Ion speciation

[1] The pH of a solution of a monoprotic weak acid can be expressed in terms of the extent of dissociation.

After rearranging the expression defining the acid dissociation constant, and putting pH = −log10[H+], one obtains This is a form of the Henderson-Hasselbalch equation.

Outside the transition range the concentration of acid or conjugate base is less than 10 % and the colour of the major species dominates.

The pH regions in which the species exist overlap extensively since the difference between successive pKa values is small.

A large number of computer programs for the calculation of equilibrium species concentrations have been published.

The ratio of acid, AH and conjugate base, A − , concentrations varies as the difference between the pH and the p K a varies, in accordance with the Henderson-Hasselbalch equation .
Species concentrations calculated with the program HySS for a 10 mM solution of citric acid . p K a1 = 3.13, p K a2 = 4.76, p K a3 = 6.40.