Equilibria of extraction of chromium(III) from alkaline solutions with trioctylmethylammonium chloride (Aliquat 336)

Experimental investigation was performed on analysis of phenomena and equilibria of liquid–liquid extraction of chromium(III) from alkaline media with trioctylmethylammonium chloride (TOMA-Cl). The effect of the presence of selected electrolytes in the aqueous phases was examined. Stoichiometry of chromium(III) complexes in the organic phase and their conditional extraction constants were determined. Ion pair-type complexes, containing mononuclear tetrahydroxochromate(III) anions and trioctylmethylammonium cations, are the prevailing species in the organic phase at initial concentration of extractant exceeding initial concentration of chromium(III). The effects of initial ionic strength of the aqueous phase, aggregation of extractant in the organic phase, and concentration of some electrolytes in the feed solution on extraction equilibria of chromium(III) were estimated. At equilibrium conditions aggregation of TOMA-Cl in the organic phase depends on initial ionic strength of aqueous phase and on initial concentration of extractant as well on equilibrium concentration of chromium(III) complexes in organic phase. Values of conditional extraction constants of tetrahydroxochromate(III) anions decrease with initial concentration of electrolytes in the aqueous phase and with initial ionic strength. Conditional extraction constants also depend on the kind of electrolytes present in the feed aqueous solution and on aggregation of TOMA-Cl in the organic phase. The dependences established on the base of experimental data allow for the estimation of equilibrium concentrations of TOMA-Cl aggregates and conditional extraction constants of chromium(III) (Kex) in studied systems. These dependences are of practical importance because they permit to design the optimal initial conditions for extraction of chromium(III).