Background: I was recently playing around with some reusable heat packs, the kind that operate by phase change of sodium acetate trihydrate and noticed that they didn't reach the temperature that I had expected (58 °C) rather, they stayed at 52 °C.
There were two such packs, roughly 50 mm × 100 mm × 15 mm and placed together with the broad sides in contact. Between them and at the center, there was a calibrated PT-100 RTD (which I certainly trust to be better than 1 °C accurate). The entire mass was then insulated with a few layers of thick wool fabric. The temperature reading became constant at 52 °C within a few minutes of crystallization being initiated in both packs and remained very stable for more than 10 minutes thereafter.
It's worth noting that although the manufacturer labeled the contents as being 'Sodium acetate (food grade)' and water, no proportions are given. Also, the contents of the pack are bright red, indicating the presences of unlisted dyes.
Question: What would be the most likely causes for the discrepancy? Contaminants, incorrect, proportions something else?
I'm ultimately interested in using this reaction to maintain ~58 °C in an experiment. I'd like to avoid the same result if I mix up a batch of this stuff from pure ingredients.