Dan and Joe:
The rate at which oxygen will condense at liquid nitrogen temperatures will depend on several physical factors (i.e. oxygen concentration in the environment, exchange rate, surface area, etc.) Yes, it can get exciting when liquified oxygen gets near combustible material.
Flammability limits (e.g. LEL/UEL or LFL/UFL) are published for air. The range will expand at both ends in a oxygen-rich environment (i.e. (LEL will be lowered, UEL will be increased). So, all bets are off when it comes to evaluating the question "Am I above the LEL for this scenario?" when you change the oxygen concentration of the environment. I would never bet the farm on a determination of LEL/UEL in an environment other than air without some serious data.
Harry
From: ACS Division of Chemical Health and Safety <DCHAS-L**At_Symbol_Here**Princeton.EDU> On Behalf Of Joseph Smith
Sent: Friday, March 12, 2021 12:15
To: DCHAS-L**At_Symbol_Here**Princeton.EDU
Subject: Re: [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen
Probably not enough O2 to exceed the LEL.
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
Daniel Reid Kuespert, PhD, CSP
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