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

In relation to the separation of rare earth metal ions (M(3+)) by solv ent extraction in the presence of diethylenetriaminepentaacetic acid ( DTPA) in an aqueous phase, the rate of dissociation of rare earth meta l-DTPA complex, i.e., MDTPA(2-)+2H(+)reversible arrow M(3+)+H(2)DTPA(3 -), was determined by the analysis of the rate of exchange reaction be tween MDTPA(2-) and Eu3+, which is described by MDTPA(2-)+Eu(3+)revers ible arrow EuDTPA(2-)+M(3+) (MDTPA(2-); DyDTPA(2-), HoDTPA(2-), TmDTPA (2-)). The exchange rate could be expressed by the following equation and the parameters alpha, beta and gamma in the equation were determin ed. Rate = )+beta[MDTPA(2-)][Eu3+]+gamma[MDTPA(2-)][H+][Eu3+] A method for estimating approximate values of alpha, that is, the rate constan ts of proton-catalyzed dissociation of MDTPA(2-) for various rare eart h metals was proposed. Based on the dissociation rate constants of DTP A complexes obtained, selectivities in solvent extraction in the prese nce of DTPA were estimated for various cases.