Analytical grade reagents were used. Solutions of certain molarities of test metals were prepared by dilution of sulphate, chloride and nitrate salts of metals (CuSO4·5H2O, ZnCl2, NiCl2·6H2O, CdCl2·H2O and Pb(NO3)2 from Sigma Co.) with double distillated water. The pH buffer solutions were obtained from Aldrich Company.