Chemistry of water -Part 28-

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3- Calibration of phosphate ions:

In wastewater and natural water, phosphorus is among the dissolved organic or mineral compounds. The following are the most important colorimetric analysis methods for measuring phosphate ions:

3- a- The method of ammonium molybdate and ascorbic acid:

In an acidic medium, complexes are formed between ammonium molybdate and phosphate, and it is reduced using ascorbic acid to obtain a compound of a stable color. The spectroscopic device is used to measure its absorbance.
The reagent is prepared by mixing 400 ml of sulfuric acid + 120 ml of ammonium molybdate solution. In a standard bottle with a capacity of 25 ml, 20 ml of the water to be treated is poured, then 4 ml of the reagent that has been prepared is added to it, and we complete with distilled water the volume of the standard bottle. After 20 minutes and at a wavelength of 690 nm, we measure the absorbance.
The result is compared with a standard curve obtained from performing the same steps on solutions of known concentration and confined between 0 and 0.5 mg/L of Phosphate.

3- b- The ammonium metavanadate method:

This method is based on the formation of a colored phosphate complex with ammonium molybdate and ammonium metavanadate in an acidic medium.

Reagents used:

  • Ammonium molybdate solution at a concentration of 50 g/L.
  • Ammonium metavanadate solution at a concentration of 2.5 g/L with the addition of a few drops of nitric acid.
  • Nitric acid.

5 ml of the above-mentioned reagents are added to 50 ml of aqueous sample and mixed well, then the absorbance is measured at the wavelength of 460 nm.


  • [AQUAPROX- Livre: Traitement des eaux de refroidissement. Imprimé en France par EMD S.A.S- 53110 Lassay-les-Chateaux. N° d'imprimeur: 15566- Dépot légal: juin 2006. N° 842- Cyclus print 90°]

  • [Introduction to Water Chemistry (Pollution- Treatment- Analysis). Dr. Nasser Al-Hayek. Publication of the Higher Institute for Applied Sciences and Technology (HIAST). Syrian Arab Republic, 2017.]

  • Folco Laverdière, Anja Holstein, Laurent Thiebaut, Robert Mallee, Guillaume Gravejat, Benjamin Des clozeaux: Les principales methodes d’analyse,1999, p5.