The pH value, according to DIN 38 38404 (Part 5, 2009 edition), is the negative decimal logarithm of the effective hydrogen-ion activity expressed in mol/l (concentration). It indicates the strength of the acidic or alkaline effect of an aqueous solution.
The scale for determining the pH value extends from 0 to 14. See Fig. 1 pH value
The pH value can be determined by various methods. The colorimetric analysis method uses indicators which show colour changes at certain pH values (titration), or - in the form of universal indicators (paper, strips etc.) - can be compared with appropriate colour comparison scales.
pH values can be measured with a particularly high degree of accuracy by the electronic method of determining pH value using electrodes (e. g. glass electrodes) and comparison solutions (buffer solutions), see also DIN 19260 and 19261.