In my (pretty long-term) experience working with lots of liquid nitrogen in a variety of situations I, also, have never witnessed accidental or unintentional enrichment of oxygen in the liquid cryogen.
In full disclosure, at the same time I have observed, even participated in, the intentional condensation of liquid oxygen by passing oxygen gas through a collection tube immersed in a bath of liquid nitrogen for various the purpose of thoughtfully and carefully arranged experiments. FWIW, it never spontaneously detonated. And yes, it was sometimes very exciting. :)
Plus, as was mentioned by another list member, there is very good justification why this process is discouraged by the blanket of gaseous nitrogen sustained by the active outflow of boiling nitrogen. (in other words, not only is the observed incidence of the process in question significant; a plausible mechanism explaining the observation exists.)
Yet, with the repetition of an anecdotal report "...We heard of an accident..." the suspicion, the story, the question is enlivened, revivified and sustained.
This thread began with a hypothetical (and doubtful) supposition that oxygen would spontaneously condense in an open-mouthed liquid nitrogen dewar followed by a questionable practice of discarding the dewar contents.
It seems (to this chemist at least) this is a non-issue for standard- and good-laboratory practice. Also, besides this anecdotal report the cited article was quite good and very informative to new experimentalists. I intend to use it with my students.
jadv
On Mon, 15 Mar 2021 1:25 PM, Yaritza Brinker wrote:
> I worked with SEM/EDS using an LN2 Dewar to cool the EDS detector for about 10yrs. Never seen it happen. However, this older article does anecdotally report it happening and resulting in ignition.
>
> ‰??‰?|We heard of an accident where ignition was due to the condensation of oxygen from room air in liquid nitrogen and subsequent elaboration of an oxygen-rich gas as the nitrogen boiled away. Because of the fire hazard, one should neither allow liquid N2 to evaporate to dryness‰?|‰??
>
> J. Bastacky, T. L. Hayes, Safety in the Scanning Electron Microscope Laboratory, SCANNING Vol. 7, 255-272 (1985)
>
> https://onlinelibrary.wiley.com/doi/pdf/10.1002/sca.4950070506
>
> Thank you,
>
> Yaritza Brinker
>
> 260.827.5402
>
> *From:* ACS Division of Chemical Health and Safety *On Behalf Of *Daniel Kuespert
> *Sent:* Friday, March 12, 2021 10:25 AM
> *To:* DCHAS-L**At_Symbol_Here**Princeton.EDU
> *Subject:* [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen
>
> *** External Email ***
>
> 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.
>
> Regards,
> Dan Kuespert
> Laboratory Safety Advocate
> Johns Hopkins University
> Daniel Reid Kuespert, PhD, CSP
> dankuespert**At_Symbol_Here**me.com
> 11101 Wood Elves Way
> Columbia, MD 21044
> 410-992-9709
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