On-Line Pretreatment with Cobalt(III) Ions for a Flow Injection-Spectrophotometric Determination of Organic Phosphorus.

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Cobalt(III) oxidation was applied to an on-line pretreatment of organic phosphorus compounds in a system of flow injection analysis (FIA). An FIA manifold, comprised parts for cobalt(III) oxidation and subsequent Molybdenum Blue spectrophotometric measurement, was assembled and the analytical conditions of the system were optimized. A platinum wire (0.2mm×3m) was inserted in a pretreatment coil (1mm i.d.×5m) for accelerating the digestion of organic phosphorus compounds by cobalt(III) ions. Cobalt(III) ions could completely convert organic phosphorus (<1mg dm-3) to phosphate ions with the aid of the platinum catalyst at room temperature and in a relatively short time (average residence time in the coil: ca. 7min). The detection limit for phosphorus by the subsequent spectrophotometry was 0.005mg dm-3 (S/N=3) and the relative standard deviation of repeated measurements (n=10) was below 3% for 1mg dm-3 (as P) disodium phenylphosphate. The present method reduced the amounts of the pretreatment and coloration reagents required and the time for analysis (10-12 samples could be determined per 1h), compared to the amounts and time needed for the batchwise pretreatment method.
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