Orcein
Orcein is an organic vegetable dye from lichens and a mixture of at least 14 substances.
Occurrence and extraction
The mixture of substances obtained by alcoholic extraction from Orseille, a dye that is derived from lichens of the genus Roccella.
Composition and properties
From orcein can be chromatographically isolating at least 14 components. These can be divided into three phenoxazine chromophores:
- 7- hydroxy-2- phenoxazone
- 7- amino-2- phenoxazone
- 7 -hydroxy-2 -phenoxazine
Orcein (CI Natural Red 28) is a brown-red, microcrystalline powder that is practically insoluble in water, benzene, chloroform or ether, in alcohol, acetone or glacial acetic acid with a red color and in dilute alkali solutions with blue-violet color is soluble. In this respect, an alcoholic ( ethanol ) solution of orcein as litmus as an indicator of alkaline ( blue-violet ), neutral ( purple) form and acidic solutions ( red) suitable.
Use
Orcein was in ancient times, in the Middle Ages and the early modern period similar to the brazilwood an important supplier for the red color of fabrics. Similar to Brazil wood, the color faded very quickly. Due to the lack of suitable dyes orcein but was used frequently. After the collapse of the Roman Empire, the use of orcein in Europe was largely forgotten. Only in the Middle East was dyed with this substance on. By 1300 the use of orcein was rediscovered as a colorant by a Florentine merchant and played for in the following centuries back in Europe again a more important role as a coloring agent.
The basic dye is today (as well as the dye carmine ) is used among other things as acetic acid solution for staining of chromosomes or chromatids in microscopic preparations. With a solution of orcein in a sodium carbonate solution, which is weakly alkaline ( blue-violet color), can be colorless dye wool. After washing with water, the wool has an intense red-violet color. However, so dyed wool is not fast to washing.
Gallery
α - hydroxyorcein
β - amino- orcein
β - hydroxy- orcein
β -amino- orceinimin
γ - amino- orcein
γ - hydroxyorcein
γ -amino- orceinimin