Chemgineer
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Copper Sulfate Electrolysis
I have been electrolysing copper sulphate at a saturated solution to produce some sulphuric acid and as a result my liquid level drops during the
process.
Towards the end just before I get a clear colourless acid solution I keep getting a white looking crystal drop out of solution (difficult to see the
colour as it's against a blue background).
What could this be? Maybe a contaminant salt in my copper sulphate which is less soluble?
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unionised
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I think that the monohydrate of copper (II) sulphate may form. in solutions with a (relatively) large amount of acid.
It's more or less white.
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j_sum1
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That is how I used to make my sulfuric acid.
The white crystals are common, as is a slight green hue on boiling down.
I found I had best results by working at half saturation and a relatively low current such that a batch would take 24 hours to electrolyse. I would
calcilate on the basis of 10% electrical inefficiency and regulate the current accordingly.
My setup was 50g of copper sulfate pentahydrate in a plastic jam jar and using a strip of lead for the anode. It was important to not let the solution
sit because the copper would
on the cathode react with the acid produced.
I would collect several days' of acid and then boil it down to around 70% concentration.
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Chemgineer
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I think it may be copper oxide from the residual copper sulphate decomposing during the boiling process. It filters off ok but this is more difficult
as the sulphuric acid gets the the point it destroys the filter paper.
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Tsjerk
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Copper oxide is black, I'm sure it is the monohydrate. Also copper oxide dissolves in sulfuric acid.
It just means your solution is too concentrated to dissolve all copper, dilute your solution like j_sum1 suggests and you won't see it again.
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Jome
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I electrolyzed a solution of copper sulfate a few days ago. It turned rather strongly green, but that might be residual copper sulfate as well as iron
from a ceramic spacer I had between the electrodes. The graphite anode, however, got eaten some time during 15 hour run despite a low 4-5 mA/cm^2
current density.
I'm fascinated by the use of lead electrodes; do they flake off or form power/suspended particles or do the PbO2 behave nicely?
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Tsjerk
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What voltage did you use? I would say you shouldn't run above 5V.
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Jome
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I used 2.6V, got a current of around 0.3-0.4 A for a 6x60mm cylindrical graphite anode. I did however start off a few hours with wrong polarity (but
at 1.7V) due to the circuit being mislabeled. I used 25g (0.1 mol) CuSO4 in 200ml's of water.
Maybe my anodes are actually oversized pencil-leads or something, meant for drawing...
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j_sum1
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I highly recommend ditching the graphite anode and using Pb. You will have something that lasts a lot longer without disintegrating, produces an acid
that needs no filtering and in general is a lot more manageable. Actually, you get a lovely PbO2 coating that may make it a useful electrode for
other applications.
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woelen
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I do not believe that the white material is the monohydrate. Copper sulfate turns blue, even in the presence of a small amount of water and only in
concentrated H2SO4 it turns white. So, in aqueous solution, you certainly won't get lower hydrates of copper sulfate. I myself crystallized copper
sulfate from slightly acidified solutions by slow evaporation of water. In that situation you get saturated solutions and from those, I always get
bright blue crystals, not some pale blue powdery material.
It is interesting though to know what the pale blue solid might be. Maybe it is copper peroxodisulfate, which is pale blue. This, however, is quite
soluble in water, but maybe in highly concentrated solutions of copper sulfate it is less soluble and then precipitates as pale blue solid. On
electrolysis of acidic concentrated sulfates, formation of peroxodisulfate ions at the anode is quite common, especially if the solution is not too
warm.
You could try to isolate some of the pale blue solid (no need to purify it, if a few cubic millimeters can be obtained as a mud, then it is enough).
Add this solid to 1 ml of 50 - 70% HNO3 and then add a tiny pinch of AgNO3 (just a few crystals). If peroxodisulfate ion is present, then the solution
turns deep brown, due to formation of silver(III) ions.
[Edited on 6-9-21 by woelen]
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Chemgineer
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As far as electrodes go I am actually using a pair of MMO anodes scavenged from those small ebay hypochlorite makers. I have 2 connected in parallel
for surface area as they are only small. MMO is more tolerant of acid compared to platinum I believe.
For the cathode I just have a small copper pipe, which keeps growing bigger as more copper precipitates.
[Edited on 6-9-2021 by Chemgineer]
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Tsjerk
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Quote: Originally posted by woelen  | Maybe it is copper peroxodisulfate, which is pale blue. This, however, is quite soluble in water, but maybe in highly concentrated solutions of
copper sulfate it is less soluble and then precipitates as pale blue solid.
[Edited on 6-9-21 by woelen] |
Could it be insoluble in sulfuric acid? I believe the point where it was seen by chemgineer was when the blue color was already gone.
Another possibility is that the solution is warm when it gets concentrated with sulfuric acid, warm enough to dissolve enough copper to form a
precipitate later, but anhydrous enough not to turn blue, and then on cooling precipitates a lower hydrate of copper sulfate. I think sulfuric acid
could be so dehydrating at higher temperatures; even anhydrous ethanol can dehydrate copper sulfate at 30-40 degrees, it turns pale blue.
[Edited on 7-9-2021 by Tsjerk]
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j_sum1
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| Quote: Originally posted by Chemgineer | As far as electrodes go I am actually using a pair of MMO anodes scavenged from those small ebay hypochlorite makers. I have 2 connected in parallel
for surface area as they are only small. MMO is more tolerant of acid compared to platinum I believe.
For the cathode I just have a small copper pipe, which keeps growing bigger as more copper precipitates.
[Edited on 6-9-2021 by Chemgineer] |
Good luck with that. I started off with MMO and ruined them on my first run with H2SO4. Passivated Ti substrate just would not conduct.
It might be you have collected some MMO sulfate precipitate.
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