[8] A few early contributors to our understanding of optical
activity and polarimetry include: Malus, Arago, Biot, Drude, Herschel,
Fresnel, and Pasteur.
------------------------------
Subject: 21. Molecular and Structural Modelling
Supplied by: Dave Young (young@slater.cem.msu.edu)
21.1 What hardware do I need to run modelling programs?
There are two types of programs that are referred to as molecular
modeling programs. This first is a program which graphically displays
molecular structures as Lewis structures, ball & stick, etc. The second
is a program which does a calculation to tell you something about the
molecule, such as it's energy, dipole moment, spectra, etc.
For an introductory description of various types of computations,
see http://www.cem.msu.edu/~young/topics/contents.html
There are many programs of both sorts available for a large range
of machines. The speed, memory, graphics and disk space on the machine
will determine how big of molecules can be modeled, how accurately and
how good the images will look. There are a few programs that will run
on a 286 PC with windows. There are some fairly nice things that can be
done on a 386 with about 8 MB of RAM and windows. The professional
computational chemists are generally using work stations and larger machines.
Currently many computational chemists are using machines made by
Silicon Graphics (SGI) ranging from the $5,000 Indy to the $1,000,000
power challenge machines. These are all running Irix, which is SGI's
adaptation of Unix. SGI is popular for two reasons; first that the power
is very good for the price, second that SGI's run the largest range
of chemical software. However, you will find some computational chemistry
software that can run on almost any machine.
As far as graphics quality, the SGI Onyx (about $250,000) is about
the top of the line. Even if you find a machine that claims to have better
graphics than this, chances are you won't find and chemistry software that
can utilize it.
For chemical calculations there is no limit to the computing
power necessary. There are some calculations that can only be done
on the biggest Cray's or massivly parallel machines in the world. There
are also many calculations which are too difficult for any existing
machine and will just have to wait a few years or a few centuries.
21.2 Where can I find a free modelling program?
The single best place for public domain modelling software
is probably the anonymous ftp server at ccl.osc.edu in the directory
pub/chemistry/software. "ccl" stands for "computational chemistry
list server" and is a list frequented mostly by professional
computational chemistry researchers. This machine contains their
archives with quite a bit of information as well as software.
For work stations and larger, the program GAMESS (General Atomic
and Molecular Electronic Structure System) can be obtained as source
code from Mike Schmidt at mike@si.fi.ameslab.gov GAMESS is a quantum
mechanics, ab initio and semiempirical program. It is powerful but
not trivial to learn how to use.
The COLUMBUS program for work stations and larger can be obtained
by anonymous ftp from ftp.itc.univie.ac.at It is a HF, MCSCF and
multi-reference CI program. This is probably the most difficult program
to use that is in use today since it requires the user to input EVERY
detail manually. However, because you control everything there are some
calculations that can only be done with COLUMBUS.
CACAO is an extended Huckel program available by anonymous
ftp at cacao.issecc.fi.cnr.it
21.3 Where can I find structural databanks?
21.4 Where can I find ChemDraw or ChemWindows
For ChemDraw (Macintosh, Windows, UNIX)
CambridgeSoft Corporation
875 Massachusetts Avenue
Cambridge, MA 02139
Phone: (800) 315-7300 or (617) 491-2200
Fax: (617) 491-7203
Internet: info@camsci.com
http://www.camsci.com
For ChemIntosh or ChemWindows
SoftShell
1600 Ute Avenue
Grand Junction, CO 81501
Phone: (970) 242-7502
Fax: (970) 242-6469
Internet: info@softshell.com
http://www.softshell.com
------------------------------
Subject: 22. Spectroscopic Techniques
All of these are covered in texts on instrumental Analysis [1-4], and I
will eventually include a paragraph about each.
22.1 Ultra-Violet/Visible properties and terminology
22.3 Nuclear Magnetic Resonance properties and terminology
22.4 Mass Spectrometry properties and terminology
22.5 X-Ray Fluorescence properties and terminology
22.6 X-Ray Diffraction properties and terminology
22.7 Fluorescence/Phosphorescence properties and terminology
------------------------------
Subject: 23. Chromatographic Techniques
23.1 What is Paper Chromatography?
Paper chromatography was the first analytical chromatographic technique
developed, allegedly using papyrus (Pliny). It was first published by Runge
in 1855, and consists of a solvent moving along filter or blotting paper.
The interaction between the components of the sample, the solvent and the
paper results in separation of the components. Most modern paper
chromatography is partition chromatography, where the cellulose of the
paper is the inert support, the water adsorbed ( hydrogen bonded ) from air
onto the hydroxyl groups of the cellulose is the stationary phase. If the
mobile phase is not saturated with water, then some of the stationary phase
water may be removed from the cellulose resulting in a separation that is
a mixture of partition and adsorption. Paper chromatography remains the
method of choice for a wide range of coloured compounds, and is used
extensively in flower colour research. The technique is suitable for any
molecules that are significantly less volatile than the solvent, and many
examples and references are provided in Heftmann [1].
23.2 What is Thin Layer Chromatography?
Thin layer chromatography involves the use of a particulate sorbent on an
inert sheet of glass, plastic, or metal. The solvent is allowed to travel
up the plate with the sample spotted on the sorbent just above the solvent.
Depending on the sorbent, the separation can be either partition or
adsorption chromatography ( cellulose, silica gel and alumina are commonly
used ). The technique came to prominence during the late 1930s, however it
did not become popular until Merck and Desaga developed commercial plates
that provided reproducible separations. The major advantage of TLC is the
disposable nature of the plates. Samples do not have to undergo extensive
cleanups as they would for HPLC. The other major advantage is the ability
to detect a wide range of compounds cheaply using very reactive reagents
( iodine vapours, sulfuric acid ) or indicators. Non-destructive detection
( fluorescent indicators in the plates, examination under a UV lamp ) also
means that purified samples can be scraped off the plate and analyzed by
other techniques. There are special plates for such preparative separations,
and there are also high-performance plates that can approach HPLC resolution.
The technique is described in detail in Stahl [2] and Kirchner [3].
23.3 What is Gas Chromatography?
Gas chromatography is the use of a gas to carry the sample through a column
consisting of an inert support and a stationary phase that interacts with
sample components, thus it is usually partition chromatography, however
there are also a range of materials, especially for permanent gas and
light hydrocarbon analysis that utilise adsorption. The simplest partition
systems consisted of a steel tube filled with crushed brick that had been
coated with a high boiling hydrocarbon. Today the technique uses very narrow
fused silica tubes ( 0.1 to 0.3mm ID ) that have sophisticated stationary
phase films ( 0.1 to 5um ) bonded to the surface and also cross-linked to
increase thermal stability. The ability of the film to retard specific
compounds is used to ascertain the "polarity" of the column. If benzene
elutes between normal alkanes where it is expected by boiling point ( midway
between n-hexane and n-heptane ), then the column is "non-polar" eg
squalane and methyl silicones. If the benzene is retarded until it elutes
after n-dodecane, then the column is "polar" eg OV-275 ( dicyanoallyl
silicone ) and 1,2,3-tris (2-cyanoethoxy) propane. In general polar columns
are less tolerant of oxygen and reactive sample components, but the ability
to select a select different polarity columns to obtain satisfactory peak
resolution is what made GC so popular.
The column is placed in an oven which has exceptional temperature control,
and the column can be slowly heated up to 350-450C ( sometimes starting at
-50C to enhance resolution of volatile compounds ) to provide separation of
wide-boiling range compounds. The carrier gas is usually hydrogen or helium,
and the eluting compounds can be detected several ways, including in flames
( flame ionisation detector ), by changes in properties of the carrier
( thermal conductivity detector ), or by mass spectrometry. The availability
of "universal" detectors such as the FID and MS, makes GC a popular tool in
laboratories handling organic compounds. There are also columns that have a
layer of 5-10 um porous particulate material (such as molecular sieve or
alumina ) bonded to the inner walls ( PLOT = Porous layer open tubular ),
and these are used for the separation of permanent gases and light
hydrocarbons. GC is restricted to molecules ( or derivatives ) that
are sufficiently stable and volatile to pass through the GC intact at the
temperatures required for the separation. Specialist books on the production
of derivatives for GC are available [4,5].
There are several manufacturers of GC instruments whose catalogues and
brochures provide good introduction to the technique. (eg Hewlett Packard,
Perkin Elmer, Carlo Erba ). The catalogues of suppliers of chromatography
consumables also contain explanations of the criteria for selection of the
correct columns and conditions for analyses, and they provide an excellent
indication of the range of applications available. Well-known suppliers
include Alltech Associates, Supelco, Chrompack, J&W;, and Restek. They also
sell most of the standard GC texts, as do the instrument manufacturers.
Popular GC texts include "Basic Gas Chromatography" [6], "High-Resolution
Gas Chromatography" [7], and "Open Tubular Column Gas Chromatography" [8].
There are Standard Retention Index Libraries available [9], however they
really only complement unambiguous identification by mass spec. or
dual-column analysis.
23.4 What is Column Chromatography?
Column chromatography consists of a column of particulate material such as
silica or alumina that has a solvent passed through it at atmospheric or low
pressure. The separation can be liquid/solid (adsorption) or liquid/liquid
(partition). The columns are usually glass or plastic with sinter frits to
hod the packing. Most systems rely on gravity to push the solvent through.
The sample is dissolved in solvent and applied to the front of the
column. The solvent elutes the sample though the column, allowing the
components to separate based on adsorption ( alumina, hydroxylapatite) or
partition ( cellulose, diatomaceous earth ). The mechanism for silica
depends on the hydration. Traditionally, the solvent was non-polar and the
surface polar, although today there are a wide range of packings including
bonded phase systems. Bonded phase systems usually utilise partition
mechanisms rather than adsorption. The solvent is usually changed stepwise,
and fractions are collected according to the separation required, with the
eluate usually monitored by TLC.
The technique is not efficient, with relatively large volumes of solvent
being used, and particle size is constrained by the need to have a flow of
several mls/min. The major advantage is that no pumps or expensive equipment
are required, and the technique can be scaled up to handle sample sizes
approaching a gram in the laboratory. The technique is discussed in detail
in Heftmann [1].
23.5 What is High Pressure Liquid Chromatography?
HPLC is a development of column chromatography. it was long realised that
using particles with a small particle size ( 3,5,10um ) with a very narrow
size distribution would greatly improve resolution, especially if the flow
rate and column dimensions could be adjusted to minimise band-broadening.
Pumps were developed that could handle both the chemicals and pressures
required. Traditional column chromatography ( nonpolar solvent and
polar surface ) is described as "normal" and, as well as silica, there are
columns with amino, diol, and cyano groups. If the system uses a polar
solvent ( water, methanol, acetonitrile etc. ) and a non-polar surface it
is described as "reversed phase". Common surface treatments of silica include
octadecylsilane ( aka ODS or C18), and it has been the development of
reverse-phase HPLC that has experienced explosive growth. Reverse-phase HPLC
is the method of choice for larger non-volatile biomolecules, however it is
only recently that a replacement "universal" detector ( evaporative
light-scattering ) has emerged. The most popular detector (UV), places
constraints on the solvents that can be used, and the refractive index
detector can not easily be used with solvent gradients. There are several
excellent books introducing HPLC, including the classic "Introduction to
Modern Liquid Chromatography" [10]. HPLCs can be a pain to operate, and
novices should borrow "Troubleshooting LC Systems" by Dolan and Snyder [11].
There is also a handy basic primer on developing HPLC methods by Snyder [12],
however, unlike GC, you need to search the journals ( Journal of
Chromatography, Journal of Liquid Chromatography ) to find relevant examples
to assist method development.
23.6 What is Ion Chromatography?
Ion chromatography has become the method of choice for measuring anions
( eg Cl-, SO4=, NO3- ) in aqueous solutions. It is effectively a development
from ion-exchange systems ( which were extensively developed to deionise
water and aqueous process streams ), and brings them down to HPLC size.
IC uses pellicular polymeric resins that are compatible with a wide pH range.
The sample is eluted through an ion-exchange column using a dilute sodium
hydroxide solution. The eluent is passed through self-regenerating
suppressors that neutralise eluant conductance, ensuring electrochemical
detectors ( conductivity or pulsed amperometric ) can detect the ions down
to sub-ppm concentrations. The major manufacturer of such systems is Dionex,
who hold several patents on column, suppression, and detection technology.
There are several books covering various aspects of the technique [13,14].
23.7 What is Gel Permeation Chromatography?
Gel Permeation chromatography ( aka Size Exclusion chromatography ) is based
on the ability of molecules to move through a column of gel that has pores of
clearly-defined sizes. The larger molecules can not enter the pores, thus
they pass quickly through the column and elute first. Slightly smaller
molecules can enter some pores, and so take longer to elute, and small
molecules can be delayed further. The great advantage of the technique is
simplicity, it is isocratic ( single solvent - no gradient programming ),
and large molecules rapidly elute. The technique can be used to determine
the molecular weight of large biomolecules and polymers, as well as
separating them from salts and small molecules. The columns are very
expensive and sensitive to contamination, consequently they are mainly used
in applications where alternative separation techniques are not available,
and sample are fairly clean. The best known columns are the Shodex
cross-linked polystyrene-divinylbenzene columns for use with organic solvents,
and polyhydroxymethacrylate gel filtration columns for use with aqueous
solvents. "Modern Size Exclusion Chromatography" [15], and Heftmann [1],
provide good overviews, and there are some good introductory booklets from
Pharmacia.
23.8 What is Capillary Electrophoresis?
Capillary electrophoresis uses a small fused silica capillary that has been
coated with a hydrophilic or hydrophobic phase to separate biomolecules,
pharmaceuticals and small inorganic ions. A voltage is applied and the
materials migrate and separates according to charge under the specific
pH conditions,as happen for electrophoresis.The capillary can also be used
for isoelectric focusing of proteins. The use of salt or vacuum mobilization
is no longer required.
23.9 How do I degas chromatographic solvents?
One major problem with pressurising chromatography systems using liquid
solvents is that pressure reductions can cause dissolved gases to come out
of solution. The two locations where this occurs are the suction side of the
pump ( which is not self-priming, consequently a gas bubble can sit in the
pump and flow is reduced ), and at the column outlet ( where the bubbles
then pass through the detector causing spurious signals).Note that the
problem is usually restricted to solvents that have relatively high gas
solubilities - usually involving an aqueous component, especially if a
gradient is involved where the water/organic solvent ratio is changing.
As water usually has a higher dissolved gas content, then a gradient
programme may cause the gases to come out of solution as the mobile phase
components mix.
There are three traditional strategies used to remove problem dissolved
gases from chromatographic eluants. Often they are used in combination to
lower the dissolved gases.
a. Subject the solvent to vacuum for 5-10 mins. to remove the gases.
b. Subject the solvent to ultrasonics for 10-15 mins. to remove the gases.
c. Sparge the solvent with a gas that has a very low solubility compared
to the oxygen and nitrogen from the atmosphere. Helium is the preferred
choice - 5 minutes of gentle bubbling from a 7um sinter is usually
sufficient, although maintaining a positive He pressure is even better.
Note that most aqueous-based solvents usually have to be degassed every
24 hours. Also remember that solubility of gases increases as temperature
decreases, so ensure eluants are at instrument temperature prior to
degassing.
Modern HPLCs are sold with a "solvent degassing module" that removes
undissolved gases in the eluent automatically.
23.10 What is chromatographic solvent "polarity"?
There are four major intermolecular interactions between sample and solvent
molecules in liquid chromatography, dispersion, dipole, hydrogen-bonding,
and dielectric. Dispersion interactions is the attraction between each pair
of adjacent molecules, and are stronger for sample and solvent molecules
with large refractive indices. Strong dipole interactions occur when both
sample and solvent have permanent dipole moments that are aligned. Strong
hydrogen-bonding interactions occur between proton donors and proton
acceptors. Dielectric interactions favour the dissolution of ionic
molecules in polar solvents. The total interaction of the solvent and
sample is the sum of the four interactions. The total interaction for a
sample or solvent molecule in all four ways is known as the "polarity" of
the molecule. Polar solvents dissolve polar molecules, and for normal
phase partition chromatography solvent strength increases with solvent
polarity, whereas solvent strength decreases with increasing polarity.
The subject is discussed in detail in Snyder and Kirkland [10].
------------------------------
Subject: 24. Extraction Techniques
24.1 What is Solvent Extraction?
Solvent extraction is usually used to recover a component from either a solid
or liquid. The sample is contacted with a solvent that will dissolve the
solutes of interest. Solvent extraction is of major commercial importance
to the chemical and biochemical industries, as it is often the most efficient
method of separation of valuable products from complex feedstocks or
reaction products. Some extraction techniques in involve partition between two
immiscible liquids, others involve either continuous extractions or batch
extractions. Because of environmental concerns, many common liquid/liquid
processes have been modified to either utilise benign solvents, or move to
more frugal processes such as solid phase extraction. The solvent can be a
vapour, supercritical fluid, or liquid, and the sample can be a gas, liquid
or solid. There are a wide range of techniques used, and details can be found
in Organic Vogel, Perry as well as any textbook on unit operations.
24.2 What is Solid Phase Extraction?
Solid Phase Extraction (SPE) is an alternative to liquid/liquid extraction,
which has been the method of choice for the separation and purification of
a wide range of samples in the laboratory. The sample is usually dissolved in
an appropriate solvent and passed through a small bed of appropriate
particulate adsorbent. The compounds are eluted off with small amounts of
different solvents. The major advantage is that solvent volumes are greatly
reduced. There is a newer, modified technique that is used in analytical
laboratories, called Solid Phase MicroExtraction. This immerses a fused
silica fibre coated with a stationary phase into the sample solution for
several minutes, The analytes adsorb onto the stationary phase, which is
subsequently pushed into a hot GC injector to rapidly desorb the sample.
24.3 What is Supercritical Fluid Extraction?
Supercritical fluids have been investigated since last century, with the
strongest commercial interest initially focusing on the use of supercritical
toluene in petroleum and shale oil refining during the 1970s. Supercritical
water is also being investigated as a means of destroying toxic wastes, and
as an unusual synthesis medium [1]. The biggest interest for the last decade
has been the applications of supercritical carbon dioxide, because it has
a near-ambient critical temperature (31C), thus biological materials can
be processed at temperatures around 35C. The density of the supercritical
CO2 at around 200bar pressure is close to that of hexane, and the solvation
characteristics are also similar to hexane, thus it acts as a non-polar
solvent. Around the supercritical region CO2 can dissolve triglycerides at
concentrations up to 1% mass. The major advantage is that a small reduction
in temperature, or a slightly larger reduction in pressure, will result in
almost all of the solute precipitating out as the supercritical conditions
are changed or taken to subcritical. Supercritical fluids can produce a
product with no solvent residues. Examples of pilot and production scale
products include decaffeinated coffee, cholesterol-free butter, low-fat meat,
evening primrose oil, squalene from shark liver oil. The solvation
characteristics of supercritical CO2 can be modified by the addition of an
entrainer, such as ethanol, however that then remains as a solvent residue
in the product, negating some of the advantages of "residue-free" extraction.
There are other near-ambient temperature supercritical fluids, including
nitrous oxide and propane, however there are safety issues with both of them.
There are several introductory texts on supercritical fluid extraction,
including some the ACS Symposium series [2-4]. There are also a large
number of articles on applications of the technique, including processing [5],
extraction of natural products [6], and chemical synthesis [7]. The major
concentration of information occurs in the various proceedings of the
International Symposium on Supercritical Fluids [8]. There is also a Journal
of Supercritical Fluids.
------------------------------
Subject: 25. Radiochemical Techniques
25.1 What is radiochemistry?
------------------------------
Subject: 26. Electrochemical Techniques
26.1 What is pH?
The pH scale determines the degree of acidity or alkalinity of a solution,
but as it involves a single ion activity it can not be measured directly.
pH = - log10 ( gammaH x mH)
gammaH = hydrogen ion single ion activity coefficient
mH = molality of the hydrogen ion.
As pH can not be directly measured, it is defined operationally according to
the method used to determine it. IUPAC recommend several standardised methods
for the determination of pH in solution in aqueous solutions. There are
seven primary reference standards that can be used, including 0.05 mol/kg
potassium hydrogen phthalate as the Reference Value Standard. There is an
ongoing debate concerning the relative merits of having a multiple primary
standard scale ( that defines pH using several primary standards, and their
values are determined using a cell without a liquid junction ) or a single
primary standard ( that requires a cell with a liquid junction ). Interested
readers can obtain further information on the debate in [1]. Bates [2], is a
popular text covering both theory and practise of pH measurement.
26.2 How do pH electrodes work?
Contributed by Paul Willems
The most common type of pH electrodes are the so called glass electrodes.
A special glass membrane is sensitive to variations in pH and a pH
variation creates a variation in the potential over the glass. In order
to be able to measure this potential, a second electrode, the so called
reference electrode is required. Quite often both electrodes are combined
to one "combined" pH electrode.
The glass electrode consists of a glass shaft on which a bulb of a special
glass is mounted. The inner is filled with KCl, most often at a
concentration of 3 Mol/liter and sealed. Electrical contact is provided
by the way of a silver wire immersed in the KCl.
Normally this glass electrode is surrounded by a concentric reference
electrode. This reference electrode can consists of a silver wire in
contact with the almost insoluble AgCl. The electrical contact with
the meter is through the silver wire. The contact with the solution
to be measured is by way of a KCl filling solution which is physically
in contact with the solution to be measured. In order to minimise
mixing of the solution to be measured and the filling solution, a porous
sealing, the diaphragm, is used. Alternatively other devices which
allow a slow mixing contact can also be used. Besides the "normal"
KCl solutions, often solutions with an increased viscosity, and hence
lower mixing rate are used. In stead of a liquid KCl filling, also
gel filling is used. This eliminates the necessity of low mixing devices.
The glass electrode in contact with some solution gives in respect to
the reference electrode a voltage of about 0 mV at pH 7, increasing with
59 mV per pH above 7 or decreasing with 59 mV per pH unit below 7.
Both the slope and the intercept of the curve between pH and generated
potential are temperature dependent. In fact, the potential of the
electrode is roughly given by the Nernst equation :
E = E0 - RT log [H+] = E0 + RT pH
In which E is de generated potential, E0 is a constant, R is universal
gas constant and T is the temperature in degrees Kelvin.
All pH dependent glasses are also susceptible to other ions, such as
Na or K. This gives an correction on the above equation. By this reason
the relation between pH and generated voltage becomes nonlinear at
high pH values.
Also the slope tends to diminish as the electrode wears out. At high pH the
slope tends also to diminish. As the electrode has a very high impedance,
typically 250 Mega Ohms to 1 giga Ohm, it is absolutely necessary to use a
very high impedance measuring apparatus.
The reference electrode has a potential that does normally not vary
too much. However the potential is also temperature dependent and can
also vary if the activity of the silver ions in the reference electrode
would vary. This can be the case if a pollutant enters the reference
electrode.
Calibration
From the preceding, it is obvious that a frequent calibration and
adjustment of pH meters are necessary. To check the pH meter, one
should verify if the pH shown does not differ from the "real" pH
of so called buffer solutions. At least two such solutions are required,
e.g. pH 7 and pH 4. If the difference is not acceptable, one should
adjust the reading.
To adjust, one should take care not to work too fast, so as to be sure
that the system is in equilibrium. Also the pH meter should be already
powered on for some time so as to ensure that all components are in a
thermal steady state. On should first use the buffer at pH 7 and adjust
the zero (or the intercept). Thereafter, one should use the buffer at
a different pH to adjust the slope. This cycle in repeated at least once
or until no further adjustments are necessary. Note many modern pH meters
have an automatic calibration feature. In this case one only needs to
use each buffer only once.
Errors
Although many people take a pH measurement for granted, many errors are
possible. Those can have different causes. There can be errors of the
pH dependent glass, errors on behalf of the reference, errors in the
electrical part as well as externally generated errors.
Errors of the pH dependent glass
The pH dependent glass can break or crack. Sometimes such a break is
obvious, but sometimes such a break is hard to find. If there is a
connection between the internal liquid of the pH measuring part and
the external environment, one will find a pH value close to 7, which
does not change when the electrode is put is a solution of a known
different pH. Also if one measures the electrical resistivity over
the glass membrane, one find a value which is typical below 1 mega ohm.
In that case one can only replace the electrode with a new one.
A similar case can develop if the glass wall between the inner and the
outer part of a combined electrode break. This is possible eg. in case the
outer part is made of a plastic material, which is bent. The inner part
can crack without any marks on the outside. The electrical resistivity
is over the glass electrode itself intact, but actual measuring between
both electrodes reveals as in previous case a low resistivity. The remedy
is the same as in previous case : replace the electrode.
The glass can wear out. This gives slow response times as well as
a lower slope of the mV versus pH curve. The first remedy possible is
to put the electrode in a 3 Molar KCl solution at 55 degrees celsius for
5 hours. This should revitalise to some extend the electrode. If this
does not help, one can refurbish the electrode by removing a layer of the
glass. This is done by putting the electrode for two minutes in a (plastic!)
container containing a mixture of HCl and KF (be careful, do not breath
the fumes; wear gloves). Afterwards the electrode is put two more minutes
in HCl, and rinsed thoroughly. As a part of the glass in physically removed,
the new surface will be about as good as the original new surface. However
because the glass is now less thick, this shortens the life of the electrode.
After this remedy the first days, a very frequent recalibration is
required.
The glass can be dirty. If a film of some product lays on the glass, the
glass still measures correctly but does not measure the pH in the solution
to be measured but the pH in the layer of surrounding product instead.
This is seen normally by very slow response times and obviously wrong pH
values. Also the pH may vary according to the buffer capacity and/or the
stirring rate in the solution to be measured.
If one knows exactly what product it is, one should dissolve the product
using an adequate solvent. In the general case one should normally first
dip the affected electrode a few minutes in a strongly alcaline solution,
followed by immersing it in a strong acid (HCl) solution. If this does
not help, one should try pepsin in HCl. If still unsuccessful, one can
use the HCl/KF method described in the previous paragraph.
Errors of the reference
The diaphragm of the reference can be blocked. This is seen as unstable
or wrong pH measurements. If one measures the electrical resistivity
over the diaphragm, one find high values. (Most multimeters will give
an overrange). The most common reason is that AgS did form a precipitate
in the diaphragm. The diaphragm will be black in this case. The electrode
should be immersed in a solution of acidic thiourea until the diaphragm
is white again. Afterwards the internal filling liquid of the reference
electrode should be replaced.
There is no contact over the diaphragm, due to some air bubbles. This is
seen exactly as if the diagram were blocked, except that the diaphragm
has its normal color. In this case one should make sure that the liquid
is at all times (slowly) flowing from the reference electrode towards
the liquid to measure.
A polluting substance did enter the reference electrode. This is seen as
unstable or wrong pH measurements. Often the pH at which the output of the
system is 0 mV differs considerably from pH 7. The diaphragm has its normal
color and the electrical resistivity is normal. However, quite often this
case is combined with the previous case, which invalidates the previous
statement. The remedy is to replace, eventually several times the reference
liquid. In many cases, however, the electrode will be permanently damaged.
One can prevent this to happen by choosing for gel filled reference
electrodes, double junction electrodes or by making sure that there is at
all times an net outflow of reference liquid towards the solution to be
measured.
The electrode was filled with a wrong reference solution. This is seen
as pH measurements which are shifted. Replace the reference liquid.
Errors in the electrical part
The input stage of the meter is broken. This gives random measurements.
Shorting both input wires does not make any difference. Remedy : one
should repair the meter.
The input stage seems broken. Shorting both input wires gives a stable
pH measurement of about 7. The meter can in fact be broken, but most
probably the problem is elsewhere.
The input stage of the meter is contaminated with some liquid. This gives
an almost constant measurement of about pH 7, even with the pH electrode
disconnected. Sometimes this is also seen as a pH which seems to vary only
to some proportion of what it should, when tested with two standard
solutions. In this case one should clean the contaminated part, first with
distilled water, afterwards with ethanol and dry thoroughly.
Water did enter into the connecting cable. This appear exactly as the
previous case, except that if one disconnects the cable the pH will
start to drift. The remedy is the same as in previous case, only the
contaminated part is different.
There is a short circuit in the cable. This gives similar results as
the previous case. Sometimes one does not know that in most pH cables
between the two copper conductors there are two layers which seem to
be insulators. However the inner layer is in fact an isulator whereas
the outer layer is a conductor to avoid trace electrical effects. If
this outer layer does make a connection to the inner conductor, there is
a short circuit. Remedy : make sure that there are no such contacts.
Externally generated errors
If there is a marked flux of liquid around the electrode, then there can
be a trace electric effect. This generated some potential on the glass
membrane, which is superposed on the actual pH measurement. This effect
becomes negligible for good conducting liquids. It is seldom observed.
In case the trace electric effect does influence pH measurements, one can
add a little salt to increase the conductivity or one can try to change
the flux of liquid around the electrode.
In case of ground loops or spurious currents, there are electrical currents
flowing on places where one should not expect them. Such currents can
strongly influence pH measurements. It is not unlikely to observe a pH
in the range of -15 to +20 even if the real pH is 7, just due of such
electrical phenomena. One can remove those ground loops by correctly
grounding the setup. One should also check the insulation. Often
those problems can be extremely difficult to detect and remedy.
26.3 What are ion-selective electrodes?
Ion selective electrodes are electrochemical sensors whose potential varies
with the logarithm of the activity of an ion in solution. Available types:
1. The membrane is a single compound, or a homogeneous mixture of compounds.
2. The membrane is a thin glass whose chemical composition determines the
response to specific ions.
3. The support, containing an ionic species, or uncharged species, forms the
membrane. The support can be solid or porous.
Popular texts on applications of ion-selective electrodes include
"Ion-Selective Electrodes in Analytical Chemistry" [3], and "Ion-selective
Electrode Methodology" [4].
26.4 Who supplies pH and ion-selective electrodes?
The best known manufacturer of ion-selective electrodes is Orion Research.
There are several pH electrode manufacturers, including Radiometer and
Metrohm.
------------------------------
