The potential of hydrogen. Degree of acidity or alkalinity of a chemical solution 1.

A measure of the degree of the acidity or the alkalinity of a solution as measured on a scale (pH scale) of 0 to 14. The midpoint of 7.0 on the pH scale represents neutrality, i.e., a ‘neutral’ solution is neither acid nor alkaline. Numbers below 7.0 indicate acidity; numbers greater than 7.0 indicate alkalinity. It is important to understand that pH is a measure of intensity, and not capacity; i.e., pH indicates the intensity of alkalinity in the same way temperature tells how hot something is – but not how much heat the substance carries.

More specifically, pH is the logarithm of the reciprocal of the hydrogen ion concentration in a solution. The hydrogen ion concentration is the weight of hydrogen ions, in grams, per liter of solution. In neutral water, for example, the hydrogen ion concentration is 10E-7 grams per litre; the pH is therefore 7.

Since it is hydrogen that is responsible for acidity and alkalinity, the abbreviation pH is believed to stand for either ‘power of hydrogen’ or ‘potential of hydrogen’. The neutral point of 7.0 actually indicates the presence of equal concentrations of free hydrogen and hydroxide ions.



1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston