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Symbol for the negative decadic logarithm of the H⁺ ion concentration (measured in moles per liter); a solution with pH 7.00 (1 × 10⁻⁷ g molecular weight of hydrogen per liter) is neutral at 22°C ( i.e., [H⁺] = [OH⁻]), one with a pH value of more than 7.00 is alkaline, one with a pH lower than 7.00 is acid. At a temperature of 37°C, neutrality is at a pH value of 6.80. Cf.:dissociation constant of water. [p (power or potency) of [H⁺ ]]

- blood pH pH of arterial blood; normal is 7.4 (normal range 7.36–7.44).

- critical pH the pH range, about 5.5, at which saliva ceases to be saturated with respect to calcium and phosphate, and below which tooth mineral will dissolve.

- optimum pH the pH at which an enzymatic or any other reaction or process is most effective under a given set of conditions.

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pH (')pē-'āch n a measure of acidity and alkalinity of a solution that is a number on a scale on which a value of 7 represents neutrality and lower numbers indicate increasing acidity and higher numbers increasing alkalinity and on which each unit of change represents a tenfold change in acidity or alkalinity and that is the negative logarithm of the effective hydrogen-ion concentration or hydrogen-ion activity in gram equivalents per liter of the solution <instead of saying that the concentration of hydrogen ion in pure water is 1.00 x 10^-7, it is customary to say that the \pH of pure water is 7 (Linus Pauling)> also the condition represented by such a number compare PK

a measure of the concentration of hydrogen ions in a solution, and therefore of its acidity or alkalinity. A pH of 7 indicates a neutral solution, a pH below 7 indicates acidity, and a pH in excess of 7 indicates alkalinity.