The extraction of iron (III) from hydrochloric acid solutions has been investigated using tributyl phosphate (TBP) in benzene under different conditions. The organic extracts were examined by infrared and ultraviolet absorption spectroscopies. It was found that with increasing the concentration of aqueous hydrochloric acid solutions, the distribution coefficient increased steeply at above 2 -3 mol dm -3 HCl. Consequently, the following equilibrium equation is proposed for the extraction of iron

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... (III) from hydrochloric acid solutions by TBP: FeCl 3 (aq) + HCl(aq) + 3TBP(org) HFeCl 4 ・3TBP(org). In solvent extraction of metals from acid liquors, the separation of metals from iron (III) is an important problem. Therefore numerous reseachers have studied on solvent extraction of iron (III) from acid solutions. However, since there are few investigations to clarify the extraction mechanism of iron (III), the present author has engaged systematically in the fundamental study on solvent extraction of iron (III). The extraction of iron (III) from hydrochloric acid solutions by -hydroxyoxime, such as 5, 8-diethyl-7-hydroxy-6dodecanone oxime (HR, the active component of LIX 63), has been reported previously 1) , because the information on the extraction of iron (III) by LIX 63 was little submitted, despite the fact that the discrimination of LIX 63 for copper over ferric iron was not so good as required. Furthermore, when dihexyl sulphoxide (DHSO) is used as an effective extractant for precious metal such as mercury, the problem to separate precious metal from iron (III) has been investigated in the extraction system from chloride media 2) . In the present paper, the work is extended to the extraction of iron (III) from acid solutions by neutral organophosphorus compound such as tributyl phosphate (TBP). TBP is using in various industrial fields, especially such operations have been remarcable in the nuclear field for conspicuous extractability of uranium and in the processing of radioactive products from nucler reactors. For the extraction of some base metals, however, there is a particular case in the extraction of iron from chloride solutions by TBP prior to the separation of cobalt from nickel by tertiary amine 3) . In the extraction of iron (III) from acid solutions by TBP, the extraction efficiency for iron (III) of TBP is in the order of the extraction systems HCl > HNO 3 >> H 2 SO 4 4) . As it seems that TBP is superior in the extraction of iron (III) from hydrochloric acid solutions, the extraction system for chloride media has been undertaken in this study. 2.Experimental 2・1 Reagents The TBP (Daihachi Chem. Ind. Co., Ltd.) was purified by usual method 5) and diluted with benzene. Aqueous solutions of iron were prepared by dissolving ferric chloride(FeCl 3 ・6H 2 O) in hydrochloric acid of the required concentration. In general the aqueous metal concentration was 1 g dm -3 ferric chloride except for the loading test. Other chemicals were of analytical reagent grade. 2・2 Extraction and analytical procedures Equal volumes (15 cm 3 each) of the aqueous and organic phases were shaken for 10 min in 50 cm 3 stoppered conical flask at 293 K. Preliminary experiments showed that equilibration was complete in 10 min. The mixture was separated by centrifugation, and aliquots of both phases were analysed to determine the distribution coefficient (E o a , the ratio of the equilibrated concentration of iron in the organic phase to that in the aqueous phase). The concentration of iron in aqueous solution was determined by EDTA titration using xylenol orange (XO) as indicator. The chloride concentration and the water content of the organic phase were determined by the use of Volhard's method and Karl Fisher's titration, respectively. 2・3 IR and UV spectral measurements The infrared (IR) spectra of the organic extracts were determined on JASCO models IRA-1(4,000 -650 cm -1 ) and IR-F(700 -200 cm -1 ) using a capillary film between thallium halides or polyethylene films. The ultraviolet (UV) absorption spectra were recorded on a Hitachi model 340 spectrophotometer using 1.0 × 1.0 cm fused silica celle. 3.Results and discussion 3・1 Extraction isotherms by TBP The extraction of iron (III) from hydrochloric acid solutions by TBP in benzene was carried out by varying the aqueous acidity. The distribution coefficient rises steeply with increasing the initial concentration of hydrochloric acid at *