Cobalt(III) Oxidation Pretreatment of Organic Phosphorus Compounds for a Spectrophotometric Determination of Phosphorus in Water

Abstract

Cobalt(III) oxidation was applied to the decomposition of organic phosphorus compounds as a pretreatment of a spectrophotometric determination of organic phosphorus. A cobalt(III) solution (ca. 0.16 mol dm⁻³), prepared through the electrolysis of a 0.2 mol dn⁻³ cobalt(II) sulfate solution containing 5 mol dm⁻³ sulfuric acid, was mixed with a sample solution containing a phosphorus compound at a volume ratio of 1:5, and allowed to stand for oxidation. After excess cobalt(III) ions had been reduced to cobalt(II) ions with ascorbic acid, the concentration of phosphate ions in the sample solution was determined by Molybdenum Blue spectrophotometry at 880 nm. The cobalt(III) ions could completely convert the mg dm⁻³ level of phosphorus of organic compounds having a C-O-P or C-P bond into phosphate ions through the treatment within 10 min at room temperature, and did not interfere with the subsequent determination. Although phosphonic and phosphinic acids could be oxidized by cobalt(III) ions, condensed phosphates required heating at 95–100°C during the cobalt(III) treatment for complete hydrolysis. Since the present method was free from any interference from coexisting organic and inorganic substances, except at a relatively high level of reducing substances, it is applicable to organic phosphorus determinations in real water samples.