The product digests the sample with potassium persulfate and oxidizes all the phosphorus contained in it to orthophosphate. In an acidic medium, in the presence of antimony salt, orthophosphate reacts with ammonium molybdate to form a phosphomolybdic heteropolyacid, which is immediately reduced by ascorbic acid to form a blue compound. The total phosphorus content in the water samples was then measured by spectrophotometry.