Along with universal indicator, litmus paper is one of the most commonly encountered pH indicators in school chemistry lessons. Unlike the range of colours produced by the former, litmus is pink-red in acidic solutions and blue in alkaline solutions. This graphic highlights its complex origins in lichens and the chemical changes that account for its colour change.
Litmus has a long chemical history, going back to before chemists even established definitions for acids and alkalis. It’s claimed that a Spanish alchemist, Arnaldus de Villa Nova, was using litmus as early as the 14th century, but it wasn’t in widespread use until two centuries later. At this time, production was carried out primarily in the Netherlands, and it’s possible that our name from it comes from the Dutch lackmoes, roughly translating as ‘lacquer pulp’. Another etymological link is the Old Norse litmosi, or ‘moss dye’.
This name almost betrays litmus’s origins: it’s extracted from particular species of lichen. The only flaw in this lingual link is that lichens and mosses are distinct from each other; mosses are plants, and lichens are definitely not. Lichens aren’t even a single organism, but rather a symbiotic relationship between a fungus and one or two species of algae.
Litmus is produced from extracts from several species of lichen which grow on rocks. A British Lichen Society bulletin from 1982 references a company in the Netherlands that it claims, at the time, was “probably the only company now in the western world that produces litmus from lichens.” Today, production appears to be slightly more widespread, with lichen species from California to Mozambique being used to extract it.
The species of lichen that litmus is derived from all contain chemical compounds called orsellinic acid depsides, which can be converted to another compound, orcinol. Adding ammonia and oxygen to orcinol produces a mixture of dyes collectively known as orcein dyes, historically used to colour wool and silk. In the case of litmus, lime (calciym hydroxide), potash (potassium carbonate) and gypsum (calcium sulfate) are added as well to produce the final dye.
Litmus dye is not one single dye, but a mixture of long, polymeric molecules. Of these, the primary constituent consists of several hydroxyorcein units linked together, as shown in the graphic. Litmus dye gets its colour from repeating sections of these larger molecules.
Subtle structural changes in these sections are sufficient to cause the colour change. In acidic solutions below pH 5, the addition of a proton (hydrogen ion) to the nitrogen atom causes a red-pink colour. In alkaline solutions above pH 8, the loss of a proton from one of the oxygen atoms produces a blue colour. In between these two extremes, at neutral pH the litmus dye is a purple colour.
As those who’ve used it in school laboratories will know, litmus paper isn’t just limited to testing the pH of solutions. If dampened, it can be used to test the acidity of gaseous substances, too. For example, ammonia will turn red litmus blue, while chlorine will turn blue litmus red – though, as a bleaching agent, will ultimately turn it white.
Other uses of litmus include as a test to distinguish between different types of bacteria in milk. ‘Litmus test’ is also a chemical term that has passed into common use as a phrase meaning “an opinion or action that is thought to show more general opinions or likely future actions.”
References and further reading