Add time:08/25/2019         Source:sciencedirect.com

The dissociation kinetics of lanthanide-N-methylethylenediamine-N,N′,N′-triacetate (MEDTA) complexes have been studied in aqueous acetic acid/ sodium acetate solutions using Cu2+ ions as sequestring agent. The reaction mechanism involves a fast protonation equilibrium (KH) of a mixed complex Ln-MEDTA-OAc−, followed by a slow rate determining internal proton transfer (ks) where Ka is the acid dissociation constant of acetic acid and KM the stability constant of the mixed complexes Ln-MEDTA-OAc. KM values increase and the product ksKH decreases with decreasing ionic radii of the trivalent rare earths. Formal rate constants for the formation reaction are calculated from the dissociation rate and the stability of the mixed complex; however, this reaction is too fast to be followed by the stopped-flow spectrometer.

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