The Analytical Application of Sulfur Analogues of β-Diketones. I. The Extraction Spectrophotometric Determination of Cobalt(II) with 1,1,1-trifluoro-4-(2-thienyl)-4-mercaptobut-3-en-2-one (STTA)

Abstract

Abstract The solvent extraction of cobalt(II) with 1,1,1-trifluoro-4-(2-thienyl)-4-mercaptobut-3-en-2-one (abbreviated as STTA) in cyclohexane has been investigated, and a new spectrophotometric method for the determination of minute amounts of cobalt has been developed on the basis of the results obtained in the extraction study. When a sample solution the pH of which had been adjusted to 6.0–6.5 with hydrochloric acid or sodium hydroxide solution was shaken with a 10−3m STTA organic solution, the extraction of cobalt(II) proceeded rapidly and quantitatively. The amount of cobalt up to 8 ppm in the final organic phase could be determined by measuring the absorbance at 480 or 490 nm against the reagent blank. All other interferring ions, such as Hg(II), Cu(II), Ni(II), Pb(II), Zn(II), and Fe(III), extractable together with cobalt(II) could be stripped out of the organic phase by shaking it with 1–12m hydrochloric acid, while cobalt remained in the organic phase as it was. After this treatment, the organic phase was shaken with an aqueous buffer solution of pH 9–11; thereby, the excess of the reagent was released to a great extent from the organic phase into the aqueous solution. The cobalt-STTA chelate was then the only one existing in the organic phase; therefore, the sensitivity and the selectivity of the method increased so much that a few ppb of cobalt(II) could be determined precisely by measuring the absorbance of the final organic phase at 360 nm against the reagent blank. In this work, the apparent mole ratio of STTA to cobalt in the complex formation was estimated to be 4:1, and the molar absorption coefficients of the chelate in cyclohexane were found to be 4970000 (at 360 nm), 13200 (at 450 nm), 6500 (at 480 nm), and 5500 (at 490 nm) respectively. Anions, such as oxalate and citrate, as well as EDTA interfered with this extraction. By the recommended procedure, cobalt of a few ppb in an analytical-grade reagent of nickel nitrate hexahydrate could be determined.