Litmus test (chemistry)

Litmus test (chemistry)

Litmus is a water-soluble mixture of different dyes extracted from lichens, especially "Roccella tinctoria". The mixture has CAS number 1393-92-6. It is often absorbed onto 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 color change occurring over the pH range 4.5-8.3 (at 25°C). Neutral litmus paper is purple in color. [Römpp Chemie Lexikon - Version 1.0, Stuttgart/New York: Georg Thieme Verlag 1995 (Germany)] The mixture contains 10 to 15 different dyes (erythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein and leucazolitmin). Pure azolitmin does show nearly the same effect as litmus. [E.T. Wolf: Vollständige Übersicht der Elementar-analytischen Untersuchungen organischer Substanzen, S.450-453, veröffentlicht 1846, Verlag E. Anton (Germany)]

History

Litmus was used the first time about 1300 CE by Spanish alchemist Arnaldus de Villa Nova. From 16th century on the blue dye was extracted from some lichens especially in the Netherlands.

Natural sources

Litmus can be found in different species of lichens. Formerly, the dyes would be extracted from such species as "Roccella tinctoria" (South America), "Roccella fuciformis" (Angola and Madagascar), "Roccella pygmaea" (Algeria), "Roccella phycopsis", "Lecanora tartarea" (Norway, Sweden), "Variolaria dealbata", "Ochrolechia parella", "Parmotrema tinctorum" and "Parmelia". Currently, the main sources are "Roccella montagnei" (Mozambique) and "Dendrographa leucophoea" (California).Fact|date=July 2008

Uses

The main use is to test whether a solution is acidic or alkaline. Wet litmus paper can also be used to test water-soluble gases; the gas dissolves in the water and the resulting solution colors the litmus paper.

Other reactions can cause a color-change to litmus paper; for instance, chlorine gas turns blue litmus paper white – the litmus paper is bleached. [ [http://www.ucc.ie/academic/chem/dolchem/html/elem/elem017.html UCC - Chlorine] ] This reaction is irreversible and therefore here the litmus is not acting as an indicator.

To find out if a substance is neutral, a blue and red sheet of litmus paper is needed. When the substance is placed on it, the color should remain the same.

See also

*Dipstick
*Universal indicator
*Nitrazine strips

References


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