Spectrophotometric determination of acidity constants by mean centering of ratio spectra

Abstract

In this work, an application of mean centering of ratio spectra is proposed to resolve the two-way pH-spectral data and acquire the acidity constants of diprotic acids. The method is based on the mean centering of ratio spectra in order to obtain a pH profile of the intermediate component. After recording the electronic absorbance spectra of the acids at different pH values, the contributions of H₂A and A²⁻ are eliminated from the absorbance data using a mean centering of ratio spectra method, which made feasible the determination of two successive acidity constants by computational fitting of the obtained pure pH profile of HA⁻¹. Given the acidity parameters (pK_as), the absorption spectrum of the intermediate can be reached through least squares regression. The applicability of the proposed method was evaluated by using model data. The pH-spectral data of Eriyochrome Black T (acid–base indicator) as a real system was also studied by the proposed method.