The rate of mass transfer of metal chlorides between hydrochloric acid and alkylammonium chloride solutions—III: Niobium(V) between 10.8 m hydrochloric acid and bis(3,5,5-trimethylhexyl)ammonium chloride in benzene

Abstract

A kinetic study of the transfer of niobium(V) from 10.8 M hydrochloric acid solution into benzene solutions of bis(3,5,5-trimethylhexyl)ammonium chloride (BTHA·HCl) using the single-drop method is reported. Vapour phase osmometry of BTHA·HCl in benzene shows dimeric and trimeric species to be dominant, with monomer and higher oligomers in very small concentrations. The kinetics have been explained in terms of the inter-facial reaction ▪ The reverse reaction required the assumption of a saturated interface, and log k 0 is −7.3 ± 0.1. The forward reaction was interpreted using two models: 1. 1. Replenishment of the interface with extractant is dominated by a bulk phase oligomer (BTHA·HCl) n , where where n > 10, leading to a rate law with an apparent zero order dependence on BTHA·HCl. R = k[ NbCl 5][( BTHA· HCl s] 1 n where (BTHA·HCl) s is the stoichiometric BTHA·HCl concentration. Log k a is −2.4±0.2. 2. 2. The saturation of the interface by BTHA·HCl with rapid, non-rate determining replenishment from the bulk phase provides a satisfactory explanation of the transfer of niobium(V) from the acid to the organic phase. Such an explanation is preferred for its simplicity in comparison to the first model. The conclusion is reached that a critical interfacial BTHA·HCl concentration (CIC) must exist which determines whether the interfacial monomer or bulk phase aggregates dominate the metal transfer. Below the CIC, the transfer of metal species is slowed by the need to supply extractant from aggregates to the interface. Above this critical value, the interfacial reaction is rate determining leading to an apparent zero order dependence on the extractant.