Salt Effects on Equilibrium Constants

The equilibrium-constant expression Equation (2-7), written in terms of activities of the reactants, shows no salt effect. In practice, however, an equilibrium constant usually is determined experimentally in terms of concentrations. Thus we can write 

We distinguish here between the simple K, written in terms of activities (sometimes called the thermodynamic equilibrium constant), and K, written in terms of concentrations. Since for ionic solutes the y s in the equation approach unity at infinite dilution, the usual practice is to extrapolate the experimental values of K at various ionic strengths to obtain K, which is the limiting value at infinite dilution. 

An estimate of the magnitude of the salt effect can be obtained from the Debye-Hiickel theory. Defining 

EXAMPLE 2-2 Estimate the effect of ionic strength on the two successive ionization constants of a dibasic acid composed of neutral molecules HjA. 

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