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)

ph

What is it and why does it matter?

pH: a measure of acidity (or alkalinity). Pure water has a pH of 7, acidic solutions have lower pH values and alkaline solutions have higher values.

Values of pH range from 0 (highly acidic) to 14 (highly alkaline). Where water has no net alkalinity or acidity it is said to be neutral and has a pH of 7. pH can be a little misleading unless you remember that one pH unit represents a ten-fold change. So if the pH of a water sample falls from pH 7 to pH 6, that is equivalent to a 10-fold increase in acidity. Figure 4.8 shows the pH of some common liquids.

Figure 4.8: pH scale and pH of selected liquids
Figure 4.8: pH scale and pH of selected liquids

Many compounds are more soluble in acidic waters than in neutral or alkaline waters. The pH of the wet area around roots affects nutrient uptake by the plants; pH also affects the solubility of heavy metals in water and the concentrations of total dissolved solids in rivers.

All animals and plants are adapted to specific pH ranges, generally between 6.5 and 8.0. If the pH of a waterway or waterbody is outside the normal range for an organism it can cause stress or even death to that organism.

What factors affect pH?

A wide variety of factors may have an effect on the pH of water. These include:

The pH of a waterbody varies during the course of the day as the balance between photosynthesis and respiration changes with the light intensity and temperature. Inflowing water may affect the pH of the waterbody as well: rainfall is naturally slightly acidic because of carbon dioxide dissolved in it; water running off limestone areas has relatively high pH. On the other hand, streams and lakes in coastal dune areas may have very low pH (sometimes less than 5) due to the presence of naturally-occurring humic acids.

Suggested methods, equipment and reporting

The pH of a waterbody is best measured in the field, as the samples are being taken. A pH measurement can be delayed by up to 24 hours, but only if the sample is refrigerated immediately and the sample bottle is filled completely, with no air at the top.

To measure pH, you can use either pH strips (often called indicator paper) or a pH meter.

pH strips method

pH strips are coated paper strips that change colour according to the pH of the sample. The colour can be compared to a colour scale to estimate the pH value. pH strips have a long shelf life (3 years) if stored in cool dry conditions; and give reliable results for monitoring groups. Choose pH strips that can detect changes of 0.5 units in water samples and are suitable for weakly buffered waters.

Equipment

The equipment you will need for this method includes:

Procedure

  1. Rinse the pH tube with sample water.
  2. Tear off a piece of indicator strip that is slightly longer than the tube. Leave half a centimetre of the strip sticking out the top when the tube is recapped. This enables you to easily remove the indicator strip when the test is complete.
  3. Fill the tube with sample water, put the cap on and swirl the water around the indicator paper.
  4. Wait for one minute for the full colour to develop.
  5. Place the tube on the black strip running through the middle of the pH colour indicator levels on the inside lid of the pH test box.
  6. Compare the colour on the indicator paper with the pH colours on the lid to find the pH reading.
  7. Repeat test on a field replicate water sample.
  8. Record both readings on the water quality result sheet.

Maintenance

Pour the water into the liquid waste bottle and place the pH paper in the solid waste container.

Rinse the tube with deionised water and dry it before returning it to the kit.

Calibration

You cannot calibrate pH paper, but you can check it against known reference solutions. Prolonged storage may make the paper less accurate.

pH meter method

A pH meter measures pH and temperature, and it adjusts the readings according to the temperature of the sample (pH varies with temperature). pH meters usually display results in pH units. Meters vary a great deal, but the most important part is the electrode. Buy a good, reliable electrode and follow the manufacturer's instructions for proper maintenance. Infrequently used or improperly maintained electrodes are subject to corrosion, which renders them highly inaccurate. The electrode tends to last only 1 or 2 years, so you may consider purchasing a meter with a replaceable electrode.

Equipment

The equipment you will need for this method includes:

Procedure

  1. Rinse the electrode well with deionised water.
  2. Place the electrode in the sample. Wait 2-3 minutes for the reading to stabilise but be aware that some change will occur as pH reacts with carbon dioxide dissolving from the air. Record the result on the water quality results sheet.
  3. Repeat test on a field replicate sample and record the result on the water quality results sheet.
  4. Periodically measure the pH of the calibration solution to test accuracy. If it has drifted off, recalibrate. Do not reuse buffer solutions.

Maintenance

Rinse the electrode well with deionised water, replace cap when finished.

Calibration

A good quality pH meter can detect minimum variations (sensitivity) of 0.1 pH units in river water and can be calibrated at two points giving more accurate readings over a wider range than one-point calibration meters.

Meters must be calibrated with buffer solutions before each sampling and periodically during sampling, e.g. every fifth sample, to check if the meter has drifted off calibration. Your check on the calibration standard should be within the sensitivity range, e.g. ± 0.1 pH units, of the equipment used.

If you are using a two-point calibration meter, use two buffer solutions at 4.0 and 7.0. Buffer tablets can be purchased from test kit supply companies and must be used within their expiry date. A buffer solution of pH 4.0 will last 3 months, but a solution of pH 7.0 will last 6 months if stored in a cool dark place.

Data confidence

pH strips

Minimise damage or staining of the colour chart by storing it in dark dry conditions (see Table 4.4) - water reacts with dyes in the paper.

Table 4.4: Quality control measures for pH
Relative data quality Equipment method Sensitivity (minimum change that can be detected) Calibration Type of quality control
Low or intermediate pH strips 0.5 ph units None Store pH paper in dark dry conditions
High pH meter (one and two point calibration) 0.1 pH Calibrate meter prior to each use and check against calibration standards every 2-3 hours of use 10% of tests to be internal field replicates and split samples or external field replicates

Measure a field replicate after every tenth test.

Test a Waterwatch mystery sample every six months.

pH meter

Test a Waterwatch mystery solution every 10 tests; or set aside 10% of samples to be split and tested by a laboratory elsewhere, or an external field replicate sample to be tested by a water quality professional officer (see Table 4.4).

Interpreting your results

When measuring pH, you should sample at the same time of day on each occasion, being aware of the potential effect on pH of daily changes in photosynthetic activity of aquatic plants. When monitoring estuaries, note the state of the tide and also record conductivity readings before attempting to interpret variations in pH. Note the geology and soils and land use of the catchment you are monitoring, to help interpret pH changes from one site to another.

You should monitor pH from a suspected point source of contamination both at the reference site and at your test sites. For baseline monitoring, interpretation of pH values requires some knowledge of the natural ranges likely to be found in the catchment. For example, a pH of 6.5 might be normal in some streams, but if found in a limestone catchment, would indicate a possible problem.

Changes of more than 0.5 pH units from the natural seasonal maximum or minimum in fresh water should be investigated. Many freshwater systems have a pH close to 7.0 (for example, see Table 4.5). However, the range in limestone areas is 7.0-8.5 and in non-limestone areas 5.0-7.0. In marine waters the pH should not vary more than 0.2 units from the normal values. Marine waters normally have a pH close to 8.2.

Contact your Waterwatch coordinator and ask about the relevant trigger values discussed in the revised national water quality guidelines (ANZECC/ARMCANZ 2000).

 
Table 4.5: Guidelines for water quality as it affects rivers in Victoria
  Excellent Good Fair Poor Degraded
pH range 6.0-7.5 5.5-6 or <8.0 8.0-8.5 5.0-5.5 or 8.5-9.0 <5.0 or >9.0

Source: State of the Environment Report - Victoria's Inland Waters in Waterwatch Victoria, A Community Water Quality Monitoring Manual for Victoria 1994.

Safety and waste considerations when measuring ph

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