- Litmus test (chemistry)
Litmus is a
water -soluble mixture of differentdye sextract ed fromlichens , especially "Roccella tinctoria". The mixture hasCAS number 1393-92-6. It is often absorbed ontofilter paper . The resulting piece of paper or solution with water becomes apH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turnsred underacid ic conditions and red litmus paper turnsblue under basic (i.e. alkaline) conditions, the color change occurring over thepH 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 andleucazolitmin ). 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 somelichen s especially in theNetherlands .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 2008Uses
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 stripsReferences
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