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Subject: Re: [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen
Date: Friday, March 12, 2021 at 12:11:13 PM
Author: TILAK CHANDRA <0000058f112ac338-dmarc-request**At_Symbol_Here**LISTS.PRINCETON.EDU> 		Next by Date:
Subject: Re: [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen
Date: Fri, 12 Mar 2021 13:14:47 -0500
Author: Joseph Smith

From: Safety Emporium Support Team <esupport**At_Symbol_Here**safetyemporium.com>
Subject: Re: [DCHAS-L] Condensation of liquid oxygen in open Dewar flasks of liquid nitrogen
Date: Fri, 12 Mar 2021 13:00:43 -0500
Reply-To: ACS Division of Chemical Health and Safety <DCHAS-L**At_Symbol_Here**Princeton.EDU>
Message-ID: C686DB7E-63DD-445B-83B2-9162F841AA84**At_Symbol_Here**safetyemporium.com
In-Reply-To


There's essentially no air contacting the liquid nitrogen surface - the boiloff of the liquid nitrogen is creating an oxygen-deficient layer above the liquid.  And that gas is above 77K as well.


Presumably, if you left it long enough and the Dewar is tall, then you could potentially fractionally distill out the nitrogen leaving oxygen at the bottom (liquid nitrogen and oxygen are made by the distillation of liquid, after all).  More likely it would be a mixture of both left at the very bottom. I've never observed it and don't know anyone who ever has  - and I've seen a huge number of 1 liter tall Dewars left to evaporate near dryness overnight. But I suppose the possibility makes a case for keeping your Dewars nice and clean so there's nothing to ignite.

I would imagine condensation of liquid air onto He is a bigger issue of concern, but I've never heard of an issue there and you don't hear about MRI/NMR machines blowing themselves up.

Liquid hydrogen would be a whole other level of concern, of course.  That would be an insane risk.

Pulling air through a vacuum trap that is cooled in liquid N2 is a whole different beast, of course.  In my grad school days, one of our lab mates walked into my lab to show us the neat blue liquid he had in his trap - and we all starting waving him away and telling him to put  it back in the Dewar. Ah, the good old days.

Rob Toreki

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On Mar 12, 2021, at 12:11 PM, TILAK CHANDRA <0000058f112ac338-dmarc-request**At_Symbol_Here**LISTS.PRINCETON.EDU> wrote:

Hi Dan,

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..

Tilak
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.

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
11101 Wood Elves Way
Columbia, MD 21044
410-992-9709

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