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 basespH 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.
- Isaac Feldman. 1956. Use and Abuse of pH measurements. Analytical Chemistry 28: 1859.
- J.Mendham, R.C. Denney, J.D. Barnes, M.J.K.Thomas, and R.C..Denney. 2000. Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, ISBN 0-582-22628-7 Section 13.23, "Determination of pH"
- C.Michael Hogan. 2010. Abiotic factor. Encyclopedia of Earth. Topic ed. Emily Monosson Ed.-in-Chief Cutler J. Cleveland. National Council for Science and the Environment. Washington DC
- Roger G.Bates. 1973. Determination of pH: theory and practice. Wiley