2-(1,3-Dihydro-3-oxo-2H-indol-2-yliden)-1,2-dihydro-3H-indol-3-on

BeschreibungIndigo bildet tief dunkelblaue Kristalle mit kupferrotem Glanz. Indigo ist ab ca. 300 ℃ sublimierbar und zersetzt sich oberhalb 390 ℃. In Wasser, Ethanol, Diethylether und auch in verdünnten Mineralsäuren ist der Farbstoff praktisch unlöslich. Schwefelsäure vermag es unter Sulfonierung zu lösen, wobei bei kaltem Ansatz eine grüne, bei Erwärmen eine blaue Lösung entsteht. Zur Textilfärberei wird das wasserunlösliche Indigo mittels Natriumdithionit in das lösliche Indigoweiß überführt. Die mit der Indigoweiß-Lösung gefärbten Stoffe sind zunächst gelb und werden erst an der Luft durch die Oxidation des Indigoweiß zurück zu Indigo blau.

BeschreibungIndigo, known chemically as indigotin, is a common blue dye that has been highly valued throughout history and has played a major role in trade and commerce since ancient times. The term indigo is often used to describe many blue dyes produced from a number of plants. For example, woad, a blue dye obtained from the plant Isatis tinctoria, was used throughout the Mediterannean and Europe and is often identified as indigo. True indigo comes from the leguminous plant of the genus Indigofera. The Indigofera genus includes several hundred species, and indigo has been obtained from a number of these, but the dominant species for the dye are Indigofera tinctoria grown mainly in India and tropical Asia and Indigofera suff ructiosa from the tropical Americas. The name indigo comes from the Greek indikon and Latin indicum meaning “dye from India.” There is evidence that indigo was used several thou sand years b.c.e. Persian rugs containing indigo color exist from several thousand years b.c.e. Textile artifacts from Egyptian tombs provide evidence of indigo’s use by royalty from as far back as 2500 b.c.e. The writings of Herodotus from approximately 450 b.c.e. mention indigo’s use in the Mediterranean area.

OccurrenceIndigo is a perennial shrub found in several regions of the world.

HistoryIndigotin. The blue dye of the ancient world was derived from indigo and woad. Which plant is the oldest is a matter of conjecture. That indigo was known at least four thousand years ago is evident from ancient Sanskrit writings. Cloth dyed with indigotin (CI Natural Blue; CI 75780) has been found in Egyptian tombs and in the graves of the Incas in South America. Indigo belongs to the legume family. The two most important species are Indigo tinctoria and I. suffruticosa, found in India and the Americas, respectively. The leaves of the indigo plant do not contain the dye as such, but in the form of its precursor, a glycoside known as indican.