Water (H20) contains both H+ (hydrogen) ions and OH- (hydroxyl) ions. The pH test measures the H+ ion concentration of liquids and substances. Each measured liquid or substance is given a pH value on a scale that ranges from 0 to 14.
Pure deionized water contains equal numbers of H+ and OH- ions, and has a pH of 7. It is considered neutral, neither acidic nor basic. If a water sample has more H+ than OH- ions, it is considered acidic and has a pH less than 7. If the sample contains more OH- ions than H- ions, it is considered basic with a pH greater than 7.
It is important to remember that for every one unit change on the pH scale, there is approximately a ten-fold change in how acidic or basic the sample is. For example, the average pH of rainfall over much of the northeastern United States is 4.3, or roughly ten times more acidic than normal rainfall of 5.0-5.6. Lakes of pH 4 (acidic) are roughly 100 times more acidic than lakes of pH 6.
Human-Caused Changes in pH
In the U.S., the pH of natural water is usually between 6.5 and 8.5, although wide variations can occur. Increased amounts of nitrogen oxide (NOx) and sulfur dioxide (SO-2), primarily from automobile and coal-fired power plant emissions, are converted to nitric acid and sulfuric acid in the atmosphere. These acids combine with moisture in the atmosphere and fall to earth as acid rain or acid snow.
Acid rain is responsible for thousands of lakes in eastern Canada, northeastern United States, Sweden, and Finland becoming acidic. In many areas of the United States, the type of rocks and minerals present determine the acidity of the local water. If limestone is present, the alkaline (basic) limestone neutralizes the effect the acids might have on lakes and streams.
The areas hardest hit by acid rain and snow are downwind of urban/industrial areas and do not have any limestone to reduce the acidity of the water.
Changes in Aquatic Life
Changes in the pH value of water are important to many organisms. Most organisms have adapted to life in water of a specific pH and may die if it changes even slightly. This has happened to brook trout in some streams in the Northeast.
At extremely high or low pH values (e.g., 9.6 or 4.5) the water becomes unsuitable for most organisms. Serious problems occur in lakes with a pH below 5, and in streams that receive a massive acid dose as the acidic snow melts in the spring. Immature stages of aquatic insects and young fish are extremely sensitive to pH values below 5.
Very acidic waters can also cause heavy metals, such as copper and aluminum, to be released into the water. Heavy metals can accumulate on the gills of fish or cause deformities in young fish, reducing their chance of survival.
pH Sampling Procedure
Like the sample collected for the dissolved oxygen test, the water sample for the pH test should be collected away from the river bank and below the surface. If possible, use an extension rod sampler.
The sample must be measured immediately because changes in temperature can affect the pH value. If pH must be measured later, the sample should be placed on ice and measured as soon as possible.
If the pH of the same water sample is tested more than once, the most common pH value (the mode) should be reported, not the average value.
pH Testing Procedure
