A Kinetic Study of Isotopic Exchange between Terbium Ion and its EDTA Complex in Aqueous Buffer Systems and the Possible Role of Buffer Anion

Abstract

The nature of the species undergoing exchange and the role of the buffer anion in the medium are not known precisely from previous investigations on the isotopic exchange of rare earth metal ions with their EDTA complexes confined to acetate medium only.Using terbium-160 as radiotracer, the kinetics of self-exchange of Tb(III) in TbEDTA− has been studied in detail in 0.025 M acetate buffer. The exchange has also been investigated in various other concentrations of acetate and phthalate buffer solutions, and also in benzoate buffer.In 0.025 M ammonium acetate solution the rate of exchange (R in mol l−1 s−1) is expressed by[Formula: see text]over the concentration ranges employed. The predominant mode of exchange is the decomposition of the protonated complex TbHEDTA.It is concluded that the complex TbEDTA− is the only species undergoing exchange, although it is possible that mixed complexes formed between TbEDTA− and acetate ion may be present in the medium. Increasing charge on the buffer anion species accelerates the rate of exchange, while the effect of the buffer anion size is relatively insignificant. A small positive effect observed for the ionic strength on exchange has also been interpreted.