Use a less harsh solvent and more pumps will most likely work.
Dry-Ice Bath Based on Ethylene Glycol Mixtures
J. Chem. Educ. 2000 77 629.
Bath mixtures of ethanol and ethylene glycol in dry ice produce sustainable constant temperatures over the range from -12 to -78 °C, which show a linear relationship between the bath temperature and the volume fraction of ethylene glycol. Our bath is less toxic than previously reported mixtures of ortho- and meta-xylene, and our mixture does not intractably solidify as will dry ice slurries of xylenes. Moreover, the cost of this mixture is less than that of the xylene mixtures.
http://jchemed.chem.wisc.edu/Journal/Issues/2000/May/abs629.html
For this dry ice bath its a linear relationship,the bath temperaure rises linearly based on volume fraction of ethylene glycol.
100% Ethanol(A) = -78C
100% Ethylene glycol(B) = -12C
Temp of Bath(C)=((B/(A + B))*(-66C)) - 12C
A and B are the respective volumes
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