System of measuring the acidity or alkalinity of a substance; refers to the negative logarithm of the hydrogen ion content of the solution. pH values run from 1 to 14; a pH of 7 indicates that a substance is neutral. A value of more than 7 indicates the substance is basic (alkaline) and a value of 11 or more indicates it is very basic and is likely to cause corrosion and/or tissue damage. Likewise, a value of less than 7 indicates that the substance is acidic, and a value of 3 or less indicates it is a strong acid.