[emot]

help me about preparing an ice-salt mixture to diminishes the temperature to -25 C

[Dan]

I think you will have difficulty getting that low and maintaining it with brine-ice, maybe you can though, I've not tried. I

f I were you I'd use acetone-dry ice, you can change acetone to something else if you wan't. I always use acetone-dry ice for temperatures <-10^oC, with a decent Dewar you don't have to top up the dry ice too often.

[Rabn]

http://en.wikipedia.org/wiki/Freezing_point_depression

[AWK]

http://www.reference.com/browse/wiki/Cooling_bath

[DrCMS]

From Advanced Practical Organic Chemistry by Casey, Leonard, Lygo and Proctor

Dry ice/Carbon tetrachloride -25°C
Liquid nitrogen/o-xylene slush -29°C
Liquid nitrogen/m-toluidene slush -32°C
Liquid nitrogen/thiophene slush -38°C
Dry ice/heptan-3-one -38°C
Ice/Calciumchloride hexahydrate 4:5 ratio -40°C
Liquid nitrogen/acetonitrile slush -41°C
Dry ice/acetonitrile -42°C

[cisdiols]

ammonium chloride with ice should get it to that tmp.

[kiwi]

way better than ice-salt and obscure solvent/dry-ice for these sorts of temperatures is ethylene glycol/ethanol/dry ice mixtures. the freezing point varies relatively predictably (-80 to -10 degC) depending on the ratio of ethanol/ethylene glycol. there is a short note in the lit (J. Chem. Ed. (2000) 629-) which presents the data, but to get around -25 deg C you want about 85:15 glycol:ethanol, then stir in dry ice till you get a nice thick slurry. this doesn't stink like the mixed xylenes trick, and it doesn't set like a rock (which xylenes has a nasty habit of doing). they also can hold their temperature for quite a while without being babysat. this trick is my first choice for any cold bath warmer than -78.

[alphahydroxy]

methanol / ice / salt will get down to about -18^oC if you leave it to equilibrate for a while. Using ethanol in place of methanol should take you down to ~-22^oC if I recall correctly....