Speciation diagrams and stability constants for glutamate (Glu) with (Cr 3þ ) and (Cu 2þ ) in aqueous solutions are presented. The current study covers a larger pH-range affording accurate results, and reveal a different set of species for Cu 2þ and species not previously reported for Cr 3þ . For the Cu 2þ : 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 Cu 2þ -Glu complexes have respective values of log 110 ¼ 7.6 AE 0.2, log 11-1 ¼ 1.3 AE 0.7, log 120 ¼ 13.6 AE 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 Cr 3þ : Glu system, the overall stability constants for the complexes formed have the values of log 110 ¼ 8.34 AE 0.03, log 11-1 ¼ 1.9 AE 0.1 and log 11-2 ¼ À4.6 AE 0.1. These results for Cr 3þ 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.

Formation of the metal complexes of aspartic acid (Asp) with the chromium metal ion (Cr 3+ ) in solutions using potentiometric titrations is presented within a wide pH range (∼3.5 to ∼10.5) at 25 • C and I = 0.10 M NaNO 3 . Concentration distribution diagrams revealed that the main complex formed within this pH range is the bis Cr 3+ complex. Literature stability constant values for the Cr-Asp complexes were used to construct concentration distribution diagrams. Complexes taken into consideration were the simple one-to-one complex, the bis-complex, and the bis-mono-protonated complex, namely, Cr-Asp, Cr(Asp) 2 , and Cr(Asp) 2 H. The corresponding Log β values of these complexes were 12.46, 21.86, and 24.30, respectively. UV-Vis spectra demonstrate Cr 3+ -Asp binding. The UV-Vis spectra were collected from a system that reached a high level of equilibrium state (50 days' equilibrium time).