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

The rate of forward extraction of La(III) from nitrate-acetato medium by bis-(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272, H2A2) 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)], [H2A2](o) and [H+]−1 at the interface. The forward extraction rate constant, kF, has been measured to be 10−7.0 m/s. From the temperature dependence of rate measurement, the values of Ea, ΔS±, ΔH± and ΔG300± 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;FLa=10−7.0[La3+][H2A2](o)[H+](i)−1 kmol/m2s is of the rate expression suggests that the formation of [La(HA2)]2+at interface from La3+and HA2− 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 SN2 mechanism.