Spectrophotometric Determination of Total Phosphorus in Water

Total phosphorus is a critical parameter in water quality assessment, representing the sum of all phosphorus forms present in a sample—including dissolved orthophosphates, condensed phosphates, organic phosphorus compounds, and particulate phosphorus. Excessive phosphorus loading is the primary driver of eutrophication in lakes, reservoirs, and slow-moving rivers, making its accurate determination essential for environmental monitoring and pollution control. 

Among the various analytical techniques available, the molybdate spectrophotometric method—standardised as GB/T 11893-1989 in China—is the most widely adopted approach for routine total phosphorus determination. This method combines oxidative digestion with colourimetric detection, offering a favourable balance of sensitivity, simplicity, and reliability.

The determination proceeds through three essential stages: digestion, colour development, and photometric measurement.

All Phosphorus Forms to Orthophosphate

The first and most critical step is sample digestion. Total phosphorus encompasses multiple chemical species, but the colourimetric reaction used for detection responds only to orthophosphate ions. Therefore, all phosphorus present in the sample must first be oxidised to orthophosphate. 

This is achieved by treating the unfiltered water sample with an oxidising agent—typically potassium persulfate—under neutral conditions, followed by heating in a pressurised vessel such as an autoclave or a domestic pressure cooker. The elevated temperature and pressure facilitate complete oxidation of organic phosphorus compounds and hydrolysis of condensed phosphates. 

For samples with high organic content, a nitric acid-perchloric acid digestion may be employed as an alternative. This digestion step ensures that the subsequent measurement reflects the true total phosphorus concentration rather than only the readily detectable orthophosphate fraction.

The Molybdate-Ascorbic Acid Reaction

Following digestion, the oxidised sample, now containing phosphorus exclusively as orthophosphate, is subjected to the colour-forming reaction. In an acidic medium, orthophosphate reacts with ammonium molybdate in the presence of potassium antimony tartrate (a catalyst) to form phosphomolybdic heteropolyacid. This intermediate compound is immediately reduced by ascorbic acid to yield an intensely blue-coloured complex known as phosphomolybdenum blue, or simply "molybdenum blue". 

The intensity of this blue colour is directly proportional to the orthophosphate concentration in the digested sample, and thus to the total phosphorus content of the original water. The reaction proceeds rapidly at room temperature, and the coloured complex remains stable for a sufficient period to allow reliable measurement.