Standard 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 crude copper that is subjected to electrorefining contains tellurium as an impurity. The standard reduction potential between tellurium and its lowest common oxidation state, Te +,is... 

Zinc is another common impurity in copper. Using standard reduction potentials, determine whether zinc will accumulate in the anode sludge or in the electrol) c solution during the electrorefining of copper. 

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. 

Electrorefining in aqueous solution is used for purifying metals and for electroplating, for example, for corrosion protection by zinc coating of steel. Electrorefining of metals is only possible for those metals with a positive standard potential (Ag, Au, Cu, Table 6.19.1) or if H2 formation at the cathode (which competes with deposition of the metal) is hindered by a high overpotential (Zn, Ni). For electrochemically very non-noble metals such as aluminum with a very negative standard potential, fused salt electrolysis is needed (Section 6.19.4.2). 

Electrorefining of metals in aqueous solution for purifying metals and for electroplating is only possible for those metals with a positive standard potential (Ag, Au, Cu) or if H2 formation at the cathode is hindered by a high overpotential (Zn, Ni). 

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