Abstract
The solvent extraction of cobalt(II) and nickel(II) with 1, 1, 1-trifluoro-4- (2-thienyl) -4-mercapto-3-buten-2-one (STTA) in cyclohexane has been investigated, and on this basis a new method for mutual separation and spectrophotometric determination of the metals by back-extraction has been developed. When a sample solution the pH of which had been adjusted to 5.56.0 was shaken with a 10-3M STTA-organic solution, the extraction of cobalt(II) and nickel(II) proceeded rapidly and quantitatively. Nickel(II), however, could be stripped out of the organic phase by shaking with an aqueous solution of pH 1.5, while cobalt remained in the organic phase. After the phases separated, the organic phase was reserved for cobalt, while the aqueous phase which contained nickel was adjusted again to pH 5.56.0 and shaken with a 10-3M STTA-organic solution. Then, two organic phases, the former containing Co(STTA)3, and the latter Ni(STTA)2 was respectively shaken with an aqueous buffer solution of pH 9.510.5 to remove most of the excess of STTA from the organic phase. Cobalt and nickel up to 10-6M in the final organic phases could be determined individually by measuring the absorbance at 370 nm or 450 nm against the reagent blank. The molar absorption coefficients expressed in cm2 l mol-1 of the chelate in cyclohexane were found to be 3.5×104 (370 nm) and 1.0×104 (450 nm) for Ni(STTA)2, 9.0×104 (370 nm) and 1.3×104 (450 nm) for Co(STTA)3, respectively. Anions, such as oxalate, citrate, and hexacyanoferrate(II), as well as EDTA interfered with this extration. Nickel contents as small as 10-6M in samples of cobalt chloride hexahydrate of analytical reagent grade and in pure cobalt metal could be determined by the present method.
