Stability constants absorbance data

Leggett, D. J., SQUAD Stability quotients from absorbance data, in Leggett, D. J. (ed.), Computational Methods for the Determination of Formation Constants, Plenum Press, New York, 1985, pp. 158-217. 

The procedure used in this example begins with the preparation of a series of solutions, all containing the same concentration of metal ion and the same concentration of ligand, but having different low pH values. The absorbance of each solution is measured a plot of absorbance versus pH provides a curve that shows the extent of formation of the complex as a function of pH, similar to Figure 22-16. From this data, the stability constant can be calculated in the following manner. 

It is possible to analyse these titration curves quantitatively. The number of absorbing species in solution may be determined from the raw data by factor analysis, with evolving factor analysis often giving a good indication of the stoichiometry of the species [16]. If we have a model for the complexes formed, together with an estimate for the stability constant of each species, then we can calculate a speciation of the particles at any point in the titration from the known total concentrations of metal and ligand. This speciation may be used to obtain estimates of the molar extinction coefficients of the particles (if they are unknown), and calculate the absorbance [17] a non-linear least-squares refinement of the stability constants is then carried out to give the best fit of calculated and observed absorbance. 

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