Erythrolitmin is the active ingredient extracted from the Litmus

Litmus

Litmus or litmus test may refer to:* Litmus test, a common pH test* Litmus , a test case management tool maintained by Mozilla* "Litmus" , an episode in the first season of the television series...

 lichen

Lichen

Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

, used in chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 as a pH indicator

PH indicator

A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...

. The erythrolitmin molecule is related to the orcein

Orcein

Orcein, also archil, orchil, lacmus, Citrus Red 2, and C.I. Natural Red 28, are names for dyes extracted from several species of lichen, commonly known as "orchella weeds", found in various parts of the world. A major source is the archil lichen, Roccella tinctoria. Orcinol is extracted from such...

s, and consists essentially of several phenoxazone and orcinol residues.

Interaction with acids

The intense coloring of the molecule is generated by the absorption of specific wavelengths of light by the pi bond

Pi bond

In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...

s. These bonds are ordinarily excited by light in the orange region of the spectrum, causing the molecule to appear blue. When the molecule interacts with protons from an acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

the bonds become harder to excite and thus absorb green light which has a shorter wavelength. This is what causes the molecule to appear red in the presence of an acid.