Case in Which One of the Species Redox is Polynuclear

If the medium is buffered, the sum of the first two terms on the right-hand side of the previous equation is constant. It is equal to a quantity called the apparent standard (or normal) potential E° of the couple Mn04 /Mn + at the given pH. Therefore, Nernst s equation may be written  

As a result, a system of equations absolutely analogous to those prevailing in the preceding case may be set up, provided that the standard potential is replaced with the apparent one. The potential of the solution is calculated through the equation 

This methodology can be immediately transposed to analogous cases in which there are exchanges of protons and electrons at the same time. We must emphasize the fact that the equilibrium potential of the solution depends on the pH value, since the exponential 62 depends on E° 2, which itself depends on the pH value. This is not the case when we only have an exchange of electrons. 

If the redox reaction is accompanied by an exchange of ligand, the solution equilibrium potential may be calculated in the same way as earlier. An apparent standard potential is defined for the couple whose members exchange a ligand together with the electrons. It is constant if the concentration in the ligand is held constant. The equilibrium potential depends on this concentration. 

Actually, reaction (16.1) is not absolutely general. It is not rare among the redox couples used in chemical analysis for some of them to have a Red or Ox member that possesses several atoms of the element undergoing a change in its oxidation state. This is the case, for example, for the following half-equilibria  

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