vulture - 13-11-2009 at 13:15
Everybody's probably familiar with CO2/acetone or CO2/isopropanol cooling baths, both which reach about -78C. However, there's a lot of information
out there about other cooling mixtures with CO2 which reach different temperatures.
http://www2.bc.edu/~hoveyda/cool.html
There are numerous other examples in the literature.
Strangely enough, I've found that many listed combinations will always yield about -75C to -78C, regardless if the literature says -30C or -100C
(pentane/CO2). The only thing that I've found to come close so far is p.a. acetonitrile with CO2, which won't go below -50C.
Does anybody have any idea why I can't get these mixtures to keep their listed temperatures? Do the solvents have to be extremely pure?
S.C. Wack - 13-11-2009 at 13:53
http://www.sciencemadness.org/talk/viewthread.php?tid=5028#p...
not_important - 13-11-2009 at 14:04
Yes, that previous thread says it. Many of those baths are actually slushes of the liquid, with little dry ice content while in use. The first mix
on the bc.edu page is for the melting point of fairly pure p-xylene. You nearly completely freeze the bath by adding dry ice, stirring well, then add
just enough bits of dry ice to keep some frozen material.
The sub -76 ones are another issue. Some turn out to be done pulling a vacuum on the mix. Others appear to be actual dissolution effects between the
CO2 and liquid, similar to ammonium nitrate and water. Like the water-ice and solid salts baths, these are less stable in the temperature obtained
and the duration of that temperature; they're best for fairly short durations of cooling.