A Simple Method for the Consecutive Determination of Protonation Constants through Evaluation of Formation Curves

Abstract

A simple method is presented for the consecutive determination of protonation constants of polyprotic acids based on their formation curves. The procedure is based on generally known equations that describe dissociation equilibria. It has been demonstrated through simulation that the values obtained through the proposed method are sufficiently consistent with the actual values. In contrast with the universally known and applied Bjerrum's method, no differences in the accuracy of determination of subsequent protonation constant values are observed. The proposed method requires the value of one of the protonation constants (e.g., of the first one, K1) of the polyprotic acid. An iterative method is proposed for finding the value, starting from approximate values determined from the environment of point n = 0.5(n-average degree of protonation). Apart from high accuracy, the proposed method also has didactic advantages because it illustrates the problem of multiple acid equilibria in a visual (almost graphic) manner. The proposed method may also provide initial constant values for numerical methods of nonlinear approximation of the formation curve.