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..
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.
I've not seen any data, though, on how fast this actually happens. Has anyone ever seen data on this? If you have, please let me know. It would be useful to know how long one can really leave an open Dewar sitting around.
Laboratory Safety Advocate
Johns Hopkins University
Daniel Reid Kuespert, PhD, CSP
11101 Wood Elves Way
Columbia, MD 21044
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