A Spectrophotometric Method for Determination of Cobalt in Sea-water after Enrichment with Solid Manganese Dioxide

Abstract

Detailed procedures for the determination of cobalt in sea-water are described. These involve the pre-concentration of trace amounts of cobalt by the adsorption on solid manganese dioxide, followed by desorption with nitric acid, successive solvent extraction with 2-nitroso-1-naphthol and final spectrophotometry with nitroso-R salt reagent. During the analysis, recovery of cobalt is controlled using cobalt-58 as a tracer. This simple enrichment procedure to obtain cobalt from sea-water minimizes the risk of eventual contamination and enables relatively large volumes of sea-water to be handled with reasonable speed, which is an advantage for routine work both on board ship and in the laboratory.
The lower limit for determining cobalt in sea-water by the present method is estimated to be around 0.005μg Co/l for a 20-liter sample, while the standard deviation of the results of a single analysis would be ±0.1μg Co for the range up to 5μg Co as the total amount. The chemical form of cobalt determined by the present method is thought to be as the divalent cobalt cation, possibly in the hydrolized form, and the trivalent form, if any, being in colloidal dispersion.