Kinetics of lanthanum(III) extraction from nitrate-acetato medium by Cyanex 272 in toluene using the single drop technique

Abstract

The rate of forward extraction of La(III) from nitrate-acetato medium by bis-(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272, H 2A 2) dissolved in toluene has been measured by the single drop technique. The rate of mass transfer per unit area is found to be proportional to [La(III)], [H 2A 2] (o) and [H +] −1 at the interface. The forward extraction rate constant, k F, has been measured to be 10 −7.0 m/s. From the temperature dependence of rate measurement, the values of E a, Δ S ±, Δ H ± and Δ G 300 ± are calculated to be 71 kJ/mol, −155 J/mol K, 69 kJ/mol and 115 kJ/mol, respectively. The rate expression can be written as; F La =10 −7.0[ La 3+][ H 2 A 2] ( o) [ H +] ( i) −1 kmol/ m 2 s Analysis of the rate expression suggests that the formation of [La(HA 2)] 2+at interface from La 3+and HA 2 − is the rate-determining step for the forward extraction of La(III). Negative entropy of activation suggests that the above rate-determining step occurs via the S N2 mechanism.