Kinetics of solvent extraction of metal ions with HEDHP. III. The kinetics and mechanism of solvent extraction of Cr(III) from acidic aqueous solutions with bis-(2-ethyl hexyl) phosphoric acid in benzene

Abstract

The rate of solvent extraction of chromium(III) from aqueous sulphuric acid solutions (containing 0.05 mol dm−3 sulphate ion and 0.25 mol dm−3 acetate buffer, ionic strength, I = 0.40 mol dm−3) with bis-(2-ethyl hexyl) phosphoric acid (HDEHP or H2A2) in benzene has been measured under various conditions. The rate of backward extraction measurement of Cr(III) from organic phase to aqueous phase is not possible due to the inert property of Cr(III)–DEHP chelate. The forward rate is found to be first-order w.r.t. Cr(III) concentration in the aqueous phase and HDEHP concentration in the organic phase. The order w.r.t. H+ concentration varies from −1 to 1 over the pH range 1.5 to 5.25. The rate is found to decrease with increasing sulphate and nitrate ions concentrations in the aqueous phase. At (30 ± 1) °C, the rate expression, in the presence of sulphate, acetate, and nitrate ions, is found to be represented by:[Formula: see text]In the absence of the anions, the formation of CrHA22+ intermediate complex (Cr(OH)2+ + H2A2(0) → CrHA22+ + H2O) is the rate determining step at all acidities. The effects of the anions on the rate are discussed.