The determination of the pH of a substance is the measurement of the H+ ions found in that particular substance. pH is determined and recorded as a number between 0 and 14. Pure deionized water has a pH of 7 which is neutral. This means that the level of H+ and OH- ions in pure water are equal.
If the level of H+ ions increases, the substance is considered an acid and the pH number is below 7. If the level of OH- ions increases, the substance is considered to be alkaline or base and the pH number is above 7.
An acid has a range of 0 to any numerical value below 7. For example, 6.9 would be a weak acid. A base has a range of any numerical value above 7 to 14 with 7 being a neutral value. A one-unit change in the pH, from 4 to 3, is a ten-fold change in how acidic the substance has become. This means that a pH of 3 is ten times more acidic than a pH of 4.
In the United States the pH of most natural water systems range from 6.5-8.5, but wide variations can occur due to increases in the atmosphere of nitrogen oxides and sulfur dioxides from automobile and coal-fired power plant emissions. These oxides are converted to nitric and sulfuric acids in the atmosphere and fall to earth as acid rain or snow. This acidic precipitation can adversely affect the pH of aquatic ecosystems.
Most organisms are adapted to a specific pH level. When pH increases or decreases, the diversity of an ecosystem can be changed appreciably.