Second order global analysis: the evaluation of series of spectrophotometric titrations for improved determination of equilibrium constants

In spectrophotometric titrations, linear or near-linear dependence of concentration profiles and the existence of minor species cause difficulties in the evaluation of the data. In the first case calculated absorption spectra and in the second case the corresponding equilibrium constants are not or only poorly defined. The result is the inability to reliably fit a reasonable model to the data. In second order global analysis, a number of spectrophotometric titrations with different initial concentrations are simultaneously analysed. In this way the concentration matrix is augmented to full rank and, secondly, conditions for the significant formation of each species can be obtained. Resulting equilibrium constants and absorption spectra are notably better defined. EQUISPEC is a computer program using the matrix based MATLAB environment for second order global analysis of spectrophotometric equilibrium data. It has been successfully tested with generated data and shown to overcome linear dependence and reproduce reported stability constants for the zinc: 1,10-phenanthroline system and to considerably improve the analysis of the complexation of Cu2+ by diethylenetriamine (dien).