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What phosphate is measured?
In the context of this work, phosphate is the ion, PO43- 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. It does not include organic phospate tied up in molecules within living cells.

Note that modern convention in the UK expresses the phosphorus concentration in water not in terms of the phosphate ion (ionic mass 95) which includes four oxygen atoms as well as phosphorus, but of phosphorus (atomic mass 31). This is frequently 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. The Environment Agency uses the 'Phosphors as P' convention in its routine river monitoring.

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 and this is noted in the text. The main reason for contunuing to follow the older convention is because the testing process is one used by fisheries and measures aqueous phosphate only and It is this aqueous available phosphate that is responsible for the polltion consequences that we see. A further reason is that the equipment used is calibrated directly in parts per million of available phosphate.

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 which under adidic and reducing conditions forms phosphomolybdenum blue (right). 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 by viewing the blue colour 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 as 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. This establishes the baseline against which the colour intensity is measured. An excess amount of the molybdenum reagent pre-mixed with other chemicals (traditionally 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 second reading is taken.

Units of phosphate concentration
The colorimeter readout is calibrated directly in phosphate concentration in milligrams per litre. This is the same as parts per million (ppm) as 1 litre of water has a mass of 1 million milligrams. The ppm notation is used throughout this website. Phosphate concentrations have been classified into five different levels, each level having a different impact on the ecology of the watercourse. This is discussed further on the right.

Sampling was carried out in the field, either 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.

Care is taken to minimise suspended solids in samples. No filtering of the sample before testing is carried out. Sediment can introduce error as the solids may contain phosphate that will also react with the reagent. The literature suggests that this error is likely to be minor. On a few occasions when suspended solids are significant, the sample is centifuged and phosphate is measured before and after.

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 concentrationl 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





vialsPhosphmolybdenum blue.
Left - no reagent added
Centre - 1.00ppm phosphate standard solution
Right - Heavily polluted river sample, 2.50ppm phosphate

Pollution categories
Distinguishing crudely between the standard Environment Agency (EA) broad pollution categories of high, good, moderate, poor and bad can be done adequately with the naked eye using a colour chart. This method is quick and uses sticks imprgnated with the two chemicals and a small mixing vessel. It is ideal fro quick initial field testing to help determine whether a more accurate test is needed.

The colorimeter used gives a result accurate over most of its range to around ±4%.

A number of different colorimeters are used and from time to time they are checked against each other. In turn some are checked aganst a standard.

Survey scale categories used
Results are expressed in tems of parts per million (ppm) of phophate. This is numerically the same as mg/litre.

Note - the scale grades below are currently under review in the light of new evidence received. This review is likely to raise the category of about 20% of samples by one grade


<0.04 ppm
0.04-0.07 ppm
0.08-0.19 ppm
0.20-1.00 ppm
>1.00 ppm

This scale is adapted from recommendations of the UK Technical Advisory Group of the Water Framework Directive (Phosphorus Standards for Rivers, 2013)


Updated 13th June 23