How Sodium Acetate Reusable Hand Warmers Work

Hand warmers are now much more environmentally friendly, and warmer than ever thanks to reusable hand warmer technology, but how do they work?  Reusable hand warmers take advantage of the physics of phase change between solid and liquid states to release heat energy on demand.


Are reusable hand warmers toxic? What is Sodium Acetate, and when does it become Trihydrate?

The phase-change material inside reusable hand warmers consists of sodium acetate trihydrate, which is essentially a food-grade salt water.  Sodium acetate itself is the sodium salt form of acedic acid. It is a colorless salt with a wide range of uses.  Sodium acetate is also deliquescent, meaning it can absorb a large amount of liquid.  This allows sodium acetate to combine with three molecules of h2o to every one molecule sodium acetate, forming sodium acetate trihydrate.  This is a supersaturated, aqueous form of the salt has a unique melting and boiling points.

How Reusable Hand Warmers Work

Reusable hand warmers start with the sodium acetate trihydrate solution in the liquid state.  To get to the liquid state, the hand warmers were boiled to ensure the solution was raised above its boiling point of 136.4 °F/58 °C (to 137.12 °F/58.4 °C).  The solution must be heated well above its melting point to become a supersaturated solution.

When cooling below the melting point, this supersaturated solution will not turn to a solid form on its own, it needs help for that.  That means that the stored “latent” heat can be saved to be used whenever it’s needed.  When the phase change from liquid to solid is initiated (usually by clicking a small disc inside the reusable hand warmer), the latent heat is released and the hand warmer heats up to over 130 F.

This happens because tiny pieces of metal inside the activator create a nucleation site for salt crystals to form when the metal disc is “clicked.”  Because the hand warmer is far below the freezing point, the activation sets off a process where the entire solution freezes within seconds.  It is very educational, entertaining, and warm to witness.

How to recharge reusable hand warmers

When in solid form, all you have to do to reset reusable hand warmers is boil them in water for about five minutes.  SpongeEnergy is also developing a special reusable hand warmer charger for this purpose.

What is the melting point of sodium acetate trihydrate in reusable hand warmers?

136.4 °F/58 °C (to 137.12 °F/58.4 °C)