Litmus paper is a water-soluble mixture of various pigments extracted from lichens.
This mixture is usually absorbed by a filter paper which is then used as a pH indicator to measure the pH and alkalinity of the materials. A blue litmus leaf turns red when exposed to acid, and a red litmus leaf turns blue when exposed to an alkaline or basic substance.
It was first used in the 1300s by the Spanish chemist Arnolds de Vila Nova.
The litmus test is a quick way to determine whether a liquid or gaseous solution is acidic or alkaline. The test can be performed using litmus paper and is either red or blue.
The test is quick and simple, but it has a few limitations.
Litmus paper is useful as a base indicator for general acid, but you can get more specific results if you use an indicator that has a more narrow test range. Orcein and Azolitmin yield similar results to those found in litmus paper.