I am electrolyzing a water + magnesium sulphate solution using copper for both electrodes. I am getting a light-blue precipitate forming which I believe to be a mixture of copper hydroxide and magnesium hydroxide, which are both poorly soluble. The solution is remaining clear which tells me that basically no copper sulphate is forming.

My question is, where is the sulphur from the sulphate going?

The mixture is not becoming acidic as if sulphuric acid was forming, and I do not detect any smell of sulphur is if some sulphur oxides were being released as gas.

  • $\begingroup$ It stays in solution? $\endgroup$ – Zhe Oct 21 '18 at 2:49
  • $\begingroup$ I wasn't aware that could happen. Could the sulphate ion just sit in solution by itself? I thought it would have to be in equilibrium with a positive ion as the copper sulfate would be? $\endgroup$ – platatomi Oct 21 '18 at 2:59
  • $\begingroup$ Are you sure the solution is not becoming acidic? Precipitation of Mg(OH)2 indicates that the MgSO4 has been split into Mg(OH)2 + H2SO4. If no mixing occurs, the cathode region will become alkaline while the anode region will become acidic (and will dissolve some copper). $\endgroup$ – James Gaidis Oct 21 '18 at 3:22
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    $\begingroup$ I'm pretty sure. I don't have pH indicators but when I add a base to the mixture, there is no reaction. And when I add an acid(vinegar), almost all of the precipitate disappears as it reacts with the acid to form soluble salts (magnesium and copper acetate in this case). If any acid was formed, wouldn't it just immediately react with the hydroxides and it would go back to mostly magnesium sulphate again? There is a really huge amount of white precipitate that forms very quickly when electrolysis starts. $\endgroup$ – platatomi Oct 21 '18 at 3:36

In electrolysis of aqueous solutions sulfate ions just sit there. Protic hydrogen (or the metal in the cation if this metal is not highly electropositive like magnesium) is reduced more easily at the cathode, and water is more easily oxidized at the anode.

Sulfate ion (actually, bisulfate ion) can be oxidized electrolytically at the anode if you use sulfuric acid instead of water as the solvent. See the preparation methods for potassium peroxydisulfate.

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  • $\begingroup$ What is "protic hydrogen" ? $\endgroup$ – SteffX Dec 20 '18 at 16:07

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