Phosphate in Water

There is often a direct relationship between the intensity of the colour of a solution and the concentration of the coloured component (the analyte species) which it contains. This direct relationship forms the basis of the "colourimetric" technique. One might readily determine the concentration of a sample based on its colour intensity, simply by comparing its colour with those of a series of solutions of known concentration of the analyte species.

In some cases the colour of the solution may be due to an inherent property of the analyte itself, for example, a KMnO4 solution has a natural purple colour, the intensity of which can be readily measured. In many other cases, however, the solution colour is developed by the addition of a suitable reagent which interacts with the analyte species thereby forming a coloured complex.

Intensity of coloured solutions are normally measured on a spectrophotometer.

A beam of light of intensity Io is focused on a sample, and a portion, I, is absorbed by the analyte species.

The amount of light absorbed may be mathematically expressed as:A = log (Io/I) (1)

The absorbance, A, is related to concentration by the Beer-Lambert law:A = εcl (2)

which states that the absorbance of a solution is directly proportional to its concentration, c, as long as the solution path length, l, and the wavelength of measurement are constant. Once the Beer-Lambert law is obeyed, a plot of absorbance against concentration will give a straight line, the slope of which is the molar absorptivity, ε * length.

Colourimetric techniques are useful in the analysis of a wide range of substances. One important application is its use in determining the phosphate content in water samples.

The phosphate found in natural waters mainly exists as the orthophosphate species, PO43-, however, the polyphosphates P2O74- and P3O105- are frequently encountered. These polyphosphate species may be hydrolysed to convert into orthophosphate.