pH, the acidity of water, is one of the most commonly measured water quality parameters. However, in some source waters, such as the Bull Run Watershed, pH can be difficult to measure and sensitive pH probes must be used. To get accurate pH measurements, Portland Water Bureau’s sampling technicians follow rigorous calibration and sampling procedures. If you are having trouble with pH measurement, below are several suggestions to improve your results:
Tips for selecting an appropriate pH meter
Select the best electronic pH meter and probe for your purposes. There are different pH probes that accompany pH meters depending on the intended function. Paper pH strips or kits where you compare water color to a chart after adding a reagent may not provide accurate results.
Portland’s water is currently treated to be at a pH of 8.2. The pH of Portland’s water can vary by a small amount in the distribution system, but it generally ranges from about 7.4 to 8.4, with a median value of 7.8 to 8.1. If your pH readings are well outside of this range, recalibrate your pH meter according to the manufacturer’s directions and/or replace probes as needed.
The Bull Run Watershed is our main water source. Due to its low dissolved mineral content, water from Portland’s Bull Run source has what is called ‘low ionic strength’. Low ionic strength water is difficult to accurately measure pH. If possible, use a pH probe designed specifically for low ionic strength water to measure the pH.
Depending on the level of desired accuracy, there are different methods to test the pH of water. Our field technicians use the Hach HQ40d multi meter with an Intellical PHC281 refillable pH electrode to take pH readings. The Hach Pocket Pro is a basic, yet reliable option for customers.
The temperature of Portland’s water typically varies between 38 and 75 degrees Fahrenheit. Temperature can affect pH measurement. As temperature goes up, pH goes down, and as temperature does down, pH can increase. Some pH meters include Automatic Temperature Compensation (ATC) probes to assist in taking pH at any temperature.
Calibration of pH Meters
Ensure reference standards/buffer solutions are replaced before their expiration date.
Follow the manufacturer’s recommended calibration methods. These methods can include:
- Use fresh buffer solutions when calibrating the pH probe.
- Perform a calibration verification check using known pH standard solutions. Our technicians use four known solutions when verifying pH: 4.01, 7.00, and 10.01. A pH of 7.38 is then used to verify that the probes are calibrated and properly functioning.
- Prior to calibrating, clean the pH probes using deionized water.
- Rinse the containers used to test the known buffer solutions.
Frequently calibrate your pH probe to appropriate reference standards. Portland Water Bureau technicians perform 3-point calibrations twice daily.
Storage and Replacement of pH Probes
- If it is a ‘wet bulb’ type probe, make sure your pH probe is properly maintained by using manufacturer-recommended filling and storage solutions. Do not store your probe in low-ionized strength water such as distilled or deionized water.
- Replace your pH probe(s) according to the manufacturer’s recommendations.
Troubleshooting pH Measurements
- Ensure that the pH probe is properly submerged to the indicated line/reference junction.
- Do not use the same pH probe to measure the pH of your raw water feed that you use to measure other liquids; this can cause fouling of the probe and affect the accuracy of your measurements.
- Using old or expired pH probes can result in inaccurate results.
- Rinse the pH probes to avoid rubbing the sensitive surface, or use a lint-free paper towel to blot the probe.
- Low batteries in the meter can affect pH readings. Check and replace batteries frequently.
- Follow all other manufacturer procedures.
It is important that customers use accurate pH measurements when adjusting the water for their aquatic animals as pH measures the concentration of hydrogen ions and is measured in a logarithmic scale. A change in the pH scale of 1.0 means that there is a tenfold increase in hydrogen activity. However, because of the broad range of water quality needs for aquatic species (i.e. freshwater vs. marine water species), the Portland Water Bureau recommends consulting with an aquarium professional or specialized reference guide when caring for aquatic animals. A common reference guide: The Marine Aquarium Reference, Systems and Invertebrates. 1992. ISBN 0-939960-05-2.