Resolution of carboxylate protonation microequilibria of NTA, EDTA and related complexones

Potentiometric and NMR–pH titrations were carried out on four classical complexones to elucidate their overall and site-specific basicities. NMR–pH profiles and Bjerrumʹs functions were adjusted to compound-specific symmetries and appropriate evaluation methods were developed. Symmetry-modulated relationships between the macro- and microconstants were deduced and self-consistent sets of microconstants were determined. The inherent basicity of carboxylates surrounded by adjacent, intramolecular ammonium and carboxylate sites have been found to be in the range of 1.83–2.02 log k units, which are reduced by 0.05–0.12 log k units upon a nearby carboxylate protonation.