A water-soluble dye extracted from certain lichens. It turns red under acid conditions and blue under alkaline conditions, the colour change occurring over the pH range 4.5–8.3 (at 25°C). It is not suitable for titrations because of the wide range over which the colour changes, but is used as a rough indicator of acidity or alkalinity, both in solution and as litmus paper (absorbent paper soaked in litmus solution).