Litmus test (chemistry)
|Litmus (pH indicator)|
|below pH 4.5||above pH 8.3|
Litmus is a water-soluble mixture of different dyes extracted from lichens, specially Roccella tinctoria. The mixture has CAS number 1393-92-6. It is often absorbed on to 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. The mixture contains 10 to 15 different dyes (Erythrolein (or Erythrolitmin), Azolitmin, Spaniolitmin, Leucoorcein and Leucazolitmin). Pure Azolitmin does show nearly the same effect as litmus.
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).
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. 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 or turn purple for both.