Reactions in Concentrated Solutions The Acidity Function

In solvents of high dielectric constant such as water, the deviations from ideality caused by ion-ion interactions are reasonably small below concentrations of 0.1 ilf for 1 1 electrolytes and can be treated adequately by means of the Debye-Htickel theory. For polyvalent electrolytes or for higher concentrations of 1 1 electrolytes, or for either in solvents of lower dielectric constant, the situation is less fortunate. The deviations from ideality can become rather large, and there is no adequate theory for either correlating them or predicting them. 

Numerous attempts, both theoretical and empirical, which have been made to treat this problem have met with varying success. The chief 

The base constant Kb is expressible in terms of the activities of the various species as 

If now the base, B°, or its conjugate acid, BH+, is an indicator, or has an appreciable optical absorption coefficient, the ratio (BTI+)/(B°) is experimentally observable. The magnitude of this ratio is a measure of the tendency of the solution to transfer a proton to the neutral base B°. On solving for this ratio and expressing the result in logarithmic form we have 

If we choose infinite dilution and molarity as our standard state, then in very dilute solution as the activity coefficients approach unity, this equation reduces to the simple form  

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