Polarographic Study on Metal Ion Bi(Ⅲ) Complex System

The B（i Ⅲ）-Ligand（pyridine-2-carboxylic acid）system was studied by the differential pulse polarography（DPP）at the fixed total-ligand to total-metal concentration ratio［LT］:［MT］and varied pH. An appropriate model for the metal complex system is designed according to polarographic peak parameters，that is，the observed shift in the peak potential and the change in the peak current with pH changes in the sample solution. The polarographic experimental complex formation curve（ECFC）and theoretical complex formation curve（TCFC）were used for checking the designed metal complex model as well as optimizing stability constants. The ECFC，in which experimental parameters（a shift in a peak potential and a variation in a peak current）are included，appears to be a characteristic function for a particular metal-ligand system. The TCFC is a theoretical curve calculated from massbalance equations for the designed metal-ligand model. Using the computer analytical program，stability constants of metal complexes and species distribution diagrams are obtained by solving the mass-balance equations written for the metal complex system and fitting the complex formation curve. Five bismuth complexes MHL，ML，ML2，ML3，ML3 （ OH），and their stability constants as logβ 7.52±0.15，7.48±0.06，13.94±0.02，18.12±0.03 and 26.78±0.03，respectively，are finally reported while the TCFC fits best the ECFC.