Electrolysis of MgSO4

[Halash]

i made a saturated solution of MgSO4 and i passed an elelctric current through it. on the negetive electrode, bubbles started forming rapidly. also, little flakes would fall off (it wasn't corroding tho). on the positive electrode, a yellow precipitate was falling off. i need an explanation. is this a good way to isolate sulfur? or could the sulfur be rreacting with something else?

 

 

please help

[olmpiad]

It could be the type of electrodes you are using.

[Halash]

i was using aluminum

[YT2095]

yellow ppt? there must have been some sort of contamination then, as the ppt should be White, it`s quite possible that what you Thought was Aluminium was actualy Duralumin (an alloy of Alu and Copper).

 

no, you`ll not get Sulpher this way either.

[woelen]

At the negative electrode you get the following primary reaction:

 

2H2O + 2e ---> H2 + 2OH(-)

 

This in turn causes the following secundary reaction:

 

2OH(-) + Mg(2+) ---> Mg(OH)2

 

The bubbles are H2 gas. The white stuff is the Mg(OH)2, which precipitates with the hydroxide ions, formed at the negative electrode. The negatve electrode itself is not corroded. As you see, this is not the way to get magnesium metal!

 

 

At the anode, the following reaction occurs:

 

Al --> Al(3+) + 3e

 

The electrode slowly corrodes away. This would give rise to colorless aluminium ions and not to a precipitate. Most likely there is some impurity in the aluminium, which causes another metal ion to go in solution. You also get a side reaction:

 

2SO4(2-) ---> S2O8(2-) + 2e

 

The persulfate ion is formed. It might be that this forms a yellowish precipitate with aluminium ions. It can be yellowish under certain conditions, but I do not know the precise nature of this yellow compound.

 

You definitely do not get sulphur at the anode. Remember, the anode is strongly oxidizing and in the sulpate ion the sulphur is oxidized already to its max. oxidation state.

 

If you electrolyse a solution of a sulfide instead of a sulfate (e.g. Na2S), then you probably will get a yellow precipitate of sulphur at the anode, but in your case this most likely is an impure aluminium-compound.

[Halash]

ok thanks.... i will use carbon electrodes

[woelen]

ok thanks.... i will use carbon electrodes

If you use that, then you'll get oxygen and acid at the anode, also some peroxodisulfate, S2O8(2-).

 

At the cathode, you still get hydrogen and the white precipitate of Mg(OH)2. With vigorous stirring, that precipitate may redissolve again, because of the acid, produced at the anode. If no peroxodisulfate is formed, then the acid at the anode precisely balances the hydroxide, formed at the cathode.

 

You have no need to replace your aluminium cathode, only the anode must be a carbon rod.