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Litmus powderChemical structure of 7-hydroxyphenoxazone, thechromophore of litmus components
Substance to test chemical acidity
Litmus is awater-soluble mixture of differentdyesextracted fromlichens. It is often absorbed ontofilter paper to produce one of the oldest forms ofpH indicator, used to test materials foracidity. In an acidic medium, blue litmus paper turns red, while in a basic oralkaline medium, red litmus paper turns blue. In short, it is a dye and indicator which is used to place substances on a pH scale.[citation needed]
The word "litmus" comes from theOld Norse word "litmosi" meaning "colour moss" or "colouring moss". The word is attested only in one Mediaeval source, a Norwegian law codex from 1316 in a chapter on customs and excise duties on pelts and furs.[1] About 1300, theCatalan physicianArnaldus de Villa Nova began using litmus to study acids and bases.[2][3]
The main use of litmus is to test whether a solution isacidic orbasic, as blue litmus paper turns red underacidic conditions, and red litmus paper turns blue underbasic oralkaline conditions, with the color change occurring over thepH range 4.5–8.3 at 25 °C (77 °F). Neutral litmus paper is purple.[2] Wet litmus paper can also be used to test for water-soluble gases that affectacidity orbasicity; the gas dissolves in thewater and the resulting solution colors the litmus paper. For instance,ammonia gas, which is alkaline, turns red litmus paper blue.
Litmus can also be prepared as anaqueous solution that functions similarly. Under acidic conditions, the solution is red, and under alkaline conditions, the solution is blue.
Chemical reactions other than acid–base can also cause a color change to litmus paper. For instance,chlorine gas turns blue litmus paper white; the litmus dye isbleached[4] becausehypochlorite ions are present. This reaction is irreversible, so the litmus is not acting as an indicator in this situation.
The litmus mixture has theCAS number 1393-92-6 and contains 10 to around 15 different dyes. All of the chemical components of litmus are likely to be the same as those of the related mixture known asorcein but in different proportions. In contrast with orcein, the principal constituent of litmus has an average molecular mass of 3300.[5] Acid-base indicators on litmus owe their properties to a7-hydroxyphenoxazonechromophore.[6] Somefractions of litmus were given specific names includingerythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein, and leucazolitmin. Azolitmin shows nearly the same effect as litmus.[7]
A recipe to make litmus out of the lichens, as outlined on aUC Santa Barbara website says:[8]
Details are difficult to find because the processes were kept secret.
Stir the lichens from time to time and the color changes from red to purple and finally blue after about four weeks. The lichens are then dried and powdered. At this stage the lichens contain partly litmus and partlyorceinpigments. The orcein is removed by extraction withalcohol, leaving the pure blue litmus. It is marketed as blue lumps, masses, or tablets, after mixing with colorless compounds such aschalk andgypsum. Litmus paper is paper impregnated with this substance.
Red litmus contains a weakdiprotic acid. When it is exposed to a basic compound, thehydrogen ions react with the added base. The conjugate base formed from the litmus acid has a blue color, so the wet red litmus paper turns blue in an alkaline solution.
^abcNeupert, Manfred (January 31, 2013). "Lackmus".Römpp Lexikon Chemie (in German).
^Surber, Christian; Humbert, Philippe; Abels, Christoph;Maibach, Howard (2018). Surber, Christian; Abels (eds.).pH of the Skin: Issues and Challenges - Current Problems in Dermatology. Basel: Karger Medical and Scientific Publishers. p. 3.ISBN9783318063851. Retrieved1 May 2023.In the early 14th century, the Spanish scholar, Arnaldus de Villa Nova (1235–1311), began to use litmus for studying acids and bases. This compound, extracted from a lichen, had been used as a dye since at least the time of the Vikings, but he was the first person known to use it as a test of acidity.
^O'Leary, Donal (2000)."Chlorine".The Chemical Elements. Archived fromthe original on 2008-12-21.
^Beecken, H.; E.-M. Gottschalk; U. v. Gizycki; H. Krämer; D. Maassen; H.-G. Matthies; H. Musso; C. Rathjen; Ul. Zdhorszky (2003). "Orcein and Litmus".Biotechnic & Histochemistry.78 (6):289–302.doi:10.1080/10520290410001671362.PMID15473576.S2CID41944320.
^H. Musso, C. Rathjen (1959). "Orcein dyes. X. Light absorption and chromophore of litmus".Chem. Ber.92 (3):751–753.doi:10.1002/cber.19590920331.
^E. T. Wolf: Vollständige Übersicht der Elementar-analytischen Untersuchungen organischer Substanzen, S. 450–453, veröffentlicht1846, Verlag E. Anton (Germany).
