Specific ion interaction theory

Specific ion Interaction Theory (SIT theory) is a theory used to estimate single-ion activity coefficients in electrolyte solutions at relatively high concentrations. It does so by taking into consideration interaction coefficients between the various ions present in solution. Interaction coefficients are determined from equilibrium constant values obtained with solutions at various ionic strengths. The determination of SIT interaction coefficients also yields the value of the equilibrium constant at infinite dilution.

The need for this theory arises from the need to derive activity coefficients of solutes when their concentrations are too high to be predicted accurately by Debye-Hückel theory. These activity coefficients are needed because an equilibrium constant is defined in thermodynamics as a quotient of activities but is usually measured using concentrations. The protonation of a monobasic acid will be used to simplify the exposition. The equilibrium for protonation of the conjugate base, A⁻ of the acid, may be written as

for which

where {HA} signifies an activity of the chemical species HA etc.. The role of water in the equilibrium has been ignored as in all but the most concentrated solutions the activity of water is a constant. Note that K is defined here as an association constant, the reciprocal of an acid dissociation constant.

Each activity term can be expressed as the product of a concentration and an activity coefficient. For example,

where the square brackets signify a concentration and γ is an activity coefficient. Thus the equilibrium constant can be expressed as a product of a concentration quotient and an activity coefficient quotient.

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