Ways to take a pH measurement

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