Complexation equilibria of cephalexin with HgII, ZnII and CdII

Abstract

The proton ligand stability constants (log K H 1 , log K H 2 ) of antibiotic drug cephalexin and stepwise stability constants (log K 1 log K 2 ) of its complexes with Hg I I , Zn I I and Cd I I have been determined potentiometrically in ethanol-water systems [20 : 80, 40 : 60 and 60 : 40 (%v/v] at 25, 35 and 45°C and 0.05, 0.10 and 0.20 M (KNO 3 ) ionic strength. The values of log K H 1 and log K H 2 as well as log K 1 and log K 2 decrease with the increasing ionic strengths while increase with increasing ethanol concentrations. The values of thermodynamic stability constants (log β 2 , μ = 0) have been found to be 11.02, 9.56 and 12.98 in case of Zn I I , Cd I I and Hg I I respectively at 25°C in 20: 80 (%v/v) ethanol-water mixtures. Based on this data, the stability order for various metal ions was found to be Hg I I > Zn I I > Cd I I . The values of ΔH and AS are both negative, thereby, indicating that the complexation is favored by enthalpy changes.