universal indicator

In chemistry, a mixture of pH indicators, used to gauge the acidity or alkalinity of a solution. Each component changes colour at a different pH value, and so the indicator is capable of displaying a range of colours, according to the pH of the test solution, from red (at pH 1, strong acid) through green (neutral) to purple (at pH 13, strong alkali).

The pH of a substance may be found by adding a few drops of universal indicator and noting the colour, or by dipping in an absorbent paper strip that has been impregnated with the indicator.