Spectrophotometric Determination of Total Phosphorus in Water

Total phosphorus is a critical parameter in water quality assessment, as it directly indicates the degree of eutrophication in water bodies. It encompasses all forms of phosphorus present in a sample, including dissolved and particulate, organic and inorganic phosphates. 

The most widely adopted method for its determination is the ammonium molybdate spectrophotometric method, which is specified in the Chinese national standard GB 11893-89. This method is valued for its simplicity, high sensitivity, and accuracy, making it suitable for surface water, wastewater, and industrial effluents.

Principle of the Method

The determination of total phosphorus by this method involves a series of chemical transformations that ultimately allow for quantitative measurement.

The first step is oxidative digestion. In this step, an unfiltered water sample is digested with an oxidizing agent, typically potassium persulfate, under neutral conditions. This process converts all forms of phosphorus in the sample—whether organic or inorganic, dissolved or particulate—into orthophosphate. For samples with high organic content, a nitric acid-perchloric acid digestion can be used as an alternative.

Following digestion, a colorimetric reaction takes place. In an acidic medium, the orthophosphate reacts with ammonium molybdate in the presence of an antimony salt (such as potassium antimonyl tartrate) to form a phosphomolybdic acid complex.

This complex is then immediately reduced by ascorbic acid to form a blue-colored complex, commonly known as "molybdenum blue". The intensity of this blue color is directly proportional to the concentration of orthophosphate in the sample.

Finally, the spectrophotometric measurement is performed. The absorbance of the blue complex is measured using a spectrophotometer at a wavelength of 700 nm, with water used as a reference. By comparing the sample's absorbance to a calibration curve prepared from standard phosphate solutions, the total phosphorus concentration in the original water sample can be determined.

Key Procedural Steps

The analytical procedure can be broken down into several key steps.

First, for sample collection and preservation, about 500 mL of water is collected and immediately acidified with sulfuric acid to a pH of 1 or less to inhibit microbial activity that could alter the phosphorus content.

Second, for digestion, an aliquot of the sample (typically 25 mL) is placed in a digestion tube with the oxidizing agent and heated, usually in a pressure cooker or autoclave at 120–124°C. This step is crucial for the complete conversion of all phosphorus forms to orthophosphate.

Third, for color development, after the digested sample has cooled, ascorbic acid and molybdate reagent are added. The solution is then allowed to stand for about 15 minutes to ensure the full development of the blue color.