Determination of ammonium citrate-soluble cobalt in soils and sediments

A rapid field method for the determination of cold-extractable cobalt in soils and sediments has been developed for use in geochemical prospecting. Readily available cobalt is dissolved by treating the sample with an aqueous ammonium citrate-hydroxylamine hydrochloride solution; the filtered citrate extract is then treated with 2-nitroso1-naphthol to form a pink-colored cobalt complex which is concentrated in a floating organic layer by a 1 to 3 mixture of carbon tetrachloride and cyclohexane. After the organic extract is shaken with potassium cyanide solution to eliminate interference by copper and nickel, the intensity of the pink to red color is estimated by comparison with a standard series.

A concentration range of 1 to 60 ppm is obtained when a 0.2-g sample is used but this range is varied easily by adjustment of sample size and volume of organic solvent. A minimum of 100 determinations per 8-hour day can be made, and because only simple, readily portable items of equipment are required, the method is adapted to use under rough field conditions, at the sample site if desired.