Chapter 4 - From the Field to the Lab

Measuring pH in Lakes and Streams

Field Sampling Considerations

Because pH values can change rapidly, this parameter must be measured in the field immediately after collecting the sample.

Measurement Methods

There are three methods for measuring pH; a probe and meter, litmus paper, and a field kit. The most accurate and reliable method is the probe and meter. This method is no less convenient than the other methods, but requires a more expensive piece of equipment.

Probe and Meter

Calibrate the probe and meter according to the manufacturer’s directions. Use of two buffers (pH 7 and 10) for calibration is recommended.
Sample water can be collected in any glass or plastic container. Collect enough sample water so that you can submerge the tip of the probe. Rinse the probe with sample water before placing it in the sample.
Place the probe in the sample and wait for the meter to equilibrate. If the meter needs to be manually adjusted to correct for temperature – you’ll know it does if it has an extra temperature knob – adjust it to the temperature of the sample before allowing it to equilibrate. The meter will have come to equilibrium when the signal becomes steady. If it is taking a long time to equilibrate, you may try gently stirring the probe. However, do not agitate the sample since this may cause changes in the pH.
Read the pH directly from the meter according to the manufacturer’s directions.

Litmus Paper

Litmus paper is simply a strip of colored paper that is soaked in sample water. The paper turns a different color depending upon the pH of the solution. It provides a very coarse measurement of pH – it is fine for making simple determinations, but it is too coarse a measurement for allowing comparisons between sampling dates or stations.

QA/QC Considerations

Follow the manufacturer’s instructions for storage and preparation of the probe. Most probes need to be kept moist during storage – this is important! Rinse the probe with distilled water and blot dry between all samples.

The probe must be standardized with known buffer solutions every 3 hours and whenever a major change in the pH of the sample water is expected. (In natural waters where there is no large influence from an effluent discharge or other potential source for pH change, major pH change is not likely to be a problem.) Follow the manufacturer’s instructions for calibrating. Use two standards for calibration, a neutral standard and either an acidic or basic (alkaline) standard, depending upon the expected pH range of the samples. In natural fresh waters, standards with pH 7.0 (neutral) and pH 10.0 (base) usually are appropriate.

To assess field variation, collect duplicates at 5 to 10 percent of the stations and measure pH.

The accuracy of field kits or litmus paper can be checked by collecting a few samples to be read back at the lab with a pH probe. However, due to the time lapse and possibly rapid changes in pH, this lab check would be used only as a rough verification of results.

The next section tells you how to measure secchi disk depth in lakes.

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