How do Sodium Acetate hand warmers work?

[Al Bennett]

A friend bought a "ProHeat" hand warmer, and I can't figure out how it
works! I'm quite fascinated by it...

It's nothing more than a plastic sac filled with sodium acetate (food
grade) and water. Also inside is a metallic disk. To activate it,
you "click" the metal disk (it is slightly bowed) and an exothermic
reaction takes place in which a whitish salt (?) precipitates from the
originally clear solution...

My interest mainly concerns the metal disk - what is this made out of,
and what are its properties that begin this impressive reaction?

I'm sure someone can answer this for me quickly!

Thanks,
-Al-

[John C. Huffman]

Actually its not a chemical reaction as such. The solution
is supersaturated when "inactive" and the metal disk does
nothing more than cause nucleation for crystalliztion to
occur. The crystallization process is quite exothermic,
leading to the heat. By placing the pouch in boiling
water, the salt is redissolved, ready to be used again.
You could probably initiate the raction by sharply hitting
a corner with something.
--
|===================================================|
| John C. Huffman Senior Scientist |
| Indiana University Molecular Structure Center |
| huf...@indiana.edu http://www.iumsc.indiana.edu |
|===================================================|

[George D. Parks]

The sac contains a supersaturated solution of sodium acetate. The
solution is very stable, but once it starts to crystalize, it goes
very quickly. I've kept soltions around for weeks an months, and I
used to start them crystalizing by throwing in a tiny crystal.

The clicker probably creates a mechanical "shock" that initiates
crystallization. The heat comes because it's an exothermic process.

George Parks

[George D. Parks]

[MARSHALL DUDLEY]

"John C. Huffman" <huf...@indiana.edu> writes:

-> Actually its not a chemical reaction as such. The solution
-> is supersaturated when "inactive" and the metal disk does
-> nothing more than cause nucleation for crystalliztion to
-> occur. The crystallization process is quite exothermic,
-> leading to the heat. By placing the pouch in boiling
-> water, the salt is redissolved, ready to be used again.
-> You could probably initiate the raction by sharply hitting
-> a corner with something.

I have one of these, actually a heating pad. However I am confused by the way
this one is working now. If I leave it a few days after "melting" the acetate
it turns into a white slush. It appears that it has partially crystalized, and
it fully of a mushy crystal/liquid acetate. If you click the clicker then it
will go completely crystaline and get hot like it suppose to, but I can't
figure out what the slush is. If there were any crystals in the slush then it
should have seeded the solidification and converted, but it doesn't.

Marshall

[Dirk Friedrich]

Sodium acetate is much better soluble in hot than in cold water. A saturated solution prepared in
hot water will usually separate crystalline sodium acetate decahydrate upon cooling. This
crystallization produces an exotherm. If the solution is prepared with exclusion of any impurities or
other influences that would trigger crystallization, a supersaturated solution can be obtained. The
plastic bag of the handwarmer contains such a supersaturated solution, in which crystallization can
then be induced to create an exotherm. In case of the handwarmer, it is triggered by a mechanic
shock created by łclicking˛ of the metal disk, and the ensuing crystallization liberates heat. The
disk is probably just made of steel. Great idea!