pH
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pH, a measure of the acidity or the basicity of an aqueous solution. The symbol "pH" represents
the power (or exponent) of the hydrogen ion concentration, (H +), in units of moles of H+ per liter
of solution. It is defined as the negative logarithm of that concentration, that is, pH = −log[H+].
The definition of pH was formulated originally to obtain values that are simple, often positive,
numbers for moderately weak acidic or basic solutions and for dilute solutions of strong acids or
bases. Thus, a pH of 1 represents a hydrogen ion concentration of 10−1 moles of H+ per liter of
solution, while a pH of 7 represents a concentration of 10−7, and a pH of 12 represents a
concentration of 10−12.
Pure water, at 25 °C, has a pH of 7.00 and is neutral—neither acidic nor basic. Its concentration
of H+ ions is exactly equal to the OH− (hydroxide) ion concentration. An acidic solution is one
that has a greater abundance of H+ ions than of OH− (hydroxide) ions; a basic solution has more
OH− ions than H+. Thus, based on the negative logarithmic relationship in the above formula,
acidic solutions have pH values less than 7.00 at 25 °C, and basic solutions have pH values
greater than 7.00.
The fact that pH is a logarithmic measure has the important consequence that each unit of pH
represents a factor of 10. In the case of rainwater, for example, rain with a pH of 4 is 10 times as
acid as rain with a pH of 5. Similarly, rain with a pH of 3 is 100 times and rain with a pH of 2 is
1,000 times as acid as rain with a pH of 5.
Zvi C. Kornblum
The Cooper Union
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How to cite this article:
MLA (Modern Language Association) style:
Kornblum, Zvi C. "pH." Encyclopedia Americana. 2010. Grolier Online. 24 July. 2010
<http://ea.grolier.com/article?id=0310170-00>.
Chicago Manual of Style:
Kornblum, Zvi C. "pH." Encyclopedia Americana. Grolier Online
http://ea.grolier.com/article?id=0310170-00 (accessed July 24, 2010).
APA (American Psychological Association) style:
Kornblum, Z. C. (2010). pH. Encyclopedia Americana. Retrieved July 24, 2010, from Grolier
Online http://ea.grolier.com/article?id=0310170-00
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