Multiparametric curve fitting-XIII Reliability of formation constants determined by analysis of potentiometric...

The formation (protonation) constants log K(i), of the acid H(j)L are determined by regression analysis of potentiometric titration data when common parameters (log K(i), i = 1,..., j) and group parameters (E(0)', L(0), H(T)) are refined. The influence of three kinds of error on the protonation constants has been investigated: error from the strategy of minimization, random error, and error from uncertain estimates of group parameters. An analysis of variance of the log K(i), matrix was made for 7 identical titrations and 8 computational strategies, or of 7 identical titrations and 8 different options of group parameters to be refined. The influence of the standard potential E(0) of the glass-electrode cell on the systematic error in log K is greater than that of the acid concentration (L(0)) or the concentration of titrant used (H(T)). The ill-conditioned group parameters should be refined together with the common parameters (K(i)), otherwise the estimates of log K(i), are not accurate enough. Two ways of calibrating the glass electrode cell were compared. Internal calibration (performed during titration) was more accurate than external calibration done separately. Of the programs tested ESAB and ACBA are the most powerful because they permit refinement of group parameters and internal calibration. Citric acid was chosen as model substance.