Kinetic Studies of the Solvent Extraction of Metal Complexes. II. The Rate of the Extraction of Iron(III) with Thenoyltrifluoroacetone into Carbon Tetrachloride

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Abstract
The rate of the solvent extraction of iron(III) in 4.0 M perchlorate ionic media with 2-thenoyltrifluoroacetone (TTA) into carbon tetrachloride has been measured under various conditions. The rate was found to be first order with respect to the concentration of iron(III) in the aqueous phase, first-order with respect to TTA in the organic phase, but independent and inversely first-order with respect to the hydrogen-ion concentration. From these results, it was concluded that the rate-determining step in this extraction is the formation of the first complex between the undissociated form, TTA(HA), and Fe3+ in the higher acid concentration range, but that the reaction between the anionic form, TTA(A−), and Fe3+ also becomes a rate-determining step as the acid concentration decreases. It was also found that chloride ions accelerate the extraction; this was interpreted by more rapid reactions between TTA (in the both forms, HA and A−) and an iron(III) chloride complex. From these results, the rate constants for the rate-determining reactions were calculated to be KHA=100.57 M−1 min−1 and KA=106.84 M−1 min−1 in 4.0 M(H, Na)ClO4 at 25 °C by means of these equations: −d[Fe3+]⁄dt=KHA[Fe3+][HA] (where [HA] is the total concentration of both enol and keto forms of uncharged TTA in the aqueous phase) and −d[Fe3+]⁄dt=KA[Fe3+]·[A−]. The FeCl2+ species reacts with both HA and A− more rapidly than does the Fe3+ species, and the rate constants of the reactions of the former are in both cases higher than those of the latter by a factor of 28.

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