What happens at the inert anode in an electrolytic cell?

Question

Recently I did an experiment using graphite as the anode and iron as the cathode for an electrolytic cell. I tried plating different metals such as copper, nickel, and zinc with CuSO4, NiSO4, and ZnSO4 respectively. I know that normally when the anode is made from the same material as the metal being used for plating, the anode is decomposed. What I am wondering is what happens at the anode when it is made of an inert material like graphite? Is the SO4 ion oxidized there, what does it become?

During the experiment, I have also observed bubbles forming at the anode, and I hypothesized that it may be oxygen from the electrolysis of water, but I had no way to be sure.

Answer 1

There is oxidized oxygen in water

$$\ce{2 H2O -> 4 H+(aq) + O2(g) + 4 e-}.$$

$\ce{HSO4-}$ in sulphuric acid solution can get oxidized as well at high current density and therefore high enough anodic potential:

$$\ce{HSO4-(aq) -> S2O8^2-(aq) + 2 H+(aq) + 2 e- }$$

This is the industrial way to produce potassium peroxodisulphate.