Why do acids stain litmus paper red

Sour makes red

Litmus, a pigment obtained from lichen, has been used as an indicator of acids and bases for centuries

Up until a few decades ago, litmus tincture or paper soaked in litmus were an important tool in every laboratory. The dye from lichen is a so-called acid-base indicator: it changes its color from blue to red when it changes from a basic to an acidic environment.

Litmus already played a role with the alchemists. The doctor and alchemist Arnaldus de Villanova is said to have used the indicator in his laboratory as early as the beginning of the 13th century. However, it was not until the 16th century that the blue dye was obtained on a larger scale from different types of lichen from the litmus lichen family, the Rocellaceae, which occurs in several continents around the world. Litmus was not only used in the chemical laboratory in the past, but was also used to color make-up and food such as wine, liqueur, baked goods, sugar paper or cheese.

The extraction from the lichens - a symbiotic community of algae and fungi - is a rather complicated process, the details of which have often been kept secret. First, the lichens are dried and ground into powder. This is left to ferment for a few weeks in a solution with sodium or potassium carbonate and ammonia. There is a gradual change in color. The finally blue mass is dried, ground again to powder and mixed with plaster of paris or lime. The slightly crumbly substance can then be further processed accordingly, for example in a solution.

In its pure form, the dye is dark blue. However, if you put it in a solution, its color depends on the so-called pH value, the concentration of protons in the solution. The more such hydrogen ions there are in the liquid, the more acidic the solution. Litmus solution is red when the pH is below four and blue when it is higher than 8. Between these two values, the dye goes through various shades of purple.