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#1 2008-03-18 00:36:09

orgopete
Administrator

reaching -5C temperature

I am going to answer your question differently than asked. If you wished to cool something to -5C, you actually want your cooling bath to reach a temperature lower than -5C. If you wish to hold something at -5C, then you want to have a bath that will maintain -5C. What is the difference? If you are doing a reaction, then as the reaction proceeds, heat is often (generally?) increased in the reaction. That comes from the reaction being exothermic (the most common reason for cooling in the first place) or from addition of another reagent. If your cooling bath did not cool lower than -5C, then your addition would have be done very slowly because equilibration would be slow to achieve if the temperature differential were small. In that case, you would be better to use a cooling bath that reached lower than -5C.

If you are doing a reaction and you wish to hold something at -5C for a long period of time, then you want a cooling bath that will maintain -5C. That is best done with a solvent that melts at -5C. This is the same principle as ice to maintain 0C. "The Chemists Companion" contains a table listing several baths that can maintain varying temperatures.

A ratio of NaCl to ice would be difficult to maintain unless you were to freeze the brine first so the concentration of the salt in water would not change. Personally, because I was lazy, I generally used acetone to make ice cooling baths for less than 0C. It is fast and easy.

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