pH

It is a measure of the activity of hydrogen ions (H+) in a solution and, therefore, defines whether it is an acid or alkali.

The term pH means hydrogen potential and it has a numerical value between 1 and 14, without units. Solutions with a pH less than seven are acidic, while those with a pH greater than seven are alkaline. pH 7 is considered neutral because it is the accepted pH of pure water at 25°C, although strictly, pure water cannot be assigned a pH value since it is non-ionic.