chem geek wrote:Copper staining usually occurs when the copper ion level is above 0.3 ppm though this is dependent on pH. 0.3 ppm copper is 4.7x10-6 moles/liter concentration. So at this concentration, the half-reaction has an oxidation potential of -0.18V.
I'm not sure that I see why the copper ion concentration is relevant. The ions are not being oxidized; the solid copper is being oxidized. The solid copper is exposed to the water, which contains the oxidizers, but the copper is not in solution.
You're talking about oxidation and about staining, which are two different things. First, the solid copper is oxidized to Cu(+2) ions, and then it combines with anions, such as oxides or hydroxides to form stains. Copper stains are not caused by oxidation; the copper is already oxidized.
The copper can exist in the water as ions until the water becomes saturated with a particular copper product, such as copper (carbonate, borate, cyanurate, phosphate, oxide, hydroxide etc.)
Iron can be further oxidized from iron II to iron III. However, I don't think that copper undergoes further oxidation once it is oxidized from solid copper to copper (+2) ions.