Ecology Laboratory 5: Chemical Properties of Water

Phosphorus in Water

• Phosphorus compounds or orthophosphate ions (PO4) exist in solution in different proportions of PO43-, HPO42-, and H2PO4- ions, depending on pH, Eh, salinity, and temperature.

Determination of Phosphorus

• The most commonly used method for determining phosphorus employs ammonium heptamolybdate, which extracts inorganic ions and a fraction of organic orthophosphates.
• This method cumulatively indicates the various phosphorus species as "reactive phosphorus (RP)".

Expression of Phosphorus Concentration

• When expressing the concentration of orthophosphate ions or RP, it refers to the concentration of the element phosphorus alone.
• Many authors prefer to indicate the concentration of phosphorus with symbols such as P-PO4.

Method of Analysis

• The method is based on the reaction of orthophosphate with ammonium molybdate in an acid medium (sulfuric acid).
• The reaction forms a yellow phosphomolybdic complex.
• The complex is then reduced with ascorbic acid to a strongly colored blue compound, catalyzed by the antimonyl ion (SbO+).
• Absorbance is measured at a wavelength of 885 nm.

Reagents

• Ammonium heptamolybdate: 3g in 100 ml of distilled water.
• Sulfuric acid (H2SO4): 14 ml + 90 ml of distilled water (total volume 104 ml).
• Ascorbic acid: 5.4 g in 100 ml of distilled water.
• Antimonyl potassium tartrate: 0.136 g in 100 ml of distilled water.
• Mixed Reagent: combine reagents in the order shown, in a ratio of 1:2.5:1:0.5.
• Mixed Reagent is stable for about 6 hours and must be added to samples in a volumetric proportion of 10%.