Why Measure pH?

The Main Points

  1. pH is an important parameter for assessing the health of a waterbody.
  2. pH changes can indicate external stresses such as acid rain or discharges
  3. pH can predict what kinds of chemicals can be released from the sediment into the water column.

The Details

pH is a term used to indicate the alkalinity or acidity of a substance as ranked on a scale from 1.0 to 14.0. Acidity increases as the pH gets lower.

Water pH has profound effects on many chemical and biological processes.

As we have seen with the other water quality parameters (i.e. conductivity, dissolved oxygen, and turbidity), different organisms flourish within different ranges of pH. Most aquatic animals prefer a range of 6.5-8.0. Values outside this range stress the organisms and lead to a number of adverse effects. For example, reproduction rates can fall. Low pH can also allow toxic elements and compounds to become mobile and "available" for uptake by aquatic plants and animals. This can produce conditions that are toxic to aquatic life, particularly to sensitive species like rainbow trout. Changes in pH can be caused by atmospheric deposition (acid rain), weathering of the surrounding rocks, and certain wastewater discharges.

The pH of the water column can also affect the chemistry of the lake, river, or pond. Increased amounts of nutrients can be released from the sediments under alkaline conditions than under acidic conditions. Conversely, metals can be released from sediments under low pH or acidic conditions.

The pH scale measures the logarithmic concentration of hydrogen (H+) and hydroxide (OH-) ions, which make up water (H+ + OH- = H2O). When both types of ions are in equal concentration, the pH is 7.0 or neutral. Below 7.0, the water is acidic (there are more hydrogen ions than hydroxide ions). When the pH is above 7.0, the water is alkaline, or basic (there are more hydroxide ions than hydrogen ions). Since the scale is logarithmic, a drop in the pH by 1.0 unit is equivalent to a 10-fold increase in acidity. A water sample with a pH of 5.0 is 10 times as acidic as one with a pH of 6.0, and pH 4.0 is 100 times as acidic as pH 6.0.