2
$\begingroup$

When sodium acetate trihydrate hot packs are regenerated soon after activation, they regenerate to a clear liquid upon boiling (typically with an air bubble that was not present in the new pack)

On the other hand, when activated hot packs are stored in the crystallized form for a while, regeneration by boiling in water yields a mixture of clear liquid and very thin clear glossy plate-like crystals almost like mica sheets, plus large air (gas) pockets.

I suspect that the bubbles arise due to diffusion of room air through the flexible plastic envelope of the pad. When a new pad is activated and sodium acetate trihydrate crystals form, small gas spaces are left between the crystals which are necessarily filled only with water or acetic acid vapors which have low vapor pressure at room temperature. The low gas pressure in between the crystals (I estimate < 3 kPa) would create a large driving force for gas diffusion through the polymer envelope.

The crystals could be sodium acetate anhydrous formed because of water vapor loss by diffusion through the polymer envelope, but I'm not as sure on this. Perhaps they are sodium hydroxide formed because of acetic acid loss by diffusion? The liquid remaining around the crystals can still be activated to crystallize like a new hot pack, so the crystals are definitely not the trihydrate.

I'm looking for a reality check on these speculations, ideas on how to find out what the crystals are (ideally without needing sophisticated lab equipment), and advice for restoring hot packs containing these flat crystals to the initial clear liquid state. Photos below. Many thanks!

Old hot pack with crystals Old hot pack just after activation; notches in expanding crystal field represent air bubbles

$\endgroup$

1 Answer 1

2
$\begingroup$

Hi random geologist here with relatives having a heat pack factory 1; bubbles form most prominently the longer you leave the pouch in crystallized state 2; if heated sodium acetate dissolves in its own crystal water (water that is built in as H2O molecule into the crystal structure) so if you heat your pouch above 58°C it dissolves and if you do it correctly you dissolve every crystal nucleus (otherwise the pouch crystallizes again whilst cooling) so with 0 nucleii to form crystals from your pouch cools storing the latent heat of crystallization (264 kJ/kg) in it (heavily undercooled liquid) when you activate your pouch acicular crystal aggregates sprout out of your metal plate. (the more your liquid is undercooled the less it cares in what form it crystallizes, first its skeletal crystals then dendritic (snowflakes) then acicular). I hope this helps

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.