Sodium Ball of Death

An interesting problem encountered by wafflesforlife on /r/chemistry.

I recently inherited responsibility for the solvent stills in my lab and, even more recently, had to quench, clean and reset our THF still. I used a standard sodium-benzophenone system and was happy to see a beautiful purple color when all was said and done. Now, after several uses of the still, the sodium has clumped into a giant ball (2″ in diameter) and floats around in the still, even at room temperature. The exposed sodium still appears fairly metallic and the still has maintained its nice purple color. However, my concern is that the exposed sodium no longer has a THF “buffer” surrounding it and could be more reactive towards any contaminants that could get introduced. Also, I figure with roughly half of its surface area out of the solution, the sodium will not be as effective at drying the still. Has anyone else encountered this problem? After such a short period of time, I would rather not go through the trouble of quenching and setting the still up again, but maybe I just need to sack up and do it for the good of the group. Any advice would be appreciated!

Taken from: Floating sodium ball in THF still. Is it safe?

Mitch

8 Comments

  1. We use Na/benzophenone for our thf still. That is WAY too much sodium for a flask that size IMO. What I would do is probably to either 1. top off the still and hope the ball eventually gets consume or 2. (the safer way) take the still down *VERY CAREFULLY* and restart one with more appropriate amount of sodium.

    • I agree that you need to quench the still and start over (or use alternative drying methods). 2 g of sodium is plenty for 1 L anhydrous thf (maybe up to 5 g if you don’t have a press to make wire). What you have there really looks like a lab fire waiting to happen. Curly arrow’s tips may also useful – http://curlyarrow.blogspot.com/2008_09_01_archive.html

  2. I really really really don’t get why people like to use stuff like this. Molecular sieves get you way drier (4.1 ppm) than Na/benzophenone ever will (43 ppm), plus without the work and hazard. What’s not to like?

    http://pubs.acs.org/doi/full/10.1021/jo101589h

    A handy list of what works for other solvents: http://chemknowhow.com/forum/viewtopic.php?p=562#562

    • Our group uses Na/Benzophenone in order to remove molecular oxygen from Et2O and THF. FPT would work as well though (it just adds an extra step).

      • Or N2 bubbling either, which you could also safely leave unattended for less work. If Na/benzophenone doesn’t take out all the water, how can you be sure it removes all the dissolved O2 as well? Smacks of dogmatic chemistry voodoo to me.

  3. an interesting paper about how to dispose sodium and potassium with water in lab:
    http://pubs.acs.org/doi/abs/10.1021/ic010594m

  4. What I would do: distill all THF out, add a small amount of toluene, then heat to reflux. Na would melt at that temperature. Stir vigorously and you will break the big ball to small balls, then distill all toluene, add new THF then you will end up with good Na surfaces without quenching. Maybe 3 times of small THF addition-removal process to completely eliminate toluene is necessary if you don’t like some of it in the THF.

  5. Next time don’t fit your flask with a stirrer bar, this will avoid this same situation happening again lol

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