Determination of carbonate in the presence of hydroxide : Part I. Analysis of first-derivative potentiometric curves

Mathematical consideration of the course of neutralization of a solution containing a moles of a strong base (e.g. sodium hydroxide) and a moles of a salt of a weak dibasic acid (e.g. sodium carbonate) with a strong acid (e.g. hydrochloric acid) shows that the first derivative of the titration curve should contain three maxima. The first, corresponding to neutralization of most of the hydroxide, is small (with height proportional to a^{$\text{1}\text{2}$}), displaced slightly from the equivalence point, and disappears for a less than 0.01 mol l^-1. The second corresponds to the conversion of most of the CO₃^2- to HCO₃^- and its height is almost independent of a. The third corresponds to the conversion of HCO₃^- to H₂CO₃, with height proportional to a^{$\text{1}\text{2}$}. The two minima are independent of a and of the dissociation constants of the weak acid. These conclusions were examined experimentally and were extended to the titration of hydroxide contaminated with a small amount of carbonate.