Spectrophotometric analysis of simultaneous protolysis equilibria

Simultaneous protolysis equilibria {A figure is presented} can be spectrophotometrically analysed easily by means of absorbance diagrams. The dissociation constants K₁ and K₂ can be determined directly from distinct points on the absorbance diagram by correlation with a photometric titration plot of absorbance vs. pH. Furthermore, from the absorbance diagram the ratio K₂ K₁ can be evaluated from the ratio of corresponding sides, and pH-measurement is not necessary. The titration systems of a mixture of 2-nitroso-l-naphthol-4-sulphonic acid with P-nitrophenol, and a mixture of P-nitrophenol with o-nitrophenol have been analysed by these methods, and are described as examples.