As we near the end of our year long exploration of water quality, we are going to spend this month talking about pH. This parameter helps us understand how basic (alkaline) or acidic the water is and can indicate when there’s something wrong or out of balance.
Understanding pH and why it is important requires a little bit of chemistry background. First, pH is a measure of the amount of positively charged Hydrogen ions (H+). Pure water is completely neutral with a pH of 7, which means that the number of H+ ions and OH– ions are balanced. When something has more H+ ions, the pH goes down and the solution becomes acidic. When something has more OH– ions, the pH goes up and the solution becomes basic or more alkaline.
Pure water has a pH of 7, while seawater typically has a pH of 8.1 due to the minerals from weathered rock making it more basic. However, due to the increase of CO2 in the atmosphere, this is a slight decrease from the pre-Industrial Revolution pH of 8.2 [1]. Rainwater tends to be more acidic, with an average value of 5.6 due to being at equilibrium with the CO2 in the atmosphere. The reason why atmospheric CO2 affects the pH of the ocean is that approximately 30% of atmospheric CO2 ends up dissolved in the ocean [2]. When CO2 is dissolved in water, it reacts to form carbonic acid, which ultimately increases the number of H+ ions, making the water more acidic and the pH lower.
Here in the Chesapeake Bay, the normal pH range for tidal water is 7 to 9, with the normal pH range for freshwater, such as higher up in the tributaries of the Bay, being 6.5 to 9. When pH ranges are outside of those, it can help us identify problems. For example, during algal blooms, we might see a shift to higher pH as the algae consume large amounts of CO2 [3]. After the bloom is done, the decomposition of the algae by bacteria releases large amounts of CO2 and lowers the pH. We might also see pH changes during a pollution spill event, which might include excess animal waste, leaching from mining sites, industrial effluent, waste from soap or cement manufacturing, leaching from landfills, or point source pollution such as from power plants or runoff from agricultural sites [4]. Basically, a pH reading outside the normal range is the first hint that something is wrong in the system and further testing can help reveal what is causing the shift.
Measuring pH is relatively easy. There are a number of testing strips that you can purchase for very little money that will give you a relatively accurate reading of pH, as long as the range of the testing paper/strip is appropriate for the sample you are testing. Some pH strips exist for testing human urine that only have a range of 4.5 to 9, whereas wide range pH test strips for water can detect a range from 1 to 14, which is the full scale. These testing strips rely on a chemical color change that occurs when the chemical on the strip reacts with H+ ions. However, the testing strips are only good for an estimate of pH. More accurate readings can be given by a YSI with a pH meter, which directly measures the concentration of H+ ions. You can also purchase pH meters specifically for water or soil but they will only measure pH, while a YSI can measure many other parameters, as we’ve covered throughout the year.
References:
[1] Wood Hole Oceanographic Institute; https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/ocean-chemistry/ocean-acidification/
[2] National Oceanic and Atmospheric Administration; https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
[3] National Oceanic and Atmospheric Administration; https://oceanservice.noaa.gov/facts/eutrophication.html
[4] Environmental Protection Agency; https://www.epa.gov/caddis/ph