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Author: Subject: Metabisulfite for Bisulfite Adduct Discussion
Mothman
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[*] posted on 18-8-2018 at 06:53
Metabisulfite for Bisulfite Adduct Discussion


I know that Metabisulfite and Bisulfite are not the same compounds, however, I would like to discuss how they are similar or different in solution as I'd like to use the Metabisulfite to purify some THF that I distilled out of PVC primer.

Here is what I know:
In solution, Sodium Metabisulfite exists as sulfur dioxide (SO2) and sodium sulfite (Na2SO3).
I know that to form a bisulfite adduct with a ketone (acetone and MEK), I need the bisulfite (HSO3-) anion.

Now, my question is this: Could I just dissolve the metabisulfite in water and use it as is in the hopes that water causes the sulfite to become bisulfite in small amounts? The reaction would obviously be very slow if it occurred at all, but the equilibrium would be shifted to favor products as the ketones would react continuously and readily.

If I can't just add the metabisulfite solution to the crude THF, would acidifying the solution work to form the bisulfite anion in solution or would I just get a bunch of sulfur dioxide?

Help would be appreciated, I haven't been able to find much info on this subject which strikes me as odd because it's not easy to get THF from anywhere but pvc primer these days, and an OTC way of chemically purifying it after distillation ought to be at least a somewhat discussed topic on this board. Anyways, help would be appreciated, thank you to everyone who took the time to read this.




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happyfooddance
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[*] posted on 18-8-2018 at 08:28


Metabisulfite hydrolyses to bisulfite very rapidly and completely in aqueous solutions. 1 mol metabisulfite forms 2 mol bisulfite. Also, it isn't just the bisulfite anion that is needed, the cation is also part of the adduct. What I want to know is why you think THF will form an adduct, seeing as it is an ether, and not a ketone or aldehyde...

Also, acidic or basic impurities (like your suggestion of acidifying the solution) will interfere with formation of the adduct.

[Edited on 8-18-2018 by happyfooddance]
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[*] posted on 18-8-2018 at 09:19


That's actually a really good post, PhDchemist. Didn't you say that you got some metabisulfite in your eye or something once?

[Edited on 8-18-2018 by happyfooddance]
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JJay
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[*] posted on 18-8-2018 at 09:56


This procedure won't work with THF because it's not an aldehyde or methyl ketone. I don't think it will work with cyclohexanone, but I've never tried it. The acetone bisulfite adduct is pretty water soluble... not sure about the methyl ethyl ketone adduct.

I'd check Amarego's Purification of Laboratory Chemicals for details on how to purify THF.




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mackolol
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[*] posted on 19-8-2018 at 06:54


I think he wants to get rid of ketones in his thinner and leave unreacted thf JJay
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[*] posted on 19-8-2018 at 07:27


Metabisulphite will react with hydroxide to form sulphite.
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JJay
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[*] posted on 19-8-2018 at 09:44


Quote: Originally posted by mackolol  
I think he wants to get rid of ketones in his thinner and leave unreacted thf JJay


Oh, that makes more sense. Hmm...

Bisulfite and metabisulfite are the same thing in solution; bisulfite is essentially metabisulfite that has reacted with a water molecule.

I'm not seeing any reason that it wouldn't be possible to form a bisulfite adduct with MEK and acetone with sodium sulfite at the right pH and then distill off the THF.





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Mothman
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[*] posted on 19-8-2018 at 11:52


Thank you guys for all of the responses, I do just want to form the ketone/bisulfite adduct of the impurities and then distill off the purified THF, I understand that it's just aldehydes and ketones that are affected by bisulfite :P

I was confused about the hydrolysis of metabisulfite because I had read that it produced sulfite and SO2 and not two bisulfites, but I am no longer confused and if people find themselves in a similar rut as I they can locate this thread. Thank you all!




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[*] posted on 19-8-2018 at 12:06


Oh, one other thing,
On the rhodium archive I read that PCP cooks use a bisulfite adduct of cyclohexanone for making PCP, so bisulfite should clear out any cyclohexanone from my primer as well. I did fractionate out the THF already so there should be almost no cyclohexanone in the current product but it never hurts to make sure with a bisulfite treatment. I plan to use the THF for my first grignard reaction, I already bought some BHT for stabilizing it for long-term storage, this should be fun!




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[*] posted on 19-8-2018 at 12:31


Quote: Originally posted by Mothman  
Oh, one other thing,
On the rhodium archive I read that PCP cooks use a bisulfite adduct of cyclohexanone for making PCP, so bisulfite should clear out any cyclohexanone from my primer as well. I did fractionate out the THF already so there should be almost no cyclohexanone in the current product but it never hurts to make sure with a bisulfite treatment. I plan to use the THF for my first grignard reaction, I already bought some BHT for stabilizing it for long-term storage, this should be fun!


Oh cool. I just ran across this: https://the-hive.archive.erowid.org/forum/showflat.pl?static... So it does work with cyclohexanone.






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