摘要：The aim of this work is to find out in which experimental conditions a diprotic acid H2R may be treated as a monoprotic one, HR, in order to determining their acidity constants spectrophotometrically, as the relationship between the absorbance (A) and the concentration for a monoprotic acid HR is susceptible of linear transformation. It can be argued that a diprotic acid with overlapping acidity constants may be treated as a monoprotic acid provided that some approximation is made. The simplifying assumption is that the concentration (CR) at enough low and high pH values equals to [H2R] + [HR] and [HR] + [R], respectively. A bilogarithmic method for the spectrophotometric evaluation of acidity constants of two-step overlapping equilibria may be applied on this basis. The singular straight line (bilogarithmic) complete (diprotic acid model) or single (monoprotic acid model) version requires for its application the knowledge of the limit absorbance of the intermediate specie HR (A1). A trial and error method may be applied, i.e. the best value of A1 is taken as that which minimizes the standard deviation of the corresponding regression line. An additional bilogarithmic method is proposed in this work when spectrophotometric measurements are made at the wavelength of an isosbestic point (i.e. a wavelength at which the limit absorbance of two species are equal). The method has been checked with synthetic theoretical data and model systems, and has been applied to the experimental data obtained in the literature for benzidine, vanillyl ammonium ion and isoniconitic acid, and to the A-pH experimental data for isoniconitic acid obtained in this paper.
关键词：Acidity constant; Spectrophotometric measurements; Bilogarithmic Method; Isosbestic point
