Thermodynamic Proton-Ligand and Metal-Ligand Stability Constants of Some Drugs

The thermodynamic proton-ligand (pKa) and metal ligand stability constants of clioquinol, clofibrate, nitrofurazone, and tetracycline with Cu2+, Zn2+, Mn2+, Mg2+, and Ca2+ have been determined at 35°C in 50% ethanol-water media. An empirical pH correction for mixed-aqueous media has been applied. The metal-ligand stability constants were determined by following the Bjerrum Calvin titration technique as applied by Agrawal to mixed-aqueous solvents. The effect of the basicity of the ligand and the order of stability constants is discussed. The stability constants of the divalent metals follow the order: Cu2+>Zn2+>Mn2+>Mg2+>Ca2+ with all the drugs.