DISSOCIATION RATES OF VARIOUS HEAVY RARE EARTH METAL-DIETHYLENETRIAMINEPENTAACETIC ACID COMPLEXES

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In relation to the separation of rare earth metal ions (M3+) by solvent extraction in the presence of diethylenetriaminepentaacetic acid (DTPA) in an aqueous phase, the rate of dissociation of rare earth metal-DTPA complex, i.e., MDTPA2– + 2H+ ↔ M3+ + H2DTPA3–, was determined by the analysis of the rate of exchange reaction between MDTPA2– and Eu3+, which is described by MDTPA2– + EU3+ ↔ EuDTPA2– + M3+ (MDTPA2–; DyDTPA2–, HoDTPA2–, TmDTPA2–). The exchange rate could be expressed by the following equation and the parameters α, β and γ in the equation were determined.Rate = α[MDTPA2–][H+]2 + β[MDTPA2–] [EU3+] + γ[MDTPA2–][H+][EU3+]A method for estimating approximate values of α, that is, the rate constants of proton-catalyzed dissociation of MDTPA2– for various rare earth metals was proposed. Based on the dissociation rate constants of DTPA complexes obtained, selectivities in solvent extraction in the presence of DTPA were estimated for various cases.
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