Studies on the composition and kinetics of chromium(III)-alanine system

Kinetics of the complex formation of chromium(III) with alanine in aqueous medium has been studied at 45, 50, and 55°C, pH 3.3–4.4, and μ = 1 M (KNO₃). Under pseudo first-order conditions the observed rate constant (k_obs) was found to follow the rate equation:

Values of the rate parameters (k_an, k, K_IP, and K) were calculated. Activation parameters for anation rate constants, ΔH^≠(k_an) = 25 ± 1 kJ mol⁻¹, ΔH^≠(k) = 91 ± 3 kJ mol⁻¹, and ΔS^≠(k_an) = −244 ± 3 JK⁻¹ mol⁻¹, ΔS^≠(k) = −30 ± 10 JK⁻¹ mol⁻¹ are indicative of an (I_a) mechanism for k_an and (I_d) mechanism for k routes (‥substrate Cr(H₂O) is involved in the k route whereas Cr(H₂O)₅OH²⁺ is involved in k′ route). Thermodynamic parameters for ion-pair formation constants are found to be ΔH°(K_IP) = 12 ± 1 kJ mol⁻¹, ΔH°(K) = −13 ± 3 kJ mol⁻¹ and ΔS°(K_IP) = 47 ± 2 JK⁻¹ mol⁻¹, and ΔS°(K) = 20 ± 9 JK⁻¹ mol⁻¹.