Thermodynamic and spectroscopic study of Al³⁺ interaction with glycine, l-cysteine and tranexamic acid in aqueous solution

In this paper a thermodynamic and spectroscopic study on the interaction between Al³⁺ and glycine (Gly), l-cysteine (Cys), tranexamic acid (Tranex) is reported. Speciation models have been obtained by processing potentiometric titration data to determine stability constants of the species formed in aqueous solution at T=298.15K, 0.15≤I/molL^-1≤1 in NaCl. Thermodynamic formation parameters have been obtained from calorimetric titration data, at T=298.15K, I=0.15molL^-1 using NaCl as ionic medium. Al³⁺-Cys system was also investigated by spectrophotometric and ¹H NMR measurements. ¹H NMR experiments were performed on Al³⁺-Tranex system as well. Different speciation models have been observed for the three systems. The results showed the formation of MLH, ML and M₂L₂(OH)₂ species for Gly, ML, M₂L and MLOH for Cys, MLH and MLOH for Tranex. The formed species are quite stable, i.e. for ML, logβ=7.18, 11.91 for Gly and Cys, respectively, at I=0.15molL^-1 and T=298.15K. For all the systems the dependence of formation constants on ionic strength over the range 0.1-1molL^-1 is reported. The sequestering ability of the ligands under study was also evaluated by pL_0.5 empiric parameter. For Gly, Cys and Tranex, pL_0.5=2.51, 3.74, 3.91 respectively, at pH=5, I=0.15molL^-1 and T=298.15K.