Stability Constant and Thermodynamic Parameters Determination of A Semi-synthetic Penicillin Derivative with Various Bivalent Metal Ions (Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Sn2+, Hg2+ and Pb2+): A Potentiometric Study

This present work is a report on the stability constant values of a worthy antibiotic, Ampicillin Trihydrate; D[-]-α-aminobenzyl penicillin; ABP; a semi-synthetic derivative of penicillin with various biologically vital bivalent metal ions (Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Sn2+, Hg2+ and Pb2+) recorded in water-ethanol medium 50% V/V at three different temperatures (298K, 308K, 318K)maintaining ionic strength at 0.1M KNO3. The effect of temperature was also studied and the corresponding thermodynamic functions ∆Gº, ∆Hº and ∆Sº were calculated and discussed. It is significant to study the stability constants of drugs with these metal ions as they may already be present in human body. The proton-ligand and metal-ligand stability constants were determined by Calvin-Bjerrum pH titration techniques as modified by Irving and Rossitti. This analysis was carried out in order to determine the pk1H and log k1, values at 298 K, 308 K and 318 K. The metal to ligand mole ratio was maintained at 1:5 in order to fulfil the maximum coordination number of metal ion. The titrations were done in a titration cell with three openings, kept immersed in a thermostated bath.

The formation curves were plotted by taking value of (the degree of formation of metal-ligand complexes) against pH of the system. These plots indicate that these metal ions form 1:1 complexes with ABP. The results show that the pK1H values increases with an increase in temperature. The order of metal-ligand stability constants (log k1) has been found to be Cu2+ > Zn2+ > Cd2+ > Pb2+ > Sn2+ ≈ Hg2+ > Co2+ > Ni2+.