A Brief Discussion of pH

Typically, pH is a measure of the acidity or basicity of a substance or solution. In technical terms, pH is defined as the negative base-10 logarithm of the hydrogen ion concentration or activity expressed in moles per liter of solution. The reason why a logarithmic scale is used is because in most natural waters the hydrogen ion activity is too small to be easily expressed in common concentration units such as milligrams or micrograms per liter. The pH range is typically reported within a range of 0 to 14 with 7 being neutral, less than 7 being acidic, and greater than 7 being basic or alkaline. Another way of looking at this is that solutions are considered acidic when the hydrogen ion (H⁺) concentration or activity is greater than the hydroxide (OH^-) concentration or activity. When the hydroxide (OH^-) activity or concentration is greater than the hydrogen ion (H⁺) activity or concentration, the solution is considered basic or alkaline. Expressing pH in terms of a logarithmic scale therefore means every whole unit change is actually a ten-fold change from the preceding unit. It should be noted that while the pH range is commonly from 0 to 14, it is possible to have pH values less than 0 and greater than 14 in extreme situations or environments.

For further information on pH, how it is measured, and its importance refer to the following:

USGS Office of Water Quality National Field Manual
Wikipedia definition of pH
USGS Water Science for Schools Water properties: pH
Water on the Web: pH

Last updated on January 13, 2009