摘要：Spectrophotometric methods are both sensitive and suitable for studying acidity constants in solutions. These methods imply the direct determination of the mole ratio of acid-base conjugate pairs through absorbance (A) measurements in a series of solutions of known pH. The evaluation of acid dissociation constants is very simple, if the species involved in the equilibria may be obtained in pure form (i.e. the limit absorbances A2 and A0 of H2R and R species respectively, are known), and do not overlap (i.e. A1, the limit absorbance of the specie HR is also known). The situation is more complex when the two ionizing groups of a substance lie within three pKa units of one another; the absorbance of the intermediate specie HR cannot then be determined experimentally and calculations being necessarily involved. Based on the expression of the absorbance as a function of the concentration for a diprotic acid, it is possible to calculate the pH values for which the absorbances coincide with the mean of the limit values of the absorbances corresponding to the different species: A*=(A2+A1)/2 and A**=(A1+A0)/2. Then the limit values of pH* and pH**, and the parameters α = pKa1-pH* and β = pH**- pKa2 are calculated, checking in turn under what conditions a diprotic acid can be treated as a monoprotic one from a spectrophotometric point of view. Nevertheless, in order to apply the above expressions A1 must be known, which can be made by the Polster method, i.e. by measuring the absorbances of varying pH solutions at two wavelengths λ1 and λ2, using orthogonal regression method Aλ1 versus Aλ2 (similar errors are assumed in both axis). In this work, a method of evaluation of acidity constants based on the rearrangement of the A versus pH expression is applied which implies the use of a straight-line (y=a0+a1x method) in order to separate the variables Ka2 (=1/a0) and Ka1 (=a0 /a1). The method presupposes the prior knowledge of A1, which may be previously obtained by the Polster method. The theory developed in this paper has been successfully applied to the experimental data reported in the literature for the resorcinol system.
关键词：Acidity constant; Spectrophotometric methods; Polster method; Resorcinol system
