pH: Percent Hydrogen
pH measures the concentration of hydrogen ions, or protons (H+), in a solution. The higher the proton concentration, the more acidic the solution. The lower the proton concentration, the more basic the solution. pH is calculated by taking the negative log of the proton concentration. For example, if a solution had a proton concentration of 1.0 * 10-7, the pH of that solution would be seven, or neutral. An acidic solution, on the other hand, would have a pH in the range of zero to seven, while the pH of a basic solution would range from seven to fourteen. Each whole number change on the pH indicates a tenfold change in the proton concentration. For example, a solution with a pH of four is ten times more acidic as a solution with a pH of five. Some common examples of acids include citrus fruits, vinegar, and hydrochloric acid (HCl). Some common bases include baking soda, detergent, and household ammonia.
pH of Rainfall
Natural rainfall is slightly acidic, with a pH of about 5.6. This natural acidity results from the reaction between carbon dioxide and water vapor in the atmosphere to produce carbonic acid: CO2 + H2O → H2CO3. However, if enough sulfur and nitrogen oxides are present in the atmosphere, the pH of rainfall will drop below five, and at this point it can be classified as acid rain.
pH of Aquatic Environments
Most freshwater aquatic ecosystems have a natural pH ranging from six to eight. However, acid rain can cause this number to decrease significantly, and in doing so can produce many harmful ecological effects.
Most fish thrive within a pH range of six to seven. However, as this pH decreases and approaches five, plankton and mosses may invade, as these species prefer acidic conditions. Some species of fish have difficulty competing with the plankton and mosses for light and nutrients, and as a result their populations begin to decrease. Below a pH of five, entire populations of fish disappear, undecayed organic matter accumulates at the bottom of the lake or stream, and mosses dominate areas near the shore. Below a pH of four-and-a-half, the ecosystem is essentially devoid of fish.
Thinkquest Team "Fish," March 2005, Disclaimer and copyright information
