Thermodynamic study on the interaction of ampicillin and amoxicillin with Ca2+ in aqueous solution at different ionic strengths and temperatures

A potentiometric and UV spectrophotometric study on Ca2+−ampicillin and −amoxicillin systems in NaCl aqueous solution is reported. The potentiometric investigation was carried out at different ionic strengths, I = (0.15 to 1) mol kg−1, and temperatures, T = (288.15 to 310.15) K. The most reliable speciation models include two species, namely, CaLH and CaL for the Ca2+−ampicillin system and CaLH2 and CaLH for the Ca2+− amoxicillin system. The spectrophotometric results obtained for both of the systems are fully consistent with potentiometric ones, confirming the formation as well as the stability of the complex species. The dependence of formation constants of the species on ionic strength over the range I = (0.15 to 1) mol kg−1 in NaCl and on temperature over the range T = (288.15 to 310.15) K is also reported. pL0.5 empirical parameter (i.e., the concentration of ligand required to bind 50% of the metal cation present in the trace) was employed to evaluate the ability of both of the ligands to sequester Ca2+ under different conditions of pH, ionic strength, and temperature. For example, pL0.5 = 1.82 and 2.88, for ampicillin and amoxicillin, respectively, were obtained under physiological conditions.