ISPE Glossary of Pharmaceutical and Biotechnology Terminology

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The pH value of an aqueous solution is a number describing its acidity or alkalinity. A pH is the negative logarithm (base 10) of the concentration of hydrogen ions (equivalent per liter). The pH value of a neutral solution is 7. An acidic solution has a pH less than 7, while a basic solution has a pH greater than 7, up to 14.


The negative log of the hydrogen ion concentration, is a measure of the concentration of hydrogen ions (H+) in a water-based solution. The more hydrogen ions that are present, the lower the pH and the more acidic the solution.

Publication Source: ISPE Baseline® Guide, Vol. 4: Water and Steam Systems (Second Edition)

Publication Date: 2011


A means of expressing hydrogen ion concentration in terms of the powers of 10; the negative logarithm of the hydrogen ion concentration. This is a measure of whether the water is acidic or basic. pH is a measure of the hydrogen ion concentration in the water.

Publication Source: ISPE Good Practice Guide: Technology Transfer

Publication Date: 2003

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