On the use of dimensionless parameters in acid-base theory. III. The molar buffer capacity of trivalent protolytes.

The general equation for the buffer capacity of trivalent weak acids is derived by way of the mean valence. It is valid for trivalent bases and ampholytes as well. Special interest is devoted to protolytes with symmetrically spaced pK values since both phosphoric acid and citric acid have this property. The symmetry implies a mathematical simplification which makes it possible to deduce the locations and magnitudes of the maxima and minima. Thus it is shown that the buffer capacity curve has three maxima and two minima for delta pK values bigger than 1.29.