Potentials near an Electrified Interface

The development of surface charge at the interface between soil particles and the soil solution is a reflection of inhomogeneities in the molecular environment of the interface, as discussed in Sec. 3.1. These molecular inhomogeneities also influence the thermodynamic properties of both the charged species in the soil particles and those in the soil solution. In particular, the distribution of a charged species between the two bulk phases, regardless of their composition, is determined by the electrochemical potential of that 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 measurement of the electrochemical potential can be illustrated by considering the following soil clay suspension-soil solution system into which two identical electrodes are immersed  

The single vertical line refers to the interface between a silver-silver chloride electrode and either a soil suspension or a soil solution. The double vertical line marked W refers to a membrane that is impermeable to the soil colloidal anion, but permeable to dissolved ions and water. There may be ions other than Na and Cr in the suspension or in the solution, but they hre assumed not to interfere with the behavior of the 

According to the thermodynamic conventions for the assignment of emf values to electrode assemblies, summarized in Table 3,3, the emf across the silver-silver chloride electrode pair is given by the equation 

Equation 3.19a illustrates the general rule that differences in electrochemical potential for a charged species can be measured by determining the 

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