Equilibrium electrorefining

Table 2 lists standard electrode potentials for some metals and some other reactions common in electrorefining and electrowinning. The metals with high equilibrium potential are noble metals. They are often difficult to dissolve but deposit easily. The metals with low equilibrium potentials are active metals that dissolve easily but are more difficult to reduce. 

The reactions (20) to (22) form the copper equilibrium on the electrode surfaces. Concentration of Cu(I) on the cathode surface affects the deposition rate. The maximum net rate of Cu+ production is at about —50 mV versus Cu/CuSC>4 and at higher overpotentials it decreases. Disturbing the Cu(II)—Cu(I)—Cu equilibrium can cause the formation of copper powder, but this is more a problem on the anode. For the current densities commonly used in electrorefining, the cathode overpotential is between 50 and 100 mV. The system is mainly charge transfer controlled and the effect of mass-transfer polarization is small. If Cu(I) concentration on the cathode surface decreases, mass-transfer polarization will increase, causing more uneven deposit. 

For electrorefining the standard potential of lead at each electrode will be the same and the equilibrium voltage (at zero current) between the two lead electrodes is therefore zero. In practice it is desired to transfer lead from the anode to the cathode and a current will flow, giving a potential drop across the solution path between the two electrodes as well as overpotentials at each electrode. This will define the starting voltage for a refining cell. 

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