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System-wide Monitoring Program

pH

The pH of a solution is a measure of its acidity. The pH is defined as the negative logarithm of the hydrogen ion concentration in solution, and the pH scale ranges from 0 to 14. Distilled water is neutral and has a pH of 7.

Solutions with a pH less than 7 are acidic, and those with pH greater than 7 are basic (alkaline). Because the scale is logarithmic, when the pH increases or decreases by a whole number, the acidity changes by a factor of 10. For example, a solution with a hydrogen ion concentration of 104 has a pH of 4 and is ten times more acidic than a solution with a pH of 5.

Knowledge of pH is important because most aquatic organisms are adapted to live in pHs between 5.0 and 9.0. The pH in an estuary will tend to remain fairly constant because the chemical components of seawater resist large changes in pH; dissolved carbonate minerals present in seawater tend to minimize or “buffer” pH changes by reacting with the ions that change pH. Biological activity, however, may significantly alter pH levels in an estuary.

The process of photosynthesis removes carbon dioxide (CO2) from the water. Since CO2 becomes carbonic acid when dissolved, the removal of CO2 results in a higher pH (i.e., reduced acidity). Therefore, as algal populations increase during the growing season, pH levels tend to rise. During large increases in the populations of planktonic algae (or “blooms”), pH levels may increase significantly. For example, a pH of 10 was reported during a 1983 algal bloom on the Potomac River estuary.