Let's move on to the purification/refining (impure metal made anode). We have to consider the following when purifying copper by electrolysis. Resources are from:
www.iupac.org/didac/Didac%20Eng/Didac03/Content/R20.htm
- Ag, Au and Pt, all precious metals, have a lower reduction potential than Cu. They are not oxidised to ions at the anode. As the copper ions are formed, the anode crumbles away allowing the precious metal impurities to fall as sludge to the bottom of the cell.
- Sn, Bi and Sb have a larger reduction potential than Cu. They are therefore oxidised at the anode but their ions react with the electrolyte to form insoluble oxides and hydroxides. These are also deposited in the sludge.
- Pb is also oxidised, but forms insoluble PbSO4, which again sinks into the sludge.
- Fe, Ni, Co and Sn are oxidised at the anode, but remain as ions in the electrolyte. This is a consequence of the careful choice of electrode potential, which ensures that only the copper ions are reduced at the cathode, the other metal ions needing a higher electrode potential to be reduced.
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