Equilibrium models of Cr³⁺ and Cu²⁺ with glutamate

Abstract

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 β₁₁₀ = 7.6 ± 0.2, log β_11-1 = 1.3 ± 0.7, log β₁₂₀ = 13.6 ± 0.2. Attempts to refine the stability constant for the mono-protonated metal complex (log β₁₁₁) 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 β₁₁₀ = 8.34 ± 0.03, log β_11-1 = 1.9 ± 0.1 and log β_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.

Keywords:

• Glutamate
• Chromium
• Copper
• Equilibrium constants
• Hydrolytic species
• Speciation diagram
• UV–Vis

Acknowledgements

This work was supported from NSF under Grant No. HRD-0411493. The NMR material was supported from NSF under Grant No. 0443627 to the University of Memphis. The work was also supported in part by program SEED from the American Chemical Society.