I never come across condensation of LN2 into an open nitrogen Dewar when used for the Schlenk line traps, although I used a cotton cloth around the Dewar to lower the LN2 evaporation. This is very rare. I believe the evaporating nitrogen dilutes the oxygen that is present on top of the liquid nitrogen, further lowering the temperature that would be required to condense oxygen at that pressure. Good luck..

From: ACS Division of Chemical Health and Safety <DCHAS-L**At_Symbol_Here**Princeton.EDU> on behalf of Daniel Kuespert <0000057d3b6cd9b7-dmarc-request**At_Symbol_Here**LISTS.PRINCETON.EDU>
Sent: Friday, March 12, 2021 9:24 AM
To: DCHAS-L**At_Symbol_Here**Princeton.EDU <DCHAS-L**At_Symbol_Here**Princeton.EDU>
Subject: [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen

An interesting question occurred to me today: When you leave a Dewar flask of liquid nitrogen open to the atmosphere, such as when you're using it to cool the trap of a Schlenk apparatus, oxygen will condense in it since the boiling point of oxygen is 90 kelvins vs 77 K for LN. How fast this happens will depend on the air-exchange into the flask, so if it's covered loosely, presumably the dynamics will slow down. Eventually, though, the flask contains liquid air, not liquid nitrogen, which could become excessively exciting for someone who empties the flask by dumping it out somewhere near something combustible.


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