Litmus test (chemistry)

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Litmus (pH indicator)
below pH 4.5 above pH 8.3
4.5 ↔ 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 colour 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 (Erythrolein (or Erythrolitmin), Azolitmin, Spaniolitmin, Leucoorcein and Leucazolitmin). Pure Azolitmin does show nearly the same effect as litmus.[2]

Litmus powder

Contents

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

[edit] History

Litmus was used the first time about 1300 AD by Spanish alchemist Arnaldus de Villanova. From 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).[3]

Parmelia sulcata

[edit] Uses

To test the pH of some gases using litmus paper, wet litmus paper is needed. The gas dissolves in the water and thus the reaction can be seen with the eye.

The most common litmus reactions are when used under acidic or basic (i.e. alkaline) conditions. Blue Litmus Paper turns red when dipped into an acid, Red litmus paper turns blue when dipped into a base. Yellow litmus paper also exists. However, these aren't the only reactions. For example, chlorine gas turns blue litmus paper white—chemically speaking, the litmus paper is bleached. This latter reaction is irreversible and the litmus here is therefore not acting as an indicator.

To find out if a substance is neutral, one needs a blue and red sheet of litmus paper. When the substance is placed on it, the colour should remain the same for both. Also, this will identify substances with a pH over 4.5, but under 8.3 (in other words, weak acids and alkalis).

Litmus paper is often used in electronics to determine if damage to electronics was caused by the user (i.e. moisture damage) in the case of repairs claimed under warranty.