Methods Manual for Salt Lake Studies/pH

Authors: PSJ Coleman,

pH measurement overview

pH is an important indicator of biological stability in waterbodies. pH is the measurement of hydrogen potential of a liquid. In common language it is a measure of the acidity or alkalinity, and it is reported in measures that range from 1 to 14. When liquids have a pH of 7 they are considered neutral, while pH values less than 7 are acidic and values more than 7 are alkaline or basic.

pH readings may rise with rising salinity, as many of the salts found in Australian salt lakes are alkaline. High pH readings may also indicate the presence of algal blooms, and is this case the high pH may be a by-product of planktonic photosynthesis. Peats, organic litter or decomposition of sulfates (eg gypsum) by certain bacteria may result in low (<5) pH readings.

Methods

pH may be measured using pH strips (with a resolution of 0.5 pH units), pH liquid test kits (with a resolution of 0.2 units), by titration, or with a probe/multi purpose meter (with a resolution of less than 0.1 pH units). The pH strips are the cheapest and most rugged option but are less precise than the other three methods and some types of strips are sensitive to salinity changes.

Use of a pH meter

Calibrate the meter using in-date calibration solution, following the directions in the pH meter manufacturer's manual. When the meter is calibrated, rinse the probe in distilled water. Insert the pH probe and temperature probe (if separate) into the brine sample you wish to measure and stir gently. Wait until the reading stabilises and record reading in the laboratory daybook.

Use of pH strips

pH strips may become inaccurate or not react at all as they age, or if they are exposed to water. If your packet of strips becomes wet, dispose of them.

Test your pH strips before each monitoring event to make sure that they provide an accurate reading in a reasonable length of time. This is done by immersing one of the strips into a fresh solutions of known pH values (buffer solution - pH 7 and pH 10), checking that the strip provides the correct reading, and then disposing of the used strip. Should your test strip not perform adequately, dispose of the entire pack and open a new one.

In the field, obtain a sample of the brine and place a pH strip into it. Leave the strip in the water until no further colour change occurs (up to 5 minutes). Remove pH strip from the sample and immediately compare the colours on the strip with those on the packet, and record the results.