is the number arises from a measure of the activity of hydrogen ions (or their equivalent) in the solution. The pH scale is an inverse logarithmic representation of hydrogen proton (H⁺) concentration.  pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25°C with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). When a pH level is 7.0, it is defined as 'neutral' at 25°C because at this pH the concentration of H₃O⁺ equals the concentration of OH^- in pure water. pH is formally dependent upon the activity of hydronium ions (H₃O⁺).

The pH scale ranges from 0 (very acidic) to 14 (very alkaline or base), with a pH of 7 indicating a neutral solution equivalent to the pH of distilled water.