Micelle-mediated extraction of cobalt and its spectrophotometric determination in rocks, soils, sediments and sea-bed polymetallic nodules

Raghbendra Thakur, Pranab Kumar Tarafder and Raj Ranjan Jha

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The surfactant sensitized bottle-green complex (1:2:1) formed of cobaltthiocyanate-cetyltrimethylammonium bromide is suitably extracted into an organic solvent, ethylacetate, at a pH range, 6-8 and the absorption measurements were carried out at 630 nm (ƛmax). The Beer’s law is obeyed in the range from 0.05 to 10 µg per mL. The molar absorptivity and Sandell’s sensitivity of the present system were found to be 2.2 x 103 L.mol-1cm-1 and 0.026 µg cm-2, respectively. No other element forms complex of similar colour with thiocyanate at this pH range. The method was applied to a number of samples like rock, soil, sediment, polymetallic nodules etc., as well as Certified Reference Materials and the values were found highly satisfactory. The interference of Fe3+ which formed a reddish complex at acidic pH (<7) was eliminated when the complex was extracted at pH 8.0. The method is highly useful for the determination of cobalt in geological samples having its concentration from µg/mL to percentage level. As the method has been made highly selective for spectrophotometric determination of cobalt, it might be applied to almost all types of geological sample of diverse matrices.