COBALT(III) PRETREATMENT FOR TOTAL PHOSPHORUS DETERMINATION

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Abstract: A fast and simple pretreatment method of phosphorus compounds using Co³⁺ has been studied for the spectrophotometric determination of total phosphorus in water samples. Sample was mixed with a pretreatment solution (0.05 mol dm^-3 CoSO₄ in 1 mol dm^-3 H₂SO₄) in an electrolytic cell at the volume ratio of 4:1. Highly reactive Co³⁺ was continuously generated through the electrolysis of Co²⁺ during the pretreatment. Aliquot of the solution was injected to the system of flow injection - Molybdenum Blue spectrophotometry in order to evaluate phosphate ion (PO₄^3-) produced. Organic phosphorus compounds are completely decomposed to PO₄^3- within 2 min by Co³⁺ under moderate condition (70°C). The Co³⁺ is also effective for the acid hydrolysis of polyphosphates; diphosphate and tripolyphosphate are converted to PO₄^3- within 8 min through the pretreatment. Possible mechanisms of the pretreatment are discussed based on the strong oxidizing activity and the Lewis acid nature of Co³⁺.