Litmus test (chemistry)
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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.[1] The mixture contains 10 to 15 different dyes. Most of the chemical components of litmus are likely to be the same as of orcein, but in other proportions. In contrast with orcein, the principal constituent of litmus has average molecular weight of 3300.[2] Acid-base indicators on litmus owe their properties to 7-hydroxyphenoxazone chromophore.[3] Some fractions of litmus were given names (erythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein and leucazolitmin). Azolitmin does show nearly the same effect as litmus.[4]
| Litmus (pH indicator) | ||
| below pH 4.5 | above pH 8.3 | |
| 4.5 | ↔ | 8.3 |
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[edit] History
Litmus was used for the first time about 1300 AD by Spanish alchemist Arnaldus de Villa Nova. From the 16th century on, the blue dye was extracted from some lichens, especially in the Netherlands.
[edit] 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).[citation needed]
[edit] Uses
The main use is to test whether the 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. For instance, ammonia gas, which is alkaline, colors the litmus paper blue.
Other reactions can cause a color-change to litmus paper; for instance, chlorine gas turns blue litmus paper white – the litmus paper is bleached.[5] This reaction is irreversible and therefore here the litmus is not acting as an indicator.
[edit] See also
- Dipstick
- Universal indicator - Modern paper measures pH 1 to 14 with distinct colors for each. The color chart is usually included with the strips.
- Nitrazine strips - Measures pH about 4.5 to 7.5 with more precision
[edit] References
- ^ Römpp Chemie Lexikon - Version 1.0, Stuttgart/New York: Georg Thieme Verlag 1995 (Germany)
- ^ Beecken, H., E-M. Gottschalk, U. v Gizycki, H. Krämer, D. Maassen, H-G. Matthies, H. Musso, C. Rathjen, Ul. Zdhorszky. Orcein and Litmus. Biotechnic & Histochemistry 2003, 78, no. 6, pp 289-302
- ^ H. Musso, C. Rathjen. Orcein dyes. X. Light absorption and chromophore of litmus. Chem. Ber. 1959, 92, pp 751-753
- ^ E.T. Wolf: Vollständige Übersicht der Elementar-analytischen Untersuchungen organischer Substanzen, S.450-453, veröffentlicht 1846, Verlag E. Anton (Germany)
- ^ UCC - Chlorine
