Kinetics and mechanism of di-n-butylphosphoric acid transfer between toluene and aqueous phases

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Abstract
The rate of inter-phase distribution of di-n-butylphosphoric acid (HDBP) between toluene and dilute aqueous acid or alkali has been studied, using a quiescent interface, stirred mass-transfer cell. The initial rate of organic to aqueous (o → a) partition is first order with respect to total initial organic phase HDBP concentration; the order with respect to initial aqueous phase hydrogen ion activity ({H+}a, i) changes from zero at low {H+}a, i to –1.0 at high {H+}a, i. A mechanism is proposed in which the rate controlling process is diffusion of the singly ionized HDBP dimer (HD– 2) from the interface into the aqueous phase; consistency with initial rate laws is demonstrated and information is obtained on the state of interfacially adsorbed HDBP species. In the absence of mineral acid the initial aqueous to organic (a → o) transfer rate is third order with respect to total initial aqueous phase HDBP concentration (Ca, i). Variation of {H+}a, i at constant Ca i showed the order with respect to {H+}a, i to be 1.0. A mechanism, consistent with experiment, is proposed in which the interfacial formation of (HD– 2) is the rate-controlling process. Initial rate data are used for the first time in liquid-liquid extraction to predict successfully equilibrium distribution ratios. The correlation shows that transfer mechanisms remain the same under equilibrium and non-equilibrium conditions. A treatment was developed for derivation of “absolute” extraction rates and it is shown that under certain conditions during net o → a transfer, the mechanism of a → o transfer changes from the observed for initial rates.

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