Rapid and sensitive colorimetric determination of cobalt(II)

A highly selective and sensitive spectrophotometric determination of cobalt (II) was developed. 7-Nitroso-8-hydroxyquinoline-5-sulfonic acid sodium salt was used as the chromogenic reagent for color development. Although other metals form colored chelates with the ligand, it was possible to develop a selective method using McIlvaine's pH 8 citric acid-phosphate buffer. Under these conditions, iron(II), iron (III), copper (II), zinc (II), and manganese (II), minerals likely to be compounded with cobalt (II) in geriatric formulations, do not interfere with the precision of the method or the color development. Calcium (II) and magnesium (II) do not form colored chelates with the used ligand. Hormones, vitamins, and additives likely to be present along with the cobalt ion in pharmaceutical formulations do not interfere. The sensitivity is 0.37 mug of cobalt (II)/ml of sample solution.