The Formation Constants and Composition of Tetraethylenepentamineheptaacetato Nickel(II) Complexes measured by a Potentiometric Titration Method

Abstract Stability constants for nickel(II) complexes of tetraethylenepentamineheptaacetic acid (TPHA: H7L) were determined by the pH titration method. In an aqueous solution (μ=0.1) three uninuclear complex species, NiH2L, NiHL, and NiL, and a binuclear compound, Ni2L seem to exist. The structures of uni- and bi-nuclear chelates are discussed with their visible absorption spectra. Formation of the hydroxo complex was not observed on the titration curves. In the present investigation no evidence was found for species of Ni2HL either. The formation constants of the complexes stated above have been calculated as follows, (at 25±0.1°C). logKML=16.18, logKMHL=10.00, logKMH2L=6.16, logKM2L=17.35 and logKML2=27.78.