SIMULTANEOUS CHARACTERIZATION OF EXTRACTION EQUILIBRIA AND BACK-EXTRACTION KINETICS - USE OF ARSENAZO-III TO CHARACTERIZE LANTHANIDE BIS(2,4,4-TRIMETHYLPENTYL)PHOSPHINIC ACID COMPLEXES IN SURFACTANT MICELLES

A novel ''metallochromic indicator method'' that simultaneously yields kinetic and extraction equilibrium information for metals has been developed. The use of arsenazo III to monitor the kinetics of dissociation of lanthanide-bis(2,4,4-trimethylpentyl) phosphinic acid (HBTMPP) complexes in polyethylene glycol tert-octylphenyl ether (Triton X-100) micelles is described as an example. The distribution and the dimerization constants of HBTMPP in these micelles resemble its values in the CHCl3-H2O system but the extraction equilibrium constants of the lanthanides, K(ex), are 4-6 orders of magnitude larger than the values in CHCl3-H2O. Two series of lanthanide-HBTMPP dimer complexes having ratios of 1:2.5 and 1:3 are extracted into the micelles, and for both series, the K(ex) values for the heavy lanthanides are higher than for the light lanthanides. The rate-limiting step in the dissociation of these complexes is their attack by H+ and appears to proceed exclusively in the micellar phase under the conditions employed. The rate constants for the dissociation reaction are larger for the lighter lanthanides than those of heavier lanthanides. For a given metal, the 1:2.5 metal-dimer complex dissociates more readily than the 1:3 dimer complex.