Sciencemadness Discussion Board

Anodic oxidation of polyatomic anions

sakshaug007 - 22-4-2009 at 21:55

Hello Everyone,

I decided to start a topic regarding the mechanisms and products associated with anodic oxidation of polyatomic anions (e.g., ClO4-, NO3-, SO42- etc.) in various solvents.

This may seem simple to many of you but for some reason or another I've never thought about the products formed from electrolytic oxidation of polyatomic anions since there can be a host of reactions that can occur depending on the reactivity of the solvent and temperature, pH etc.

Therefore, my first question pertains to what products would form from the oxidation of NO3- either from electrolysis of a molten nitrate salt or from a hypothetically inert solvent. My best guess is that NO2 (or NOx) and O2 gases form and simply leave the solution (see attachment) is this true? If so, would a similar decomposition occur with ClO4- in which O2 and Cl2 gases are evolved?

Any information would be great.
Thanks!


[Edited on 23-4-2009 by sakshaug007]

Attachment: anodic NO3- decomposition.ppt (135kB)
This file has been downloaded 641 times

sakshaug007 - 14-5-2009 at 18:18

So I know that sulfate can oxidize to peroxodisulfate at the proper voltage, but what would peroxodisulfate oxidize to under a higher voltage?

woelen - 20-5-2009 at 01:47

This would oxidize to SO3 and O2. Possibly also some SO4 and O3. For persulfate the question, however, only is hypothetical, because all known persulfates decompose to pyrosulfate and oxygen, well before they could be melted. I know of no solvent, which could dissolve persulfate and would be sufficiently inert to withstand the extreme oxidizing conditions.

Your topic nevertheless is quite an interesting one. I also have wondered what happens with many anionic species when electrolysis is performed. I have tried with many in aqueous solution with graphite or platinum anode, but unfortunately, water is severely limiting, because with most anionic species, simply oxygen is produced at the anode.

Some examples which I tried in particular:
- potassium dichromate in somewhat acidic solution: Formation of oxygen at anode, no formation of peroxo species (these can very easily be detected due to their intense blue color at medium to low pH).
- sodium bicarbonate: Formation of a mix of oxygen and carbon dioxide
- sodium acetate: Formation of a mix of ethane and carbon dioxide
- sodium monochloroacetate: Formation of a complicated mix of CO2, CO and in later stages Cl2 as well.
- tetrachloromanganate(II) (solution of MnCl2 in conc. HCl): Formation of a very dark green/brown manganese(III) or manganese(IV) species. Only small quantities of chlorine gas are formed in such a solution.

I also have DMSO and tried a little with this solvent, but this solvent is oxidized easily, even more so than water. I really would like to have a solvent, which is not easily oxidized.

Hydragyrum - 20-5-2009 at 02:52

Quote: Originally posted by woelen  
- sodium bicarbonate: Formation of a mix of oxygen and carbon dioxide

This is a coincidence - I was wondering to myself about a week ago whether carbon could be deposited by reducing bicarbonate (or carbonate) - I suspected not as else wouldn't I have heard of it? Carbon may not be so exotic, but plating it out would be really useful I think.

Hydragyrum - 20-5-2009 at 02:56

Quote: Originally posted by woelen  
I also have DMSO and tried a little with this solvent, but this solvent is oxidized easily, even more so than water. I really would like to have a solvent, which is not easily oxidized.

Have you tried HMPA? (that is hexamethylphosphoric triamide, (Me2N)3P=O). It is a true wonder-solvent, you can even make blue solutions of alkali metals in it, like ammonia.

sakshaug007 - 20-5-2009 at 07:45

Quote: Originally posted by Hydragyrum  

Have you tried HMPA? (that is hexamethylphosphoric triamide, (Me2N)3P=O). It is a true wonder-solvent, you can even make blue solutions of alkali metals in it, like ammonia.


This definitely sounds like an interesting solvent that I would like to attempt the electroplating of lithium in. Do you happen to know where to obtain any?

[Edited on 20-5-2009 by sakshaug007]

sakshaug007 - 20-5-2009 at 07:49

Quote: Originally posted by woelen  
I also have DMSO and tried a little with this solvent, but this solvent is oxidized easily, even more so than water. I really would like to have a solvent, which is not easily oxidized.


I didn't realize DMSO oxidized easily. What are the products formed from oxidation?

UnintentionalChaos - 20-5-2009 at 08:14

Quote: Originally posted by sakshaug007  
Quote: Originally posted by woelen  
I also have DMSO and tried a little with this solvent, but this solvent is oxidized easily, even more so than water. I really would like to have a solvent, which is not easily oxidized.


I didn't realize DMSO oxidized easily. What are the products formed from oxidation?


Dimethyl sulfone. Also known as MSM (methylsulfonylmethane), which is also available in bulk from health food stores.

The_Davster - 20-5-2009 at 08:19

DMSO is also easily reduced electrolytically to dimethylsulfide. Learning this firsthand was not pleasant.

sakshaug007 - 20-5-2009 at 11:29

I found an article that addresses the electrochemical oxidation and reduction of DMSO. It specifies that it is "particularly" stable in cathodic reduction and "fairly" stable in anodic oxidation with, yes, the primary product being dimethyl sulfone. Could anyone write the electrode potential equations for this reaction? Is it dependant on the electrolyte present? As the article also states DMSO is reactive to the alkali metals (aside from lithium). Will DMSO still undergo electrochemical redox in the presence of an unreactive electrolyte (e.g. triethylbenzylammonium perchlorate)?

woelen - 20-5-2009 at 12:28

Today I also tried electrolysis of CuCl2 in conc. HCl with platinum electrodes. This does not give any interesting results. Hydrogen is produced on the cathode, chlorine is produced on the anode. The solution remains green/yellow and clear.

I also tried electrolysis of excess WO3 (until no more dissolves), dissolved in a solution of NaOH. This solution has composition Na2O.nWO3 (I don't know the value of n, I just kept adding WO3 until no more dissolved). When such a solution is electrolysed, then nothing interesting happens, hydrogen at the cathode, oxygen at the anode. No molybdenum blue is produced at the cathode and this surprises me. But on the other hand, when I add Na2S2O3 to this solution, then nothing happens beside dissolving of the solid, but as soon as a drop of acid is added, the solution turns deep blue. Probably a similar thing is true for electrolysis.

That HMPA solvent sounds very interesting. I'll see whether I can obtain some of that, I never heard of it before.

Hydragyrum - 20-5-2009 at 13:29

HMPA was 'discouraged' a few years back - apparently, it is a carcinogen... but someone should still produce/sell it I imagine (sorry, I know all this stuff from years ago, but at present my chemistry collection is not very advanced - memories of a golden age, as it were...).

If one was courageous, it probably can be made from POCl3 and Me2NH, but be warned that HMPA needs a vacuum if you want to distil it.