Thermodynamics of substituted rhodanine III: Potentiometric and spectrophotometric studies of complexes of some transition metals with 3-phenylsulfonamidorhodanine

The dissociation constants of 3-phenylsulfonamidorhodanine (PSR) were deter mined potentiometrically in 0.1 M KCI and 20% (v/v) ethanol-water. The step wise stability constants of the complexes of Mn2+, Co2+, Ni2+, Cu2+, Zn2+, La3+, Ce3+, Gd3+, UO22+ and Th4+ with PSR were determined. The stabilities of the complexes were found as follows: Th4+ > UO22+ > Gd3+ > Ce3+ > La3+ > Mn2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The thermodynamic parameters for PSR and its complexes were evaluated and discussed. The dissociation process is no n-spontaneous, endothermic, and entropically unfavourable. The formation of the complexes was found to be spontaneous, exothermic or endothermic (depe nding on the metal), and entropically favourable. The stoichiometries of th e complexes were determined spectrophotometrically and indicate the formati on of 1:1 and 1:2 (metal:ligand) complexes.