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The majority of natural waters have a pH somewhat above 7 along with the presence of carbonate and bicarbonate salts. In general, deviation from neutral pH 7 is primarily the result of hydrolysis of salts and strong bases and weak acids or of weak bases and strong acids. Dissolved gases such as CO2, H2S, and NH3 may also influence pH. The loss of gases may change the pH of water if samples are not analyzed soon after collection.

Not only is the hydrogen ion itself potentially toxic to aquatic life, but it is intimately related to the concentration of many other toxic substances. For example, the toxicity of nickel cyanide and sodium sulfide to fish increases as the pH decreases from 9 to 6, and the tolerance of fish to low concentrations of dissolved oxygen varies with pH. Usually the pH values of most inland waters with productive fish populations range from about 6.7 to 8.6.


1.Insert the pH probe into any of the analog ports on the LabQuest interface and turn on the power.

2. Rinse the tip of the pH probe with distilled water.

3. Insert the tip of the probe into the sample to be tested. While gently swirling the probe, wait for the reading to stabilize. Record the pH.

4. Rinse the probe with distilled water before taking another measurement.