Water quality indicator: pH
Updated: Oct 22, 2019
What is pH?
By definition, pH is the negative logarithm of the hydrogen ion concentration (more precisely, the activity), or algebraically pH = −log10 [H+]. Consequently, one pH unit represents a 10-fold difference in H+ concentration. For example, a solution at pH 6 contains 10 times more H+ than at pH 7.0 and 1000 times more H+ than at pH 9.
In simple words, pH is a measure of how acidic or basic water is. A pH value of 7.0 is considered neutral. A pH lower than 7.0 is considered acidic. A pH greater than 7.0 is considered basic (alkaline).
The pH of most natural waters typically falls between 6.0 and 8.5. Rain and groundwater tend to be naturally slightly acidic, but usually no lower than 6 on the pH scale.
Values of pH below 4.5 and above 9.5 are usually lethal to most aquatic organisms, and even less extreme pH values can disturb reproduction of aquatic organisms and other biological processes. One of the reasons is the pH affects the solubility of metals, salts and organics changing their bioavailability and toxicity to aquatic organisms.
BC Approved Water Quality Guidelines (BCWQGs) & Canadian Environmental Quality Guidelines (CEQGs) for Aquatic Life Protection
BCWQG permit “unrestricted change” within 6.5-9.0 range for freshwater and within 7.0 - 8.7 range for marine environment.
CEQG dictate that “the pH of marine and estuarine waters should fall within the range of 7.0 - 8.7 units unless it can be demonstrated that such a pH is a result of natural processes”.
See original CEQG factsheet about pH here.
pH Sample collection: best practices
pH changes rapidly when sample is exposed to air due to changes in the carbon dioxide (CO2) concentration, so it's best 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 (e.g., groundwater, 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 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.
pH Measurement: best practices
The most accurate method to measure the pH of a water sample is by using pH meters. Handheld, portable meters are inexpensive and are quick and easy to use, and therefore are often preferred for field measurements. However, regardless of the cost of the device, if the electrode is not maintained properly, accuracy of the 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 least two of three buffers (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 probe 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 4.0 and 7.0 buffers periodically to ensure that the pH meter is not drifting off calibration. If it has drifted, recalibrate it.