-40 Degrees Celsius

[airwalkery2k]

Hi,
I am doing some undergraduate work in Sweden, and for one of my tests, I need to place a material at -40 to about -50 degrees Celsius for a short period of time for thermal cycling. Unfortunately, my access to equipment is rather limited here

The current method we're using is just wrapping it in a cloth and dripping liquid nitrogen onto it until the thermocouple inside the cloth says -40 C. Rather unscientific, if I do say so myself. So we're trying to come up with a quick and simple solution, as buying the proper lab freezer around here would take much longer than I have time to wait for.

A few others and I have thought up a way to use our liquid nitrogen in another way, basically creating an insulated container for the liquid nitrogen and placing the samples in another compartment near the nitrogen at a place our thermocouple says is -40 to -50 Celsius.

Is there a better way to attain -40 to -50 degrees Celsius?

Thanks!

[seycyrus]

Pour a sizeable quantity of Ln2 is a thermos sized dewar. Suspend your mateiral with attached thermocouple on a string and suspend it above the surface of the Ln2, adjust the distance (height above the surface), until you hit your target temp.

[FredGarvin]

Pour a sizeable quantity of Ln2 is a thermos sized dewar. Suspend your mateiral with attached thermocouple on a string and suspend it above the surface of the Ln2, adjust the distance (height above the surface), until you hit your target temp.
That's exactly what I was thinking. Dewars are pretty cheap and easy to get.

[chemisttree]

You will want to use a low temperature bath in a dewar. You should use acetonitrile/N2 for -41C. Dry ice/acetonitrile will give you -42C. m-Xylene/N2 will give you -47C. Cyclohexanone/dry ice will give you -46C.
Pick one.

Be sure that you use enough cold stuff (N2 or CO2) to freeze the solvent but not so much that you will freeze all of the solvent. As long as there is a mixture of frozen solvent and liquid solvent, the temperature will hover pretty close to the reported freezing point.