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how should i set up an electrolysis cell in order to convert magnesium sulfate into sulfuric acid? Answered

after finding out copper sulfate is expensive everywhere, ive decided to try making my own, from sulfuric acid which i intend to make form magnesium sulfate, aka epsom salts.
i am going to try and react copper oxides with any formed sulfuric acid then crystalize it and remove it, but, im not exactly sure on how i should set up this cell, to ensure the formed magnesium oxide/hydroxide doesnt react with the sulfuric acid again,
as well as what the best voltage/currect would be to minimise polution/ errosion from my lead electrodes (just plain old lead, i plan on forming lead dioxide in this reaction also).

any help would be greatly appreciated, also whoever solves my problem will get a best answer form me.

18 Replies

Jack A LopezBest Answer (author)2011-06-01


Magnesium sulfate is used as the electrolyte to prepare copper sulfate. A magnesium sulfate solution is electrolyzed with a copper anode to form copper sulfate, magnesium hydroxide, and hydrogen:

The Wiki article does not go into more detail than this, but I can guess that this is the sum of these reactions:

Moreover I am guessing the Mg(OH)2 falls out immediately because it is almost insoluble in water (0.012 g/ L)  compared to like  for CuSO4 (250 g/L). 

Also guessing that both electrodes are made of copper.

And you want to convert the copper sulfate, CuSO4, into sulfuric acid, right?

I think you can do this with the same cell, provided there is some way to get the Mg(OH)2 out of there.  I mean if the pH starts dropping, I don't think the that magnesium hydroxide will stay insoluble.  The low solubility I quoted, (from the Wiki article on  magnesium hydroxide) is for pure water.  I expect it will dissolve in acid, and thus try to return to the MgSO4, "epsom salt",  it started from.

So getting the Mg+2 ions out of your way is important, and I don't know if that will be easy or hard.  It might be as easy as just pouring off the liquid part of your cell every so often.

By the way, the way the ancients made sulfuric acid from copper sulfate was by heating dry CuSO4 in a retort, then condensing the vapors, which contain water and SO3, and maybe SO2 also?  Regarding SO2 and SO3
I'm not sure about the thermodynamic mojo determining which one will emerge from roasting some thing that has sulfur in it, but SO3 is the gas you want for making sulfuric acid.  Also I read in a forum one time about somebody getting some good results for homemade sulfuric acid by roasting either CuSO4 or FeSO4.

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shomas (author)Jack A Lopez2017-05-22

I think this might work. Start with two jars. One just water, and the other dissolve epson salt. Join the two jars with a salt bridge. I believe the magnesium ions will migrate to cathode jar while sulfer ions will stay in the anode jar.

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Jack A Lopez (author)shomas2017-05-23

In a comment I wrote here,


about a year ago, I upped some pictures and description of an easy two-compartment electrolysis setup using two plastic cups bridged by a piece of folded, wet, paper towel.

The salt being electrolyzed in this setup is aqueous sodium chloride. At the anode Cl- is oxidized to Cl2 gas. At the cathode, water is reduced to H2 gas and OH- ions.

anode: 2 Cl− = Cl2(g) + 2 e− -1.36 V
cathode: 2H2O + 2 e− = H2(g) + 2 OH− −0.8277 V

That's a guess, at what is happening chemically. The numbers for standard electrode potential, I got from Wikipedia, here,


Anyway, I touched some pH paper to the electrolyte in each cup, and the colors show a very large difference in pH. So the wet paper towel bridge seemed to be doing a good job of keeping the anolyte and catholyte from mixing.

However, there are big energy losses incurred in pushing current through the bridge. Essentially it acts like a big resistor, electrically. Or that is sort of the explanation I am using to explain why the potential across this cell (about 18V at 1 A) is so much larger than the standard potential calculated for this reaction (1.36+0.8277 ~= .2.2 V)

(Also using a voltmeter, I can put the probes on different places on the paper towel, for to confirm that is where most of the voltage drop is.)

I think the pros use magic ion conductive plastic membranes,


like Nafion(r) for Na+ ions, and other plastics for other kinds ions. These magic plastics have low ohmic resistance, for low electric energy loss, and can keep the contents of the two cell compartments from mixing.

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well i am going to be crystalysing out the copper sulfate, as its the only way to be sure ts pure,
yes, after i have made my mega copper sulfate crystal, i plan on converting it into sulfuric acid, though this wotn be an issue much as i plan to use the sulfuric acid to make nitric acid from calcium nitrate. the proccess of making it means ill be condensing the nitric acid off, so magnesium impurities arent going to bother me much.

quick question.
would it at all be possible to place a filter of some sorts to capture the dropping magnesium hydroxide? if its insoluable, really, it would form at the electrode, and fall off.
could i safely use filter paper without it being corroded by the acid or copper sulfate? its 100% celulose, but im not sure what could happen to it during electrolysis.

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I would expect the fluffy white Mg(OH)2 to form at the cathode, where OH- ions and H2 gas are forming , and I would expect the CuSO4 to form near the anode, where Cu metal is turning into Cu+2 ions.   

OH- ions  show up at the cathode, and probably the only reason I remember that is because of a picture I saw in a chemistry book.  I'm not sure if the attached picture is the same one I was thinking of, but the story is the same: OH- ions, the presence of which is shown by pink phenolphthalein, are boiling off the cathode along with the H2 gas.

A paper separator between the two electrodes would not hurt, but I'm not sure how necessary it is.  I think most of the Mg(OH)2 is just going to fall to the bottom of the cell.   You just pour off the blue-colored water, and that's where your CuSO4 is.

BTW, if you think big blue crystals of hydrated copper sulfate are pretty, then you should definitely see this video of some guy in London who filled up an abandoned apartment with the stuff.


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i dont mean a seperator, i want to electrolyse the lot of it in a glass jar, and sitting in the jar will be a cone of filter paperwhich will have its bottom half submerged , and in that small "pool, ill put the cathode, so that any formed magnesium hydroxide stays there, essentially meaning that i wont need to filter the end product. also, it means that any sulfuric acid formed wont be able to re-react with the magnesium hydroxide because it will be repeld by the cathode, plus would need to seep through the filter paper which would take quite some time.
i just need to be sure that somehow the filter paper wont be electrolysed or affected by the acid.
heres a crude text diagram

|\___/| <--- water line here
| \ ** / | /,\ is filter paper
| \*/ | * is insluable magnesium hydroxide.

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Well, I think if it divides the cell into more than one compartment, then it is a separator.   BTW, whatever you call it, I think it is a good idea.

Part of the reason I think it is a good idea, is that I decided to actually try just couple of copper wires dunked in jar of Epsom-salt-solution.  It started out as 500mL of water + 200g of Epsom salt (MgSO4+7H2O), and then it turned into the jar full of muck shown in the pictures below.  I let the cell run for 4 hours, then let it settle for another 4.  The last two pics are after I let it settle.  I think the copper sulfate, and the magnesium hydroxide, are both heavy and are trying to settle in the bottom of the cell together.

So a contrivance of some kind to keep them separate would be a good idea.

Also there was some buildup of I-know-not-what on the anode.  While doing this I had my power supply set for a constant current of 0.5 A.  Without the buildup on the anode, the voltage on the cell would only be about 2.8 V, but when the layer of whatever built up it would add resistance forcing the power supply to drive the cell voltage much higher, to like 9 to 12V.  I found that just shaking the anode, or hitting it against the glass, would shake off the buildup, and the voltage would fall back to 2.8 V.  This would happen about every 30 minutes, or so, and it was kind of annoying to have to shake the gunk of the anode.

Anyway, it looks the naive approach of just two side-by-side electrodes, and no separator,  is uh... well... kind of naive, and there's probably some room for improvement here.

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thankyou for doing the testing for me, one of the reasons you got the best answer which i should have given a while ago. thanks

i think ill use a small solar powerd fan to blow on the water to stir it which should keep the , what i think is magnesium hydroxide or magnesium carbonate.

anyway, i got heaps of copper (5kg rod) to use, i wonder how mucjh sulfate i can make from it.

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Dang!  The reactions I wrote did not show up in the reply above. Luckily I had the html for that reply cached on the clipboard.  Anyway,  the individual reactions going on in the cell:
  • Anode: Cu → Cu+2 + 2e-
  • Cathode: 2H+ + 2e- → H2
  • Water dissociation: 2H2O → 2H+ + 2OH-
  • Precipitation: Mg+2 + 2OH-→Mg(OH)2

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Prfesser (author)2011-06-01

This doesn't really answer your question----but If your goal is actually copper sulfate, some farm-supply stores sell it as root killer; flush down the drain to kill tree roots that are growing into the waste pipes. Don't ask for copper sulfate, they won't know what it is. Look on the labels of root killer.  If it's in a plastic jar, unscrew the lid and look at the contents.

Making copper sulfate by the route you describe may be possible but it's likely to cost more and be less pure in the long run.

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Jack A Lopez (author)Prfesser2011-06-01

I found this stuff at a bix-box hardwaremonger chain, somewhere in the former US. This product calls itself Enforcer(r) brand ROOT KILL.  Enforcer(r) is a Zep(r) brand. 

Here's the product page:

Also I took a picture of some of this stuff and part of  the label on the bottle.  Looking at the big version of this picture you can read the words:
Copper Sulfate Pentahydrate 99.0%

This bottle weighs about 2 lbs, or 1 kg, and I think I paid about 10 USD for it, at the time of this writing.

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unfortunately, i live in australia.
now dont get me wrong, i dont mean this as an insult, but in australia its pretty much impossible to get products made almost entirely out of chemicals that will destroy the environment, which the USA , (lucky) tends to ignore meaning consumers have access to such toxic chems.

i cant get it in such amounts, however, i can get magnesium sulfate for less by weight than that

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by the way, its not the mass of copper you have that's significant. you wont be using tons either way, its the surface area. a large piece of bare copper wire that's coiled will allow more surface area than just a huge rod. and will thus speed the process. :D

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shomas (author)TheMadScientist2017-05-22

since distance through an elecrolite adds resistance only the parts coil closest to the other electrode will participate in the reactions. Flattening and keeping equal distance might ensure a more even erosion of copper from the coil, so that you can use more of it up rather than have it disintegrate into one turn segments of coils

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im sorry i dont understand the relevance of this?

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shomas (author)oldmanbeefjerky2017-05-22

The rate of electrolysis is effected by surface area of the electrode.

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theegghead (author)2015-06-16

I did this a few weeks ago and it worked! I did it by making a cell divider for the anode and cathode out of a coffee filter and then electrolysing it. What happens is the copper sulfate will stay in solution whereas the Mg(OH)2 forms at the cathode, then the solution could be poured off free from the Mg(OH)2 which was caught in the coffee filter.

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theegghead (author)theegghead2015-06-16

once the CuSO4 solution is separated from the Mg(OH)2 it can be turned into sulfuric acid via electrolysis.

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