[beheada]

I'm trying to figure out how to make an ice bath which will run at -20C constantly. I know dry ice/acetone is -78C and ice water is 0C, so I'm thinking maybe a regular ice/water bath with a massive amount of salt to drop the temperature to roughly -20C? Anyone know if there's a thermodynamic equation to find out the correct amount of chloride ions to drop the temp to a specific range?

Thanks

[azmanam]

carbon tet/dry ice

http://www2.bc.edu/~hoveyda/cool.html

[Morderkerl]

if you add acetone to ice water, even a few ml, it can reduce the temperature to about -5C, so try that also, im not sure about any thermodynamic equation

[mreff555]

Freezing point depression is a colligative property. You can use the FPD equation but I don't think saltwater is going to work for that temperature. What about diluted ethanol?

FPD help
http://en.wikipedia.org/wiki/Freezing-point_depression

[Arkcon]

You can also search this forum, recipies for various cold bath temps have been posted several times.  Personally, if I need an ice bath, I use one, if I need dry ice/acetone, I'll use that.  But when I needed several intermediate temps, for a study, I used a controlled cooler.

[mreff555]

Yes, but dry Ice is cheap. I would see were 70% ipa gets you with dry ice. I know it will get down to about -7F in the freezer, with dry ice I'm sure you could drop it significantly lower.

You can also search this forum, recipies for various cold bath temps have been posted several times.  Personally, if I need an ice bath, I use one, if I need dry ice/acetone, I'll use that.  But when I needed several intermediate temps, for a study, I used a controlled cooler.

[azmanam]

isopropanol is a suitable subsitute for acetone (not as flammable).  With dry ice/isopropanol, you'll get -78.

Seriously, use carbon tet and dry ice.  the only problem is the carbon tet usually solidifies, so be ready to let it warm slowly or try to chisel it out of the solid carbon tet.

[AWK]

http://www2.uni-siegen.de/~pci/versuche/english/v105-2.html

[kiwi]

dry ice/ ethanol/ ethylene glycol. it gives baths with temperatures you can tune over the range -78 -> -10 by altering the % ethanol in the mix. the baths also keep their temperatures for quite a long time unattended. i use this for anything between zero and -78. there is a j. chem. ed. article on it out there