Physics & Chemistry


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The lichen Parmelia sulcata does not thrive in low pH settings. Source: Kristian Peters

The term pH is a convenient way to express the concentration of hydrogen ions (H+) in an aqueous solution. The lower case p in pH stands for "power of" with H being the symbol for the element hydrogen. Mathematically, it is the -log of the concentration in molarity of hydrogen ions in a solution. For chemists, the term hydronium ion (H3O+ ) is often substituted for hydrogen ion to reflect the association of a hydrogen ion with a molecule of water.

Acids and bases

caption pH values for common substances. Source: Edward Stevens

Water undergoes slight dissociation into hydrogen ions (H+) and hydroxyl ions (OH-). When the concentrations of these two ions are equal, the solution is considered neutral. If the concentration of hydrogen ions is larger than the concentration of hydroxl ions, the solution is acidic. If hydroxyl ions are in greater concentration, the solution is considered alkaline or basic. Pure water at room temperature will have a neutral pH of 7.00. Values of pH below 7.00 are found in acidic solutions while values above 7.00 characterize basic solutions.

Relation to biota

An appropiate pH value is important to life on Earth. The pH of a solution will affect mineral solubility, as well as the solubility and structure of organic molecules and protein structures. For most organisms on Earth, a pH value of five to nine is most suitable for optimal organism metabolism, but each taxon resides in an ecological niche where its specific pH optimum is different from the optimum pH for other species. Certain organisms known as extremophiles can thrive in zones of unusually high pH, temperature, pressure, or radiation environment.

The pH of natural rainwater is slightly acidic, around 5.6, because carbon dioxide dissolves in water and forms carbonic acid. Human blood has a pH of 7.3-7.5. Seawater has a pH of 7.8-8.3





Haustein, C. (2011). pH. Retrieved from


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