pH Measurement: Best Practices
pH Sample Collection
pH changes rapidly when sample is exposed to air due to changes in the carbon dioxide (CO2) concentration, so best practice is to analyze pH continuously or "when continuous monitoring is not possible, a grab sample should be collected carefully to minimize aeration and splashing. Samples should be analyzed as soon as possible after sampling, while taking extreme care not to shake the sample or stir rapidly during the measurement.
Waters that are not at equilibrium with the atmosphere such as groundwater or high-pH waters are subject to changes when exposed to the atmosphere; therefore, the sample containers should be filled completely and kept sealed prior to analysis..." (Reference 1).
pH also depends on the temperature of the tested sample, this is why it is important to measure the pH as quickly as possible after the sample collection to minimize the impact of temperature on pH reading.
See Health “Canada's Guideline Technical Document – pH” for more info here.
The most accurate method to measure the pH of a water sample in the field is by using pH meters or online pH sensors. Handheld pH meters are typically inexpensive and quick and easy to use. In-line, continuous pH sensors are also relatively common, especially in the water treatment industry.
However, regardless of the cost and complexity of the device, if the electrode is not maintained properly, the accuracy of readings can be questionable. Infrequently used or improperly maintained electrodes are subject to corrosion, which makes them highly inaccurate.
The procedure for measuring pH is the same whether it is conducted in the field or lab.
If necessary, soak the electrode in electrode cleaning solution for a few minutes to dissolve the deposits.
Rinse electrode with deionized water.
Calibrate electrode using at a minimum two, preferably three buffers (typically, pH 4.0, 7.0, 10.0). Make sure to rinse with deionized water between different buffers and after calibration.
Place electrode into a water sample.
Stir the sample with pH sensor and allow pH value to stabilize. Record the pH value.
Rinse electrode with deionized water and store in an electrode storage solution.
Measure the pH of the pH 4, 7 and 10 buffers periodically to ensure that the pH meter is not drifting off calibration. If it has drifted, recalibrate it.