Mathematical consideration of the course of titration of a weak acid with a strong base shows that buffer index versus pH curve, π = f(pH), should contain two minima and one maximum under certain conditions, which coincides with the three inflection points of the pH versus fraction titrated, pH = f(T), curve. Ionic strength is assumed to be constant in this treatment, and the effect of dilution is neglected. The theory developed is applied to the acetic acid/sodium acetate and boric acid/sodium borate systems. The central maximum in the π = f(pH) does not coincides with the pK a because of the self ionization equilibria of water. The end point inflection (when occurs) at which the slope of the pH = f(T) curve is greater (minimum buffer capacity) precedes the equivalence point.