Waterwatch Australia national technical manual

Module 4 - physical and chemical parameters
Waterwatch Australia Steering Committee
Environment Australia, July 2002
ISBN 0 6425 4856 0

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Methods (continued)

Phosphorus

What is it and why does it matter?

Phosphorus (chemical symbol P): a mineral nutrient that is essential for all forms of life

The phosphorus found in both surface water and groundwater is in a form called phosphate (chemical formula, PO4). It is naturally derived from the weathering of rocks and the decomposition of organic material, but it can also enter waterbodies in runoff or discharges - soil and fertiliser particles can carry phosphorus, and sewage is also rich in phosphorus.

Phosphates available to plants and animals are called orthophosphates, and exist in waterbodies as dissolved and particulate (suspended) and colloidal forms. Dissolved orthophosphate is immediately available to plants and animals. Particulate orthophosphate is potentially available to plants and animals. Colloidal polyphosphates are dissolved but not immediately available to plants.

Plant growth is limited by the availability of dissolved orthophosphate. A sudden increase in orthophosphate in inland waters can stimulate great increases in the growth of algae, particularly, as well as other aquatic plants. Algal blooms potentially produce toxins and also can cause large deficits of dissolved oxygen. Phosphates do not pose a human or animal health risk, so are not regulated in our drinking water.

What factors affect phosphate?

Phosphate concentrations in water are affected by:

Phosphate in domestic waste enters the waterway from leaking septic systems, sewage treatment facilities and stormwater drains. Many sewage treatment plants are allowed only a limited amount of phosphate in their discharges.

Suggested methods, equipment and reporting

Phosphate concentration is measured in milligrams per litre (mg/L). Since most results are reported in terms of phosphorus (rather than phosphate), Waterwatch has adopted the convention of reporting results as P. To convert phosphate (PO4) to phosphorus (P), divide by 3 (i.e. 0.06 mg PO4/L is equivalent to only 0.02 mg P/L).

Even small changes to low phosphorus concentrations (0.01-0.02 mg/L) can have a significant effect on the ecosystem, but existing field equipment cannot detect phosphate at concentrations below about 0.02 mg/L. There is little to be gained from monitoring if concentrations of phosphate are consistently lower than the detection limit of your equipment.

Phosphate concentration does not have to be measured on-site in the field, provided the samples can be refrigerated immediately, or frozen. The phosphate in refrigerated samples must be measured within 24 hours of sampling, and in frozen samples within 1 month of sampling. The sample bottle should be filled to three-quarters capacity.

The dissolved (or filterable) reactive test measures only dissolved orthophosphate and provides a measure of only the immediately available phosphate in the system at the time of sampling.

The total phosphorus test measures the immediately and potentially bioavailable forms of phosphorus in the sample. Total phosphorus concentrations are often many times higher than orthophosphate reported as phosphorus in the same sample. Therefore total phosphorus readings are more likely to come within the measuring range of your instrument.

Filterable reactive phosphate test

The ascorbic acid method is generally used for measuring phosphate in a sample. The ascorbic acid produces this test's characteristic blue colour.

It is best to filter the sample, but some Waterwatch groups do not have access to filters. If the sample can be filtered, the ascorbic acid method measures dissolved orthophosphate only (also known as filterable reactive phosphate). This is a useful value.

In an unfiltered sample, the ascorbic acid method is likely to give inconsistent results, because the reagents are also reacting with other substances attached to the unfiltered particles.

For measurement of total phosphorus, all the forms of phosphorus in the sample are first converted to orthophosphate by 'cooking' the sample in suitable reagents. The sample is then neutralised and the orthophosphate is measured by the standard ascorbic acid method. Because the sample is not filtered, the procedure measures both dissolved and particulate (suspended) orthophosphate.

The need for a heat source means the total phosphorus test is better done in a laboratory. Even then, it is difficult for inexperienced operators to get accurate results.

Equipment

The equipment you will need for this method includes:

Colour comparator

Colour comparators are appropriate for monitoring sites with expected high concentrations of phosphates (greater than 0.1 mg/L), including stormwater, run-off in urban streams and waste-water treatment outfalls. A colour comparator is a low-cost simple piece of equipment consisting of a blue colour spectrum. Intensity of the blue is in direct proportion to the amount of orthophosphate present. Reagents react with the water sample to result in a blue colour. The colour is matched against the spectrum to determine mg/L of phosphorus.

Colorimeter

The colorimeter is best used at sites where the expected concentrations of phosphates are relatively low (down to 0.02 mg/L). The colorimeter is a relatively expensive electronic device which measures the degree of 'blueness'. The colorimeter measures the amount of light transmitted or absorbed at a nominated wavelength.

Procedure

Field procedure for a colour comparator

  1. Collect sample.
  2. Filter it (if filters are available). If you are not filtering, you are measuring total reactive phosphate. It is important to note which method you are using so your data can be interpreted with the method your group is using.
  3. Follow manufacturer's instructions for your colour comparator.
  4. Make the closest possible colour match between the treated sample and the colour scale of the colour comparator.
  5. Record the value in orthophosphate as P on your water quality results sheet.

Field procedure for a colorimeter

  1. Follow the manufacturer's instructions for your model.
  2. 'Zero' the meter using a blank (sample minus reagents) following the manufacturer's directions. Most manufacturers will instruct you to zero the meter using a reagent blank. The blank you use will depend on what you are testing. When testing your waterway samples your blank will be the sample without any reagents added to it. When testing glassware or reagents for data confidence, your blank will be deionised water plus the reagent powder.
  3. Pour the recommended sample volume into a mixing container and add reagent powder. Swirl to mix. Wait the recommended time (usually a minimum of 10 minutes) before proceeding.
  4. Pour the first water sample into the sample cell test tube. Wipe the tube with a lint-free cloth to be sure it is clean and free of smudges or water droplets. Insert the tube into the sample cell of the colorimeter.
  5. Place the cover over the sample cell. Read the concentration of the sample, or percentage transmittance and convert to mg/L as P on the chart provided. Record your reading on the water quality result sheet. Note: if the sample concentration is below the minimum detection limit of your instrument, say 0.05 mg/L, report it as '<0.05 mg/L as P'.
  6. Pour the used samples into a designated waste bottle.
  7. Rinse the sample cell test tube and mixing container three times with deionised water. Avoid touching the lower portion of the sample cell test tube. Wipe with a clean, lint-free cloth. Be sure the lower part of the sample cell test tube is clean and free of smudges or water droplets.
  8. Be sure to use the same sample cell test tube for each sample. If the test tube breaks, use a new one and repeat step 1 to re-zero the meter.

Calibration

The procedure for calibrating a colorimeter in the laboratory is as follows.

  1. Prepare six standard solutions that are in the range of the results expected. Generally 0.00 mg/L, 0.04 mg/L, 0.08 mg/L, 0.12 mg/L, 0.16 mg/L and 0.20 mg/L will be suitable concentrations.
  2. Label six 25 mL volumetric flasks - one for each new standard solution.
  3. Pour about 30 mL of a main standard solution containing 1 mg P/L into a 50 mL beaker.
  4. Using Class A volumetric pipettes (pre-rinsed in standard solution) transfer 0-5 mL of the main standard solution from the beaker to the volumetric flasks, as shown in Figure 4.11.
  5. Fill the volumetric flasks to the line. Swirl.
  6. Analyse a portion of each of these new standard solutions in a colorimeter, as described above (see Field procedure for a colorimeter).
  7. Construct a standard curve from your measured concentrations (mg P/L), with measured concentration on the y axis and desired concentration on the x axis. The points should fall on a straight line (see Figure 4.12).
Figure 4.11: Proportion of main standard solution to concentration of new standard solution
Concentration of
new standard solution		   Volume of
main standard solution
0.00 mg/L   0 mL
0.04 mg/L   1 mL
0.08 mg/L   2 mL
0.12 mg/L   3 mL
0.16 mg/L   4 mL
0.20 mg/L   5 mL
where volume of main standard solution needed =
Desired concentration of new standard
x
Final volume (mL )of new standard
Concentration of main standard solution

Figure 4.12: Acceptable and unacceptable calibration curves for phospate
Figure 4.12: Acceptable and unacceptable calibration curves for phospate
Total phosphorus method

It is very difficult to obtain results that agree with those measured in external laboratories.

Equipment

The equipment you will need for this method is as listed above, plus:

Procedure

Follow the manufacturer's instructions for 'cooking' (digesting) and neutralising the sample before testing for orthophosphate with either a colour comparator or colorimeter, as described above.

Because of the digestion step, measurement of total P is very difficult to do under field conditions. Extreme caution must be taken when carrying out the total P test. Also take care with the disposal of waste from the reactions.

Because of the extra equipment needed, use of acid and the much longer time needed to complete a total phosphorus test you should consider only testing for orthophosphate using the ascorbic acid method on an untreated sample.

Extra safety tips for testing phosphate

Data confidence

Acid-wash cleaning method

Reused sample containers and all glassware used in this procedure must be cleaned before the first run and after each sampling run by:

  1. washing each bottle with a brush and phosphate-free detergent;
  2. rinsing three times with tap water;
  3. rinsing with 1M or 2M hydrochloric acid;
  4. rinsing three times with deionised water;
  5. when clean and dry, covering the bottle opening with aluminium foil or plastic film to avoid contamination.

If factory-sealed, disposable Whirl-pakŪ bags are used for sampling, no preparation is needed.

Interpreting your results

When you have enough measurements of phosphate through time in your waterbody, especially in comparison with a local or regional reference waterbody, you will know when a particular phosphate value does not fit the usual pattern. Alternatively, contact your Waterwatch coordinator about the water quality guidelines' (ANZECC/ARMCANZ 2000) suggestions of relevant trigger values for your type of waterbody, its environment and its designated uses.

Safety considerations and waste disposal tips when sampling and measuring phosphate

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