Ask A Scientist Chemistry Archive

name         Victoria M.
status       educator
age          20s

Question -   What is the simplest method of making iodine crystals assuming i have liquid 
iodine?
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Victoria,

Since elemental iodine sublimes rather than melts, there is no such thing as "liquid iodine." 
Very likely, what you have is tincture of iodine -- that is, a solution is made of iodine 
dissolved in alcohol. In order to recover the free iodine, you must allow the alcohol to 
evaporate. This is not as easy as it sounds because as the alcohol evaporates, the iodine 
will too. "Sublimation" means passing from the solid state directly to the gaseous state 
without first melting. Also, you should be aware that as iodine sublimes, its vapors are 
very irritating.

Regards,
ProfHoff 736
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I2 crystals from the melt: The m.p. of I2 is about 113C. At that temperature it has a 
significant vapor pressure so the process has to be carried out in sealed glassware. A 
FUME HOOD IS MANDATORY, AS WELL AS SODIUM SULFITE OR SODIUM THIOSULFATE SOLUTION FOR 
NEUTRALIZING ANY SPILLS.
The apparatus must be insulated against any drafts etc.  It requires chemical glassware. 
This will require a couple of nested Polystyrene foam ice chests with cover. Water (and organic
materials like rubber and cork) must be scrupulously avoided so the glass ware will have 
to be dried with a torch.Iodine usually comes as crystalline chips. Select a few of the best 
looking ones. They need not be large -- 1-2 mm. Attach each to a length of stainless steel 
wire (must not be a metal that reacts with molten I2 --  available from hobby, craft, or 
hardware stores.
Melt the I2 (you will need a thermometer to monitor the temperature). Heat the molten I2 to 
about 120 C. You will need a hot plate with a magnetic stir bar to slowly stir the molten I2 
so that temperature gradients are avoided. The hot plate will need to be controlled by a 
"Variac". The hot plate must be the type where you can control the temperature and stir rate 
independently. Start turning down the Variac until the temperature is slightly above the m.p.=
113 C. Immerse an I2 crystal epoxied to the end of one of the lengths. Start turning down the 
Variac one or two "notches" a day. If the seed crystal on the end of the wire has melted you
 will need to replace it with another and continue the cool down procedure where you left off. 
 IF NO OTHER SEED SITES ARE FORMED, the seed crystal will grow. The rate of cooling is 
 important, so be patient, the process may take several weeks. When the crystal is the size 
 you want, remove it and remove any molten I2 by washing it with a poor solvent for I2, like 
 cyclohexane (CAUTION: FLAMMABLE AND HOT). Mount the crystal in a sealed insulated bottle. The 
 I2 will tend to sublime and form new crystals on the walls of the container.
Unless you have some special reason for wanting I2, there are a lot of other crystals that are 
much easier to grow!!!

Vince Calder
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Hi Victoria!
It is very simple to obtain iodine crystals. You surely are talking about iodine tincture 
that is an alcoholic solution of iodine (not liquid iodine). If you let the alcohol  evaporate 
the result will be iodine crystals deposited over the recipient walls. But since solid iodine 
is a substance that sublimates, that is, passes to vapour directly from the solid state, much 
of the iodine will be lost in the open air.
But beware! iodine is a very toxic, corrosive and dangerous substance, that causes eye and skin
 burns,  may cause severe respiratory and digestive tract irritation with possible burns. Also 
 may cause allergic skin reactions and kidney and thyroid damage.

A potent antiseptic, its use was discontinued exactly because of the dangers associated with 
it.  Sorry...I would not advice you to try to work with that substance...
But thanks to ask Newton!

Mabel
(Dr. Mabel Rodrigues)
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Victoria,

When you say that you have liquid iodine at atmospheric pressure, I would say that it is 
most likely iodine crystals dissolved in an alcohol solution.  Iodine is a halogen that at 
room temperature, the  solid iodine crystal goes directly into the gaseous state (sublimes).  
For a little history of the Iodine, Iodine was first prepared in 1811 by Bernard Courtois, 
who observed purple vapors rising from an extract of kelp ashes that he had acidified with 
sulfuric acid and heated.  At that time, kelp was commonly collected, dried, and burned to 
give ashes from which sodium and potassium salts were obtained.  The purple vapor condensed 
on a cold surface, forming nearly black crystals.  Others-notably Gay-Lussac and Humphrey 
Davy-proved that the crystals were an element, and it was named after the Greek iodes, 
meaning violet.  I hope that this helps.

Sincerely,

Bob Trach
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