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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: emot on July 04, 2008, 03:39:55 AM

Title: cooling mixture
Post by: emot on July 04, 2008, 03:39:55 AM
help me about preparing an ice-salt mixture to diminishes the temperature to -25 C
Title: Re: cooling mixture
Post by: Dan on July 04, 2008, 04:19:45 AM
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 <-10oC, with a decent Dewar you don't have to top up the dry ice too often.
Title: Re: cooling mixture
Post by: Rabn on July 04, 2008, 04:50:13 AM
http://en.wikipedia.org/wiki/Freezing_point_depression
Title: Re: cooling mixture
Post by: AWK on July 04, 2008, 05:26:29 AM
http://www.reference.com/browse/wiki/Cooling_bath
Title: Re: cooling mixture
Post by: DrCMS on July 04, 2008, 12:23:33 PM
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
Title: Re: cooling mixture
Post by: cisdiols on July 07, 2008, 08:19:36 AM
ammonium chloride with ice should get it to that tmp.
Title: Re: cooling mixture
Post by: kiwi on July 08, 2008, 10:59:32 AM
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.
Title: Re: cooling mixture
Post by: alphahydroxy on September 29, 2008, 10:35:05 AM
methanol / ice / salt will get down to about -18oC if you leave it to equilibrate for a while. Using ethanol in place of methanol should take you down to ~-22oC if I recall correctly....