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Qualitative Aspects of Electrolysis

In the electrolysis process we have discussed above, we have taken molten salt as electrolyte, which contains only one cation and one anion. Now, if the electrolyte taken contains more than one cation and more than one anion (for example, aqueous solution of the ionic electrolyte), then the cation and anion that will get discharged depend on the ability of the cation to get reduced and the ability of anion to get oxidized.
The ability of an ion to get oxidized or reduced depends on the size, mass, positive charge, negative charge, etc. Thus, it is not possible to predict qualitatively that which ion would be discharged first, as one factor might enhance the ability to discharge while the other factor may hamper it. This can only be predicted on the basis of quantitative value assigned based on the cumulative effect of all the factors responsible for an ion’s ability to discharge. The value is referred to as standard potential, which is determined by keeping the concentration of ion as 1 M, pressure of gas at 1 atm, and the measurement done at 25°C. For a cation, the standard reduction potential (SRP) values are compared. The cation having higher standard reduction potential value is discharged in preference to cation with lower SRP value provided the ions are at 1 M concentration. For an anion, the standard oxidation potential (SOP) values are compared and anion having higher SOP is preferentially discharged, if the concentration is 1 M for each of the ion. The SRP values at 25°C for some of the reduction half reactions are given in Table.
Table SRP values at 25°C for some of the reduction half reactions
Reduction half cell reaction
E° in volts at 25°C
1. F2 + 2eh 2F
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When solution of an electrolyte contains more than one type of cations and anions at concentrations different from 1 M, the discharge of an ion does not depend solely on standard potentials but also depends on the concentration of ion in the solution. This value is referred to as potential, known as the reduction potential for cation and oxidation potential for anion. The relation between reduction potential and standard reduction potential is given by Nernst equation as follows:
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where ERP is the reduction potential of cation and Description: 30904.png is the standard reduction potential of cation. Thus, it is possible that a cation (A+) with lower standard reduction potential getting discharged in preference to cation (B+) having higher standard reduction potential because their concentrations might be such that the reduction potential of A+ is higher than that of B+.
When two metal ions in the solution have identical values of their reduction potentials, the simultaneous deposition of both the metals will occur in the form of an alloy.

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