Kinetic Studies of Exchange Reactions between Rare Earth Metal Ions and Their Diethylenetriaminepentaacetic Acid Complexes

Abstract

Kinetics of the exchange reactions between rare earth metal ions, Re′³⁺ (Re′³⁺; Eu³⁺, Er³⁺) and their diethylenetriaminepentaacetic acid complexes, ReDTPA^2– (ReDTPA^2–; ErDTPA^2–, YDTPA^2–, EuDTPA^2–) were studied by a spectrophotometric method. By taking into account the formation of binuclear complexes, the stability constants of which were determined experimentally, an equation expressing the initial rate of the exchange reaction was obtained as follows:

Rate= α[ReDTPA^2–][H⁺]² + β[ReDTPA^2–][Re′³⁺] + γ[ReDTPA^2–][H⁺][Re′³⁺]

where α, β, and γ are the rate constants. Moreover, the time-course of the exchange reaction up to the equilibrium stage was analyzed for the ErDTPA^2––Eu³⁺ system. The results were quantitatively interpreted by the proposed rate equations.