Types of Half-Cell Reactions

The wide variety of electrode systems or halfcells of analytical importance can be classified in terms of the nature of the components of the redox couple. Of particular importance is the metal that is used as an electron conductor. 

If the redox couple involves a metal, then the halfcell consists of a metal immersed in a solution containing the metal ions, e.g., 

The form of Equation 7-8b that applies in these cases is pEzn = PE + (l/2)log[Z 2 ] 

Metal electrodes corresponding to these halfcells are referred to as electrodes of the first kind. The activity of the metal, Zn or Ag, is unity, as will be that of every solid component (as well as H2O) of a half-cell reaction. 

A useful variant of this type of half-cell is obtained by coating the metal with a slightly soluble salt or oxide. 

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It is obvious from the Nemst Equation 7-5 that the value of the electrode potential of a redox couple will depend on the values of the activities of the reaction components. This means of course that changes in the concentrations of any of the components will likewise affect the value of the electrode potential. The precise way in which the concentration affects the potential is related to the type of half-cell reaction involved, i.e., in the half-cell reactions... 

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