[kamiyu]

Hi guys

Do you think it is possible to have a dry ice acetone bath at -40^oC???

[discodermolide]

Yes, if you control how much dry-ice you add.
But using CCl₄ dry-ice gives you about -38°C.

[kamiyu]

You mean add dry ice until the acetone becomes -40oC ? I think in this case you are saying that the cooling bath does not have any solid dry ice anymore..

If there is solid dry ice, then the temp must be -78oC, right?

Yes, if you control how much dry-ice you add.
But using CCl₄ dry-ice gives you about -38°C.

[discodermolide]

Just add small bits of dry-ice until the temp is -40°C, you can maintain this just by adding dry ice in portions to keep the temp at -40°C.

[TheUnassuming]

You can definitely use dry ice/acetone system to hit -40 ºC, but it will take a close eye and lots of maintenance to keep it there.  Its much easier to use a system that has a melting point at the temperature you want the bath to be.  CCl4 only hits -23 ºC (its freezing point), but if you use acetonitrile you can hit -41 ºC.