What phosphate is measured?
In the context of this work, phosphate is the ion, PO₄^3- in aqueous solution. It is often referred to as 'active phosphate' or 'orthophosphate'. It does not include any insoluble phosphate ions tied up in the benthic sediments or in suspension.

Note that modern convention in the UK expresses the phosphorus concentration in water not of the phosphate ion (ionic mass 95) which includes four oxygen atoms as well as phosphorus, but of phosphorus (atomic mass 31). This is usually referred to a 'phosphorus as P' and results in a record of the concentration in ppm that is approximately one third of the phosphate value. All values in this account are expressed, for consistancy, as the concentration of the phosphate ion and not phosphorus as P. In some quoted cases where authors (such as the Environment Agency) have expressed results in terms of phosphorus as P, I have converted them using the 95/31 conversion factor.

The phosphate test
The standard test for the phosphate ion in solution has remained largely the same since it was first developed in 1826 by one of the founders of modern chemistry, the Swede, Jacob Berzelius. He was the chemist behind the modern notation for chemical compounds and the first to coin chemical terms like ‘protein’, ‘catalysis’ and ‘polymer’.

The group of metallic elements known as transition metals are well known for producing intensely coloured compounds and these form the basis of the classical oil-paint colours and also of gemstones. Common examples of transition metals are iron, cobalt, nickel, silver and gold. The metal molybdenum is a less well known transition metal and its compounds can form, under the right conditions in solution, an intense blue substance with phosphate. The reagent used is ammonium molybdate whih under adidic and reducing conditions forms phosphomolybdenum blue. The intensity of the blue colour is directly related to the concentration of the phosphate in the sample.

It is quite possible to make an adequate assessment of the phosphate concentration with the naked eye but this process is much better automated using a simple device called a colorimeter. A monochromatic light beam is directed through the solution and detected; the attenuation of the beam is measured and translated on the read-out screen calibrated directly into the concentration of phosphate in the original sample.

The colorimeter is first calibrated using the test solution in an optical-standard glass vial without reagents. An excess amount of the molybdenum reagent pre-mixed with other chemicals (often ascorbic acid but in this case potassium disulfate) is added to create the right environment (acidic and reducing). The vial is then shaken then to ensure the reagents are completely dissolved, and then left a further 3 minutes for the rection to complete before the filnal reading is taken.

Sampling
Sampling was carried out in the field, from public footpaths or bridges or with the permission of the landowner, using glass vessels pre-washed with the sample solution. Testing is either done on the spot or the sample is sealed and the testing then done later the same day under laboratory conditions at a temperature between 15 and 25 ºC.

Filtering
Care is taken to minimise suspended solids in samples. No filtering of the sample before testing is carried out. This can introduce error as the solids may contain phosphate that equilibrates with dissolved phosphate. The literature suggests that this error is likely to be minor. On a few occasions when suspended solids are significant, the sample is centifuged before testing.

Variations in phosphate concentration within watercourses
It has become evident that phosphate concentrations are not necessarily consistant within the watercourses tested. Mixing of the nutrient with water may not be perfect. There may be a vertical gradation and, quite often, a concentration gradient across the river flow. Examples have been noted of consideably reduced concentrations near river edges where flow is slow and oxygenating plants are common. Sewage outflows, high in phosphate, only slowly permeate throughout the whole width of rivers. Where possible, samples are taken from the main river flow and below the surface