Litmus is a water soluble mixture of different from . It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for . In an acidic medium, blue litmus paper turns red, while in a basic or medium, red litmus paper turns blue. In short, it is a dye and indicator which is used to place substances on a pH scale.
History
The word "litmus" comes from the
Old Norse word "litmosi" meaning "colour moss" or "colouring moss"
/ref> About 1300, the
Spaniards physician Arnaldus de Villa Nova began using litmus to study acids and bases.
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Natural sources
Litmus can be found in many different species of
lichens. The dyes are extracted from such species as
Roccella tinctoria (South American),
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).
Usage
The main use of litmus is to test whether a solution is acidic or basic, as blue litmus paper turns red under
acid conditions, and red litmus paper turns blue under basic or
alkali conditions, with the color change occurring over the pH range 4.5–8.3 at . Neutral litmus paper is purple.
Wet litmus paper can also be used to test for water-soluble gases that affect
acidity or
basicity; the gas dissolves in the
water 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 an aqueous 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 is
Chemistry
The litmus mixture has the
CAS 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 as
orcein but in different proportions. In contrast with orcein, the principal constituent of litmus has an average molecular mass of 3300.
Acid-base indicators on litmus owe their properties to a
7-hydroxyphenoxazone chromophore.
Some fractions of litmus were given specific names including
erythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein, and leucazolitmin. Azolitmin shows 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).]
A recipe to make litmus out of the lichens, as outlined on a UC Santa Barbara website says:
Mechanism
Red litmus contains a weak
diprotic acid. When it is exposed to a basic compound, the
hydrogen 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.
