Sciencemadness Discussion Board

A few questions about electrolysis of CuSO4

EuropeanAnion - 7-1-2014 at 09:54


I'm thinking of a few questions about this process:

1. Can the solution be electrolized too far so that formed H2SO4 reacts further? Cleavage e.g.

2. Is CuSO4 really insoluble in anhydrous H2SO4? I can't fint solubility data for CuSO4 in H2SO4. My thought is to filter out the CuSO4 when the solution is pretty anhydrous.

3. The use of lead electrodes is said to create PbO2 at the surface; is it required that the surface initially is of PbO2 or does it become? In other words, what would happen of I just start the electrolysis with plain lead electrodes? Doesn't the radical formed simply oxidizes Pb to PbO2 in situ? Correct mer if I'm wrong.

Excuse me, my knowledge in electro chemistry isn't that deep - I'm studying organic chemistry and physics. Thanks for competent answers and comments.

MrHomeScientist - 7-1-2014 at 13:10

I'm not the most experienced in electrochemistry either, but here's my take.

1) Not that I've ever heard of. Then again electrolysis alone won't ever get you to a high enough concentration where this might be a problem.

2) Anhydrous copper sulfate is insoluble in anhydrous sulfuric acid, yes. The blue crystals you usually see are the pentahydrate, and the water can be pulled out of them by strong enough acid. I made a video on this a while back: http://www.youtube.com/watch?v=J1zwFwmANw4
However I seriously doubt electrolysis alone can yield concentrated acid. It's conceivable that the water would eventually evaporate as you electrolyze, but this would take forever. I also imagine that if most of the water did leave the solution in some way, it wouldn't be conductive any more since the acid wouldn't be ionized. The usual method is to electrolyze to form dilute acid, then boil away the water until you reach the desired concentration.

3) I believe lead dioxide electrodes require special preparation and cant be made so simply.

Those are just my first thoughts on your questions. I'd wait to hear from someone more experienced in this field. Take a look at the Technochemistry subsection for more information than you'll ever need.

[Edited on 1-7-2014 by MrHomeScientist]

Zyklon-A - 7-1-2014 at 13:17

I'm pretty sure it will never become anhydrous, you'll have to boil it at >300C for that, and even then it will still have about 2-4% water, and yes like MrHomeScientist said, ''lead dioxide electrodes require special preparation and cant be made so simply''.

EuropeanAnion - 7-1-2014 at 13:26

Quote: Originally posted by MrHomeScientist  
I'm not the most experienced in electrochemistry either, but here's my take.

1) Not that I've ever heard of. Then again electrolysis alone won't ever get you to a high enough concentration where this might be a problem.

2) Anhydrous copper sulfate is insoluble in anhydrous sulfuric acid, yes. The blue crystals you usually see are the pentahydrate, and the water can be pulled out of them by strong enough acid. I made a video on this a while back: http://www.youtube.com/watch?v=J1zwFwmANw4
However I seriously doubt electrolysis alone can yield concentrated acid. It's conceivable that the water would eventually evaporate as you electrolyze, but this would take forever. I also imagine that if most of the water did leave the solution in some way, it wouldn't be conductive any more since the acid wouldn't be ionized. The usual method is to electrolyze to form dilute acid, then boil away the water until you reach the desired concentration.

3) I believe lead dioxide electrodes require special preparation and cant be made so simply.

Those are just my first thoughts on your questions. I'd wait to hear from someone more experienced in this field. Take a look at the Technochemistry subsection for more information than you'll ever need.

[Edited on 1-7-2014 by MrHomeScientist]


Thank you. Yes, I did believe that the concentration never was high during the electrolysis.

Lead dioxide electrode preparation seems to be a problem, and not very attractive - not at least for it's unhealthy aspects.

Anyone knows what would happen if one using just plain Pb electrode?

elementcollector1 - 7-1-2014 at 14:01

The lead would at least partially dissolve away to insoluble lead sulfate - this is a component reaction of lead-acid batteries.
The concentration of the acid at the end of the electrolysis is ultimately limited by the concentration of the CuSO4. To get higher concentrations, you could use higher temperatures to get more copper sulfate into solution - but this would not be as effective as boiling the dilute stuff down.
The highest concentration of sulfuric acid that can be stores is 98.3% - this is the result of boiling down a solution as far as it will go.

chornedsnorkack - 7-1-2014 at 23:31

Quote: Originally posted by MrHomeScientist  
I'm not the most experienced in electrochemistry either, but here's my take.

1) Not that I've ever heard of. Then again electrolysis alone won't ever get you to a high enough concentration where this might be a problem.

Electrolysis of sulphate solutions is supposed to be a standard way of producing peroxodisulphates:
2HSO4-=2e-+H2S2O8
So what are the conditions that favour anode formation of peroxodisulphates rather than oxygen?
Quote: Originally posted by MrHomeScientist  

However I seriously doubt electrolysis alone can yield concentrated acid. It's conceivable that the water would eventually evaporate as you electrolyze, but this would take forever. I also imagine that if most of the water did leave the solution in some way, it wouldn't be conductive any more since the acid wouldn't be ionized.

Concentrated sulphuric acid is less conductive than dilute one, but it does undergo appreciable autodissociation. Although its viscosity means low mobility of most ions, the autodissociated ions have good mobility thanks to chainwise transfer of protons.

But concentrated sulphuric acid is said to be reduced to SO2 rather than H2 when reacting with electropositive metals. Should it behave likewise on electrolysis cathodes?
Cathode reactions balanced by anode oxidation of oxygen would be:
in dilute acid where only H2 is reduced -
1) 2H2O=2H2+O2
in concentrated acid where only SO2 is reduced:
2) 2H2SO4=2H2O+2SO2+O2
So, if at some concentration of sulphuric acid, both H2 and SO2 are formed at the same time, you could get a reaction like
3) H2SO4=SO2+H2+O2
Therefore, sulphuric acid should be electrolyzed at constant composition where the cathode gas is a bit over 50 % hydrogen and the rest SO2, so that the reactions 1) and 3) take place at the same ratio as the ratio of H2O and H2SO4 is. Electrolysis of any other composition should concentrate or dilute the acid to that concentration.