Chemistry Lab Demonstrations: Sodium Acetate Crystallization

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Recrystallization lab today.  Standard two-solvent recrystallization of an unknown carboxylic acid from ethanol/water.  My students did awesome – as far as I know everyone obtained crystals on the first try.

The demo today was crystallization of sodium acetate from a supersaturated water solution.  I had to try a few times to get a good supersaturated solution.  First, I heated up some water and tried adding anhydrous sodium acetate one scoop at a time.  That didn’t really work – it was hard to get it all into solution, and when I took it off the hot plate, it ended up crystallizing while cooling.

Then I tried loading up a round bottom with the crystalline trihydrate and spinning it on my rotovap while the hot water bath was turned up real high.  That didn’t work at all.  I was real surprised it didn’t work, but nothing went into solution.  Sodium acetate trihydrate “melts” at around 55 degC (no, contrary to some websites, this is NOT molten sodium acetate.  Anhydrous sodium acetate decomposes above  324 degC).

What ended up working for me was pouring a small layer of the trihydrate into an Erlenmeyer flask and warming that on the hot plate.  The solution was formed, and more trihydrate would be added portionwise.  Then I would add a few small scoops of the anhydrous just to get a bit more sodium acetate into solution.  This formed a nice, clear, colorless solution of sodium acetate.  It could be cooled without auto-crystalization to create the supersaturated solution.

See, at 0 degC, sodium acetate has a water solubility of 760 g/L.  Hot solvents dissolve more than cold solvents, so heating the water allows one to dissolve more than 760 g of sodium acetate in a liter (not that I was working on that large of a scale).  If cooled slowly and without addition of a nucleation site (i.e. dust particle, etc), a supersaturated solution is formed.  That is, if cooled to 0 degC, more than 760 g/L would be dissolved in water.

This supersaturated solution is just begging to crystallize.  You don’t have to coax it very much.  Even the introduction of one tiny seed crystal or dust particle can be enough to get sodium acetate to crystallize.   And once it starts crystallizing… it takes off.  It crystallizes much faster than typical crystals.  So fast, in fact, that it can be used as a demo.  You can either drop a seed crystal at one side of the flask and marvel at how fast the crystals race to the other side, or you can slowly pour it onto a seed crystal and slowly build yourself a Lot’s wife-worthy pillar of salt.

I made two supersaturated solutions, so that I’d have one as a backup.  It’s a good thing, too, because my backup solution auto-crystallized on my walk over to the ugrad lab.

One final note.  The crystallization is exothermic.  This is the exact process used in hand warmers for gloves and such.  there is a disk in a supersaturated solution of sodium acetate.  When the disk is snapped, the solution crystallized, giving off quite a bit of heat.  This warms your hands.  The temperature rises to about 130 degF (around 55 degC).  The second video shows the temperature rise.