Litmus Paper Definition

Litmus paper is filter paper which has been treated with a natural water-soluble dye obtained from lichens. The resulting piece of paper, called "litmus paper", can be used as a pH indicator. Blue litmus paper turns red under acidic conditions (pH below 4.5) while red litmus paper turns blue under alkaline conditions (pH above 8.3). Blue litmus does not change color under alkakine conditions, while red litmus paper does not change color under acidic conditions. Neutral litmus paper is purplish in color. Neutral litmus paper turns red under acidic conditions and blue under alkaline conditions.

While litmus paper may be used to determine whether an aqueous solution is an acid or a base, it not good for estimating the pH value of the liquid.

History and Composition

Spanish physician Arnaldus de Villa Nova first used litmus paper around 1300 AD. Originally, litmus was a blue dye obtained from any of a number of lichen species found in the Netherlands. Today, litmus is prepared mainly from the species Roccella montagnei from Mozambique and Dedographa leucophoea from California. However, litmus may contain from 10 to 15 different dyes.

How Litmus Paper Works

Red litmus contains a weak diprotic acid. Upon exposure to a base, hydrogen ions from the acid react with the base, producing a color change to blue. Blue litmus paper, on the other hand, already contains the blue conjugate base. It reacts with an acid to change to red.