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From: "\"Uncle Al\" Schwartz" <UncleAl0@ix.netcom.com>
Newsgroups: sci.chem
Subject: Re: Help
Date: Wed, 19 Mar 1997 08:10:53 -0800
m-35926@mailbox.swipnet.se wrote:
> Hi there !
>
> Is there anyone who could help me succeed with crystallizing
> the
> ALUMINIUM POTASSIUM SULFATE-12-HYDRATE (POTASH Alum)
> chem. pure.From powder or tiny granulates,I wish to make big crystals
> about
> 150-250 grs each crystal.
First, you are going to need a big pot...
Second, you wat to use deionized or distilled water to elimiante soluble
contamiants.
Third, you want to ultrafilter the solution to remove dust and other
nucleating particles.
Growing big crystals can be done as with making rock candy. A better
way is as follows:
__________________________ Cover to prevent contamination
| |
|- - - - - - - - - - | Surface of solvent
| |
| * | Seed crystal
|--------------------| Screen or filter mesh, preferably plastic
| |
| |
| | Solvent saturated with solute
| |
| |
| |
| * |
| *** | Excess solute
| ***** |
----------------------
/\
/ \ Small heat source limited to under solute
The idea is to get everything saturated at equilibrium (important!),
pile the excess solute hard by one corner, and apply a little heat only
to that corner. Convection currents will loft a plume of slightly warm
and therefore excess solute-containing solvent. It rises up to the
surface, cools, arches over, and descends slightly supersaturated. When
it encounters the seed crystal, BINGO!, the seed crystal grows. The
important word is "slightly." If you want massive perfect crystals
things must proceed at a slow pace to avoid trapping inclusions or
starting new crystallization seeds. The solutoon must be very clean and
gree of dust and otehr nuecleating centers to avoid triggering new
crystallization foci.
You must peridoically turn the seed crystal so that it grows evenly and
doesn't envelope the mesh. An alternative is to take a very thin and
strong monofilament (glass fiber or fishing line) and Super-Glue it to
one end of the seed crystal, then suspend the seed in the down
convection area without the mesh. The final crystal will then contain
the very thin thread (or its empty channel) within it.
If you want a nice color, add some nickel(II) sulfate or chromium(III)
sulfate to the solution before saturating with the base alum. An even
more expert voyager will also add the molar equivalents of potassium
sulfate to equal nickel or chrome alum overall.
--
Alan "Uncle Al" Schwartz
UncleAl0@ix.netcom.com ("zero" before @)
http://www.ultra.net.au/~wisby/uncleal.htm
(Toxic URL! Unsafe for children, Democrats, and most mammals)
"Quis custodiet ipsos custodes?" The Net!
Newsgroups: sci.chem
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Re: CRYSTALISATION AT -78C
Date: Tue, 27 May 1997 12:53:21 +12
In article <3389B6F3.675@uscmail.usc.es>
Gabriel Tojo <qogatojo@uscmail.usc.es> writes:
>Sofie AnnÈ wrote:
>> Can anybody give me some advise about crystalising (a liquid) at -78 °C.
>> The problem I have is that I get an oil instead of crystals. The product
>> I'm trying to recrystalise is
>> (2R,6R)-2-isopropyl-6-methyl-1,3-dioxan-4-on (ref:Helvetica Chimica Acta
>> 1987 vol. 70, p. 448), the solvent should be (?) ether:pentane.
>> I distilled my compound before crystallisation.
>> If you have any experience, please write me. I would be very greatfull.
>Sofie, you are not alone in your grievances. Crystallisation is an art
>as much as a science, and luck plays an important role. Sometimes you
>forget a solution on the bench and the following day you have nice
>crystals. Most often you try very hard and you get an oil.
>Purify your compound as carefully as possible.
>Filter the solution.
>Be patient!!
I agree that it is often a pain. The joke about bearded chemists
being more successful with crystallisations than clean-shaven
chemists has some justification.
Generally, if you are following a procedure, difficult crystallisations
usually indicate that :-
1. You have an impurity present that prevents the compound from
crystallizing. Taking a small fraction and passing it down a small
( pasteur-pipette ) chromatography column of silica with a solvent
composition close to the crystallisation solvent may provide a mg
of parent crystals which can be used to seed other batches. Once
you have crystallised a compound, and you have some seeding
crystals, it's usually easier to crystallise other batches. One of the
best way is to coat a small glass bead with a very thin film of solution,
place it in a test tube and rapidly cool it (below the solution
temperature if possible), until crystals form and then rapidly drop it
into the cooled solution. Smearing a thin film on the flask interior walls
and supercooling that can also provide some crystals for seeding.
2. Your solvent mixture should be similar to the published one, maybe
the paper is referring to another, earlier paper for details. It is
important to use the same ratio and concentration of sample.
Having too concentrated a solution may cause impurities to also
drop out - giving you the oil you started with. Try some small
scale experiments, varing both the concentration and the solvent
ratios. Ensure that your solvent can't readily evaporate - it's
important to to change the solvent/solute ratio.
3. Your glassware is too new and clean. Whilst cooling the flask,
simultaneously scratch the cooled interior surface with a glass-rod.
Minute, sharp fragments of glass act as initiation sites.
4. You are homogeneously cooling the container. Often it is preferable to
initially cool just a corner of the flask very quickly, and slowly
increase the immersion with *very* gentle, occassional swirling or
sratching with glass rod. Give the solution time to cool ( 10s of
minutes ), with occassional gentle swills. If it is still misbehaving,
add very small fragments of dry ice directly to the flask - they
sometimes provide initiation sites.
5. You aren't agitating the solution as the oil forms. If an oil starts
to form, you should very gently warm the solution until it is clear and
allow it to cool slowly whilst vigorously stirring the solution. Usually
crystals will appear, and the agitation will prevent them from occulting
the mother liquor to form the oil. The crystals can be used to "seed"
other flasks.
6. You may be too cold. If the solution becomes too viscous, then
the formation of crystals occurs very slowly. You need to change
the solvent composition slightly so that material is less soluble
in the solvent, and you can work at a slightly higher temperature.
7. Put it in a freezer for a week on so, time often heals all wounds :-).
(Seriously it does help sometimes)
8. Rapidly pull a slight vacuum very briefly to "boil" the liquid, the
emerging nucleating sites can help crystals to form, but don't try
for too long as you will change the solvent coomposition and
solvent/solute ratio
8. Ask your advisor to demonstrate their technique, and earnestly
watch their endeavours :-)
I'm sure others will offer their suggestions. Most of the above
have worked for me at one time or another, however one compound
(manool) took about two weeks of sulking before it would crystallise.
Eventually it did as a thin film between microscope slides, and
once it had reached the edge of the fim I dropped it in the solution
and decent seed crystals grew from the edges. Recrystallisation
requires a lot of patience and a little luck, but as they say " Luck is
when preparation meets opportunity ", so try several techniques.
Bruce Hamilton
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: crystal
Date: Sun, 31 Aug 1997 09:06:59 GMT
Au Sau Kheng <ausk@pacific.net.sg> wrote:
>I am trying to grow the potassium alum crystal in saturated solution. I
>managed to get a few 1.5 - 2.0 cm crystal but when they reach that size
>they become not so clear as they were smaller in size. At the same time
>the apex of each corner is not sharp but blunt.
>I changed the solution after 2 - 3 days to ensure that the solution is
>saturated and other small crystals formed were removed.
>
>How come the corners are blunt?
You have discovered the 1990s version of the crystal, with no
sharp edges to cut yourself on.
>How to get bigger but clearer crystals?
In your post, you have probably given the reason why.
The crystals can develop "veils" which make the crystal
cloudy. Close examination ( with a microscope ) would show
that these are tiny open spaces filled with the solution
the crystal is growing in. Their presence indicates that
the crystal has grown too fast - as a crystal grows, the
rate needs to slow to ensure flaws are minimised and such
spaces do not form.
I assume that you are using distilled water that has been
filtered to remove particulates. If your potassium alum
is not pure, then just harvest the crystals when they are
smaller, and redissolve them to make the saturated solution.
It's also important that you prevent concentration gradients
as the crystal grows, as these can also change the growth
rate - some systems actually have a point source of heat
to generate convection currents to mix the solution. .
The major ingredient for successful crystal growing is
patience. I strongly recommend obtaining " Crystals and
Crystal Growing " by Alan Holden and Phylis Singer,
Heineman (1960). It's old, but was part of the Heineman
Science Study Series, and may still be available. It has a
recipe for growing potassium alum crystals, along with
many excellent hints for kitchen experiments on crystals
and crystal growing.
They prepare a supersaturated solution ( 20 gms / 100 ml water ),
which is warmed, sealed in a jar, allowed to cool whilst seeded
with some potassium alum. The saturated clear liquid is decanted
off and warmed, then 4 more grams of salt are added for each
100 ml of solution and dissolved. This is sealed in a jar with
the seed crystal suspended, and stored in a location that has
a constant temperature.
Bruce Hamilton
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