Activity Coefficients of Individual Ions from Titration Data

It is shown that the activity coefficients of the anions resulting from the dissociation of a weak polyprotic acid can be obtained from titration data. Activity coefficients are determined in terms of the activity coefficient of the undissociated acid. The extracted activity coefficients are also dependent on the pH scale adopted in the titration process and are considered as operational coefficients. The computational procedure uses an exact equation representing the titration curve and a nonlinear least-squares fitting procedure. As an illustration, the activity coefficients of the anions of succinic acid are determined. The method also allows study of the variation of the activity coefficients with the ionic strength in a straightforward manner.