Equilibrium models of Cr3+ and Cu2+ with glutamate

Speciation diagrams and stability constants for glutamate (Glu) with (Cr3+) and (Cu2+) in aqueous solutions are presented. The current study covers a larger pH-range affording accurate results, and reveal a different set of species for Cu2+ and species not previously reported for Cr3+. For the Cu2+ Glu system, the most successful model that refined the potentiometric data contains the simple one-to-one complex, the bis-complex and the mono-hydroxo complex. The overall stability constants for Cu2+-Glu complexes have respective values of log 110 = 7.6 0.2, log 11-1 = 1.3 0.7, log 120 = 13.6 0.2. Attempts to refine the stability constant for the mono-protonated metal complex (log 111) that was reported in the literature indicated that this mono-protonated species did not form to an appreciable amount to be important for the model presented here. For the Cr3+ Glu system, the overall stability constants for the complexes formed have the values of log 110 = 8.34 0.03, log 11-1 = 1.9 0.1 and log 11-2 = -4.6 0.1. These results for Cr3+ system covers wider pH-range and have more accuracy than those reported previously. The NMR experiments for Glu revealed downfield shifts of all protons as pH values decrease from 11.21 to 2.85.