What is litmus paper of different colors


Violet blue powder
or pieces
molar mass varies

AGWnot specified
1 g / cm3
Melting point
decomposes when heated
Water solubility
Conc. At 20 ° C 10 to 20 g / l
--HP rates P 280.1-3
G 3
Print a labelGerman designationEnglish designation
CAS 1393-92-6litmus

The blue-violet litmus powder is a mixture of different dyes. It is also available in small pieces. The dyes are made from different white lichen species of the genus Rocella that grow, for example, in Scandinavia, the Canary Islands, Algeria or Mozambique. The powder is not very soluble in water: 10 to 20 grams are soluble in one liter of the concentrated solution, depending on the variety. Litmus powder is slightly irritating to the skin, eyes and mucous membranes.

Although litmus was already used as a reagent by alchemists in the Middle Ages, making it from lichen was a well-kept secret for a long time. The phenoxazone dye litmus does not occur in lichens, but it can be obtained from the raw materials it contains. The explanation of the structural formula did not succeed until the 1960s by the German chemist Hans Musso (1925–1988). The litmus molecule is very complex:

Litmus molecule: The middle group (marked in red) occurs three to five times in the chain.

Litmus is suitable as an acid-base indicator. At a pH value below 4.5 it is red, above 8.3 it appears blue. In between there is a transition phase that varies depending on the composition of the product. The effect of the color changes from red to blue is mainly based on the phenolic groups, which can form phenolates by releasing protons.

The shredded lichens used to be placed in an open wooden trough. There was also an aqueous solution with potash and lime. The added urine produced the necessary ammonia, which, together with the oxygen in the air, converted the substances from the lichen into a mixture of dyes. The mass was warmed up slightly, shaken regularly and changed from brown to red to blue within a maximum of three weeks. The resulting dough was formed into a cake and dried.

In today's industrial processes, ammonium carbonate is used to produce the ammonia instead of urine. Added chalk or plaster of paris will shape the powder into small pieces or cubes. The orcein dyes can also be extracted with alcohol.

Litmus used to be a widely used food coloring. As a brightener, it was used to blue laundry. Litmus milk is a solution of litmus in skimmed milk and water. In microbiology, it is used to test whether lactic acid bacteria can ferment lactose. When the color changes from blue to red, the formation of lactic acid is detected.

Enlarge image

Blue litmus paper turns red when hydrochloric acid is dropped on it.
Red litmus paper turns blue with caustic soda.

Paper soaked and dried with litmus and the aqueous, one to two percent solution have been used as indicators in chemical laboratories for a long time. Red litmus paper turns blue with alkaline solutions. "Blue" or better purple litmus paper turns red with acidic solutions.