Litmus is a water-soluble mixture of different dyes extracted from lichens, specially Roccella tinctoria. The mixture has CAS number 1393-92-6. It is often absorbed on to filter paper. The resulting piece of paper or solution with water becomes a pH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions, the colour change occurring over the pH range 4.5-8.3 (at 25°C). Neutral litmus paper is purple in colour. The mixture contains 10 to 15 different dyes (Erythrolein (or Erythrolitmin), Azolitmin, Spaniolitmin, Leucoorcein and Leucazolitmin). Pure Azolitmin does show nearly the same effect as litmus.