Complexation equilibria of some sulphoazoxines. Part VI. Protonation constants of 7-(4-carboxyphenylazo)-8-hydroxyquinoline-5-sulphonic acid by computer-assisted multi-component spectra analysis

Abstract

The protonation constants and molar absorptivities for the formation of the monomers L^3–, LH^2–, LH₂^– and LH₃, and the dimers L₂H₂^4–, L₂H₃^3– and L₂H₅^– of the metallochromic indicator 7-(4-carboxyphenylazo)-8-hydroxyquinoline-5-sulphonic acid (4-CAPAZOXS) have been determined in 0.1 M NaClO₄ using the SPOPT non-linear regression program for absorbance-pH curves at a constant wavelength and by EY608, PSEQUAD(83) and SQUAD(84) analysis of absorbance-pH spectra. The reliability of absorbance-pH curve analysis for overlapping protonation equilibria and the existence of ill-conditioned parameters in the model were examined using the diagnostic SPOPT program. The efficiency of the search for a chemical model of oligomer equilibria by multi-component spectra analysis was proved by the three programs. A reaction scheme for the monomer-dimer equilibria during protonation is suggested.