Determination of stability constants and thermodynamic parameters for interaction of thiosemicarbazones with divalent metal ions.

The Calvin-Bjerrum pH titration technique as modified by Irving and Rossotti has been applied to determine the stability constants of complexes of a series of thioligands, namely benzaldehyde thiosemicarbazone, pyridine-2-carbaldehyde thiosemicarbazone, and cyclopentanone thiosemicarbazone with divalent metal ions (Cu, Cd, Co, Ni, Pb, and Zn). The studies were carried out in 50% v/v dioxane-water at different temperatures (293 K, 303 K, and 313 K) keeping ionic strength at 0.1 mol KNO3. The thioligands undergo deprotonation of only one proton (-N2H-) of a thiosemicarbazone [R1R2C2=N3-N2(H)-C1(=S)N1H2]. Thermodynamic parameters, such as free energy, enthalpy, and entropy have also been evaluated. The investigations represent the first report of the solution phase studies in metal-thiosemicarbazone chemistry. [ABSTRACT FROM AUTHOR]