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I need to cool a tube containing liquid to −35 °C for a cold chemical experiment that will last about a week. The glass vessel I need cooled is 500 mL in volume and will be giving off $\ce{H2S}$ gas. I currently only have a fume hood. Most freezers do not seem capable of this low temperature. Most cryochemistry refrigeration seems designed for much lower temperatures, and is needlessly expensive. I realize that refrigeration is relatively simple: 1 compressor, 1 expansion valve, two heat exchangers, and electronic control/monitoring. My first impulse is to modify a freezer, but I am completely new to this. At this point I would be very happy with some general advice and perhaps a reference for custom designing these sorts of cooling devices.

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    $\begingroup$ Please note that you should never store flammable liquids or gases in a normal household refrigerator or freezer, which have various ignition sources (lights, switches, compressor motors) that could ignite vapours in the unventilated cooling compartment. The flash point of hydrogen sulfide (the lowest temperature at which it can vaporize to form an ignitable mixture in air) is approximately −82 °C. $\endgroup$ – user7951 Dec 17 '15 at 20:58
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I would suggest a Frigorific Mixture of calcium chloride (often sold as DampRid) and Ice in a cooler. You will need quite a bit of each and to moniter at least twice a day, but it wont break the bank and can reach the desired temperature.

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  • $\begingroup$ That or a Cooling Bath seem ideal. $\endgroup$ – Dale Dec 17 '15 at 21:27
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    $\begingroup$ If your going to use dry ice I would suggest a [Dewar condenser] [1] as is typically used in such cryogenic applications, noteably Birtch reduction [1]: ivyscientific.com/images/categories/CD-0054.jpg $\endgroup$ – A.K. Dec 17 '15 at 22:02

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