Potentiometric studies on the complexation equilibria between some trivalent lanthanide metal ions and biologically active 2-hydroxy-1-naphthaldehyde thiosemicarbazone (HNATS)

The chelation behaviour of some trivalent lanthanide and yttrium metal ions with biologically active 2-hydroxy-1-naphthaldehyde thiosemicarbazone (HNATS) has been investigated by potentiometric measurements at 20±0.5 deg C in 75% (v/v)dioxane-water medium at various ionic strenghts of sodium perchlorate. The method of Bjerrum and Calvin, as modified by Irving and Rossotti has been used to find out the values of n-bar (average number of ligand bound per metal ion) and pL (free ligand exponent). The formation constants of metal chelates have been computed on a PC-XT computer, using a program patterned after that of Sullivan et al. to give βn values using the weighted least-squares method. The Smin values (Smin=χ2) have been calculated. The order of formation constants of chelates was found to be: La3+<Ce3+<Pr3+<Nd3+<Sm3+<Eu3+<Gd3+<Y3+<Tb3+<Dy3+. The formation constants of the chelates formed have been correlated to size and ionization potentials of the metal ions. (authors)