Special Topic 2 Electrochemical Potentials

The introduction of ionic species into the framework of a thermodynamic analysis of chemical phenomena in soil solutions is principally a convenient 

The distribution of a charged species between two bulk phases, regardless of their composition, is determined by the electrochemical potential38 of the species, jx. The gradient of the electrochemical potential of a species drives its diffusive transfer between phases, and equilibrium with respect to this transfer is described by the equality  

The larger vertical line refers to the interface between a silver-silver chloride electrode and either a soil suspension (left) or a soil solution (right). The heavy, dashed vertical line marked W refers to a membrane that is impermeable to the soil colloidal anion, X 1, but permeable to dissolved ions and water.39 There may be ions other than Na+ and Cl in the suspension or in the solution, but they arc assumed not to interfere with the reversible behavior of the silver-silver chloride electrode toward Cl. At the left electrode (L), by convention,39 the oxidation reaction 

According to thermodynamic conventions for the assignment of emf values to electrode assemblies, summarized in Table s2.1, the emf across the silver-silver chloride electrode pair is given by the following equation  

The cell reaction is written as if oxidation occurs spontaneously at the left electrode and reduction at the right electrode. 

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These concepts are developed in greater detail in Chap. 7 of W. Stumm and J. J. Morgan, Aquatic Chemistry, Wiley, New York, 1981. As with any single-ion activity, it is always possible to relate the electron activity to a chemical potential formally (see, e.g., Eq. s2.22) and thus define an equivalent electrochemical emf (see Eq. s2.23). Accordingly, the (electro)chemical potential of an electron in aqueous solution is related to the pE value by the equation (see Special Topic 2) ... 

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