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Terms & Synonyms

Official WHO Definition

The pH of a solution is the negative common logarithm of the hydrogen ion activity: pH = −log (H+) (WHO 2007)An expression of the intensity of the basic or acid condition of a liquid (WHO 2006).

Other Definitions


Interpretations and Explanations

Most substances have a pH in the range 0 to 14, although extremely acidic or extremely basic substances may have pH less than 0 or greater than 14.The pH of water is a measure of the acid-base equilibrium and, in most natural waters, is controlled by the carbon dioxide-bicarbonate-carbonate equilibrium system (WHO 2007). An increased carbon dioxide concentration will therefore lower pH, whereas a decrease will cause it to rise. Temperature will also affect the equilibria and the pH. In pure water, a decrease in pH of about 0.45 occurs as the temperature is raised by 25°C. In water with a buffering capacity imparted by bicarbonate, carbonate and hydroxyl ions, this temperature effect is modified. The pH of most raw water lies within the range 6.5-8.5.pH is an important consideration in the management/controlling of disinfection by-product formation arising from treatment processes (Amy et al. 1987 and Stevens et al. 1976).


Amy, G.L., Chadik, P.A., and Chowdhury, Z.K. (1987) Developing models for predicting trihalomethane formation potential and kinetics. J. Am. Water Works Assoc., 79: 89.Stevens, A.A., Slocum, C.J., Seeger, D.R., and Robeck, G.G. (1976) Chlorination of organics in drinking water. J. Am. Water Works Assoc., 68: 615.WHO (2007) pH in Drinking-water: Revised background document for development of WHO Guidelines for Drinking-water Quality.WHO (2006) Guidelines for the safe use of wastewater, excreta and greywater

WHO (1996) Guidelines for drinking-water quality. Volume 2: Health criteria and other supporting information.

WHO Lexicon page (translations and examples)

See also


External Resources


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