Determination of Total Phosphorus in Steel Pipe for Sewage Recycling by High Concentration Differential Spectrophotometry

The traditional national standard method for determination of total phosphorus in recycling water and sewage by Spectrophotometry has some problems for the samples with high content of organic phosphorus, which is easily carbonized during decomposition with complicated operation and large analytical error. In this pater, Differential Spectrophotometry has been used to measure total phosphorus in recycling water and sewage with the advantages of less relative error and better accuracy and is described in this paper. Based on the orthogonal experiment, HNO₃-HClO₄ was applied to the after-treated recycling sewage in steelworks. The orthophosphate, polyphosphate and organophosphate were transferred to orthophosphate under digestion conditions. The orthophosphate reacted with vanadium ammonium molybdate in nitric acid to produce yellow dissoluble phosphorus and vanadium molybdate complexes. The color intensity was proportional to phosphorus concentration. The dosage of oxidizing agent, dissolving temperature, chromatogenic condition and measurement wavelength were optimized. The phosphorus content around 10.0-40.0 mg/L allowed the Lambert-Beer law. The recovery was 98.8%-105.0% with RSD less than 1.0% (n=5). The results were consistent with results obtained by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). Compared with the routine method of Spectrophotometry, the Differential Spectrophotometry has less relative error and higher accuracy. Compared with ICP-AES, it has a lower testing cost.