On-Line Photo-Oxidative Decomposition of Phosphorus Compounds to Orthophosphate and Its Application to Flow Injection Spectrophotometric Determinations of Total Phosphorus in River and Waste Waters

A simple and mild on-line photo-oxidative decomposition method for the highly sensitive flow-injection spectrophotometric determination of total phosphorus in river and waste waters was developed for the first time by using low pressure mercury lamps. Organic and inorganic phosphorus compounds in sample solutions were injected into a carrier solution, which merged with the decomposition solution containing sulfuric acid and potassium peroxodisulfate. The solution components were decomposed oxidatively to orthophosphate while flowing in a photo-reactor. After the decomposition, orthophosphate was determined spectrophotometrically based on the formation of Molybdenum Blue. The photo-reactor consisted of two low pressure mercury lamps, a poly(terafluoroethylene) (PTFE) tubing and a reflection mirror. The PTFE tubing was wound directly around the mercury lamps, which were covered with reflection mirror. The detection limit for phosphorus was about 0.001 mg 1-1, and the sample throuphput was about 20 samples per hour for total phosphorus determination. By using the photo-reactor, most of the phosphorus compounds were decomposed completely, whereas some condensed phosphates were less susceptible to photo-oxidation. The analytical results for practical samples obtained by the proposed method showed a good correlation with those obtained by using a heat-oxidation method, which indicates that the content of the condensed phosphates difficult to be decomposed was very scarce in the practical samples.