Polarographic studies of metal ion complexes of Ampicillin and Amoxycillin

A study of the direct current and differential pulse polarographic behaviour of metal complexes, especially nickel(II), of Ampicillin and Amoxycillin at the dropping mercury electrode, is described. In 1 M sodium acetate at pH 8.0, the nickel(II) complex is superior to those of cobalt(II), copper(II), and zinc(II) in providing differential pulse peaks which at -0.865 V and -0.840 V for Ampicillin and Amoxycillin respectively with respect to the Ag/AgCl, sat. KCl reference electrode at 25°C, are well separated from those of excess metal ion. Suitable working conditions of pH, ionic strength in sodium acetate supporting electrolyte, temperature and time of measurement after mixing of reactants were established for the nickel(II) systems. As the temperature of reaction and measurement is increased, the differential pulse peak near -0.850 V decreases roughly exponentially with time at ≥25°C. Tests showed that single electron reversible reduction occurs in the nickel(II) complexes. For quantitative determination by the differential pulse technique, linear calibration graphs were obtained over concentration ranges 3.0 x 10^-7 M to 3.0 x 10^-5 M and 4.0 x 10^-7 M to 2.0 x 10^-5 M for Ampicillin and Amoxycillin, respectively, with coefficients of variation from 2 to 3% for 6 determinations in the middle of the calibration ranges. Good agreement was found between results obtained by the differential pulse method and three methods from the British or US Pharmacopoeias.