Electrochemistry Reverse Electroplating Chemistry

[goldkiller]

I'm trying to figure out why reverse electroplating with sulfuric acid as an electrolyte only attacks gold and not nickel. Here are the equations:

2Au -> 2Au³⁺ + 6e^- V=-1.51V
Pb + SO₄^2- -> PbSO₄ + 2e^- V=0.31V

Nett Voltage=-1.21V.

For Nickel,

Ni -> Ni²⁺ + 2e^- V=0.25
Pb + SO₄^2- -> PbSO₄ + 2e^- V=0.31V

Nett Voltage=0.56V

The Nickel reaction should be spontaneous by itself. Having additional voltage should only help right?

Any insight appreciated.

 

[Geo]

it does strip nickel. just not alot until the acid heats up, then it will attack almost all metals even stainless steel.

 

[goldsilverpro]

I'm not a graduate chemist, but I did take about 10 units of college chemistry and I had no problems with following your logic. However, a lot of the data given in chemistry handbooks is based on pure systems, at specific concentrations, temperature, etc. In practice, especially with mixed systems, at various temperatures and concentrations, the results are very often difficult to predict using this data. After doing this, almost daily, for 45 years, I have come to the conclusion that it is more important to know what will occur, with what chemicals, with what mix of materials, under what conditions. Much of this can only be derived from hands-on knowledge of working with the various materials and chemicals. The basic data is often only a guide, at best. Refining, in my experience, becomes more of an art than a science - tricks and tips. Everything that happens has a scientific basis, but trying to figure this out is often a big waste of time.

Practically, I have always seen sulfuric as being 2 different acids. Strong sulfuric has totally different characteristics than weak sulfuric. As only one example, hot concentrated sulfuric will dissolve a lot of silver, as metal. Weak sulfuric won't touch it.

 

[Harold_V]

Pretty much what Chris said. I have no chemical training, yet I mastered refining at a low level, and made a career of applying that which I had learned, all with great success.

Don't over-think a stripping cell. It works because high current in the proximity of the anode converts concentrated sulfuric acid to persulfuric acid. That dissolves the gold. As the gold migrates away from the anode (the part(s) being stripped), the electrolyte reverts back to sulfuric acid, which can not hold gold in solution, so it precipitates as colloidal gold---the black substance that darkens the electrolyte.

Harold

 

[Lino1406]

Nickel is passivated in sulphuric acid,
a little HCl is needed