This involves the use of the equation;
pH = pKa + log (activity coefficent of anion) + log (D -Dha)/(Da- - D),
from the equation it is neccesary to plot pH against log(D - Dha)/(Da- -D).
This appears to be the straight line eqn which can be solved (y = mx + c) for the function pKa + log (activity coefficient of anion).
Is this approach correct or can you give an example of how to solve?