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Topic: Schlenk technique- argon inert atmosphere  (Read 411 times)

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Offline Grasshopper

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Schlenk technique- argon inert atmosphere
« on: July 08, 2019, 06:14:28 PM »
Hi, gurus of the chemical forums (fora?). Sorry if this is a stupid question (!) just trying to improve my understanding...

For... reasons, I'm currently using a fume hood that's connected to argon but not nitrogen. I seem to recall, from some murky corner of my memory, that I was once told that it's a bad idea to pull argon across a cold trap (cooled by liquid nitrogen) however I don't remember why and I can't find any info about this (so I am wondering whether my memory is faulty).

I could do with degassing some solvent, preferably by freeze-thaw. If I repeatedly freeze the solvent then pull the argon out of the vessel across the cold trap, is that a safety hazard?

Thanks!!

Offline wildfyr

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Re: Schlenk technique- argon inert atmosphere
« Reply #1 on: July 08, 2019, 08:20:18 PM »
I've never thought of it before, but I think it's because you can condense your argon. BP of argon is - 185C, BP of nitrogen is - 196C. You'll have liquid or solid argon in the trap. When you take LN2 off the trap, the argon will boil, gain a lot of volume and explode your shlenk trap.

Offline Grasshopper

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Re: Schlenk technique- argon inert atmosphere
« Reply #2 on: July 09, 2019, 05:40:43 AM »
Ah, thanks Wildfyr. So... pulling air across the cold trap is normally OK because we take care not to do it for extended periods of time, and also because oxygen is only 20% of the air... pulling a small amount of argon for a short time may be OK, but will be 100% of the gas in there and has higher boiling point than N2...

Anyway, I think I will find some other way of de-gassing my solvent just in case. I am all in favour of no explosions in the lab  ;D

Offline OrganicDan96

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Re: Schlenk technique- argon inert atmosphere
« Reply #3 on: July 09, 2019, 06:14:02 AM »
what about using dry ice/acetone instead of LN2

Offline wildfyr

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Re: Schlenk technique- argon inert atmosphere
« Reply #4 on: July 09, 2019, 10:15:45 AM »
I'd just like to note that pulling air containing oxygen is an even worse idea than pulling argon. You really shouldn't pull more than whatever the volume of your enclosed system is, plus very minor leaks that are unavoidable. If you hook your system up to a vacuum gauge and it can't get below 100-200 millitorr, you may be putting yourself in a dangerous situation.

Condensed oxygen will expand when warmed, just like argon, but there is another danger. Liquid oxygen can react with any organics inside the trap to make who-knows-what peroxy species, many of which are extremely sensitive explosives.

I did this once by being stupid and it scared the heck out of me seeing that blue liquid gently boiling in my trap. Luckily my hoodmate and I were clean workers. We just opened the vacuum lines to air and let it boil out slowly.

Offline Grasshopper

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Re: Schlenk technique- argon inert atmosphere
« Reply #5 on: July 09, 2019, 04:35:24 PM »
I don't think I explained myself very well; I'd just like to add that I'm not going to open the pump directly to either air or the argon source :) just the usual procedure i.e. attach reaction vessel to Schlenk line, freeze, evacuate the gas from the top of it, allow to thaw, etc etc.... I just had in my mind a vague memory of someone telling me (years ago) not to do this when the inert gas in the Schlenk line is Ar instead of N2, and I was wondering whether that memory was valid. On the whole I think I'd rather not take the risk if there's some potential for condensing the gas in the trap. A dry ice trap is an idea, though, thanks OrganicDan  :)

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