Determination of Total Phosphorus in Water: Principle and Practice

Total phosphorus (TP) is a critical water quality parameter that serves as a key indicator of eutrophication and pollution levels in natural waters, wastewater, and industrial effluents. TP encompasses all forms of phosphorus present in a water sample, including dissolved orthophosphate, condensed phosphates, organic phosphorus compounds, and particulate-bound phosphorus. 

Among the various analytical techniques available for TP determination, the ammonium molybdate spectrophotometric method—commonly referred to as the phosphomolybdenum blue method—remains the most widely employed due to its simplicity, high sensitivity, and excellent accuracy. This article provides a concise overview of the principle, procedural steps, and practical considerations of this classical analytical approach.

The determination of total phosphorus by spectrophotometry proceeds through a series of well-defined chemical transformations. The first and most essential step is oxidative digestion, in which the water sample is treated with an oxidizing agent—typically potassium persulfate—under neutral conditions and elevated temperature. This digestion process converts all phosphorus species present in the sample, whether organic or inorganic, soluble or particulate, into the measurable form of orthophosphate (PO₄³⁻). 

Without complete digestion, organic and particulate phosphorus would remain undetected, leading to significant underestimation of total phosphorus content.

Following digestion, the liberated orthophosphate undergoes a colorimetric reaction in an acidic medium. The orthophosphate reacts with ammonium molybdate in the presence of potassium antimony tartrate to form a phosphomolybdenum heteropolyacid complex. This intermediate complex is then immediately reduced by ascorbic acid to yield an intensely blue-colored complex known as phosphomolybdenum blue. 

The intensity of the blue color is directly proportional to the concentration of orthophosphate—and hence to the total phosphorus content of the original sample—following the Beer-Lambert law. The absorbance of the blue complex is measured using a spectrophotometer at a wavelength of approximately 700 nm (or 880 nm in some variants), with deionized water serving as the reference blank. The phosphorus concentration is then determined by comparing the corrected sample absorbance against a calibration curve prepared from phosphate standard solutions.