Determination of the ionization constants of compounds which precipitate during potentiometric titration using extrapolation techniques

It is shown that the ionization constants of diacidic compounds can be determined by utilizing potentiometric titration data, even when a precipitate forms during the titration. The two methods presented are particularly useful for compounds for which the pK alpha values are close together. A third method is presented which can be used with monoacidic compounds or compounds for which the pK alpha values are far apart and form a precipitate during the titration. Four symmetrical diacidic compounds were studied which had similar pK alpha values, and one compound was studied which had pK alpha values that were far apart. Comparison of the second pK alpha of the latter compound with that previously reported determined by a spectrophotometric procedure showed excellent correlation.