Danish biochemist S.P.L Sorensen used the measurement to represent the hydrogen ion concentration, expressed in equivalents per litre of an aqueous solution: pH = −log[H+]

The pH can be read with a less accurate litmus paper or with indicator dyes. However, it is usually measured with a pH meter that consists of a voltmeter attached to a pH-responsive electrode (usually mercury-mercurous chloride electrode) and a reference electrode (usually glass that are immersed in a solution. The voltmeter measures the potential difference between the glass and reference electrodes

The term is used in chemistry, biology, and agronomy. PH translates the values of the concentration of the hydrogen ion into numbers between 0 and 14.

• Pure water corresponds to a pH of 7, which is neutral (neither acidic nor alkaline). 
• A pH less than 7 is co...