Phosphorus occurs naturally in rock formations in the earth's crust, usually as phosphate. Of high nutritive value to plants and animals, phosphates are used in fertilizers and as animal feed supplements. They are also used in the manufacture of industrial chemicals and pharmaceuticals and as detergent builders.

High phosphate concentrations in surface waters may indicate fertilizer runoff, domestic waste discharge, or the presence of industrial effluents or detergents. Although phosphates from these sources are usually poly-phosphates or organically bound, all will degrade to "ortho" or reactive phosphates with time. If high phosphate concentrations persist, algae and other aquatic plant life will flourish eventually causing decreased dissolved oxygen levels in the water due to the accelerated decay of organic matter. Phosphate measurement is used to control scale and corrosion inhibitor levels in boilers and cooling towers.

Both methods described below measure reactive phosphate, which will give a positive reaction prior to hydrolysis, and is usually termed "ortho-phosphate."

The Colorimetric Methods. In test kits employing the vanadomolybdophosphoric acid method phosphate reacts with ammonium molybdate under acid conditions and in the presence of vanadium to form a yellow-colored product. Results are expressed as ppm (mg/L) PO₄.

Test kits employing the molybdenum blue method, for lower range determinations, utilize a stannous chloride reduction. Phosphate reacts with ammonium molybdate and is then reduced by stannous chloride to form a blue complex. Results are expressed as ppm (mg/L) PO₄.
 

.