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 colour.^[1] 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.^[2]

Litmus (pH indicator)
below pH 4.5		above pH 8.3
4.5	↔	8.3

Contents 1 History 2 Natural sources 3 Uses 4 See also 5 References

[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.

Other reactions can cause a color-change to litmus paper; for instance, chlorine gas turns blue litmus paper white – the litmus paper is bleached.^[3] This reaction is irreversible and therefore here the litmus is not acting as an indicator.

Damp red litmus paper turns blue in the presence of Ammonia gas.^{[citation needed]}

[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