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Please advice me a liquid whose freezing point is 2-5 Celsius.

I know that the solution of salt in water can have freezing point of about -3 Celsius. I would like to know if similar liquid but with freezing point slightly above zero can be obtained.

The liquid has to be created from readily available materials, like a mixture of water and something else. It also have to be non-explosive, non-poisonous and not easy to start fire on (so it is allowed to be sent via mail).

Why I need this:

A friend of mine is going to send some sweet potatoes to me to grow (this is a new culture in my county). But we fear that it can freeze. My friend will insulate the box with polystyrene. But I doubt that insulation will be useful without having a descent amount of heat inside the box.

The idea is to put some phase-change thermal accumulator inside the insulated box, so that the inside keeps above zero for several days while the temperature outside is several degrees below zero Celsius. In case temperature outside will drop below zero, the liquid will begin to freeze and while it is freezing the temperature inside will keep at its freezing point.

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closed as off topic by David Z Dec 7 '11 at 19:10

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Hi Alexey, and welcome to Physics Stack Exchange! This is a site for conceptual questions about physics, but most specific material recommendations are off topic here. – David Z Dec 7 '11 at 19:11
The question is interesting conceptually: is there a readily available water solution with a freezing point above that of water. Dilute solutions generally lower the freezing point (freezing point depression) so perhaps the best thing is deionized purified water, but this has a freezing point of 0 degrees. Benzene has a freezing point of 5.5 degrees, but you wouldn't want it near your food, same for other hydrocarbons. A negative pressure would increase the freezing point, but the difference is miniscule before sublimation. This is an interesting difficult problem-- is there antiantifreeze. – Ron Maimon Dec 8 '11 at 6:33
Some vegetable oil might be the right answer. But the nonpolar complex materials like this don't release a lot of latent heat, so they aren't a good shipping heat barrier. Heavy water is probably the best answer--- it isn't poisonous in small doses, it dilutes to safety in ordinary water, and it has a freezing point of 3.8 degrees. But it's impractical – Ron Maimon Dec 8 '11 at 6:39
Here's an identical question: . But I think I have the best answer: you pack the potatoes in a plastic bag, immersed in a big thermos filled with warm peanut oil (George Washington Carver would be proud). The oil has a melting point of 3 C. It is expensive, but not heavy water expensive. The latent heat of fusion problem, unfortnately, requires you to use about 20 times more fluid as water for an equivalent insulation. You can put a large commercial thermos with peanut oil and water in the freezer, to test over the shipping period. – Ron Maimon Dec 8 '11 at 8:25