Indigo Yellow. / Constituents of Natural Indigo. (Natural Indigo.)(CHAPTER XV. Indole Group.)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

The first application of the term "Indigo yellow" to a substance existing in natural indigos is due to Bolley and Crinsoz (Jahres., 1866, 573) who state that it is to be found in the Bengal variety, and can be isolated by sublimation. It is described as golden-yellow needles, subliming at 130, and soluble in soda lye. Crude Bengal indigo, however, gives no sublimate of this character (Perkin, Chem. Soc. Proc., 1906, 22, 198), but by submitting refined indigo, or the commercial synthetical variety to sublimation with limited access of air, a trace of a yellow compound, C₁₅H₈O₂N₂, is produced. This substance, however, is insoluble in alkaline solutions, and cannot, therefore, be the indigo yellow of Bolley and Crinsoz.

Rawson (J. Soc. Chem. Ind., 1899, J 8, 251) detected in Java indigos a yellow compound, present usually to the extent of 2-3 per cent, although in one special sample as much as about 20 per cent. occurred. This substance was soluble in alkalis with a yellow colour; on heating it partially sublimed, and had the properties of an adjective yellow dyestuff. A more recent investigation (Perkin, Chem. Soc. Proc., 1904, 20, 172) has indicated that this in reality is kaempferol - a trihydroxyflavonol known to exist (Chem. Soc. Trans., 1902, 81, 587) in the flowers of the Delphinium consolida (Linn.) and other plants. Ultimately it was shown that the leaves of the I. arrecta, from which Java indigo is prepared, contain sometimes as much as 4 per cent, of a glucoside, kaempferitrin, C₂₇H₃₀O₁₄, almost colourless needles, melting-point 201-203°, which when digested with acid gives kaempferol and rhamnose C₂₇H₃₀O₁₄ + 4H₂ = C₁₅H₁₀O₆ + 2C₆H₁₂O₅, H₂O

This compound is not hydrolysed by the indigo enzyme, and no enzyme has as yet been isolated from the plant possessing such a property. It is likely (Chem. Soc. Trans., 1907, 91, 435) that the use of sulphuric acid, when manufacturing Java indigo, may result in the contamination of the dyestuff with kaempferol. When the wet indigo sludge or "mal" is boiled in the "mal" boiler with addition of a little of the acid, the kaempferitrin present in the adhering water will be hydrolysed, and the insoluble colouring matter remain with the indigo. Samples of Java indigo more recently obtained contained only a trace (0,2 per cent, approx.) of kaempferol, whereas in a sample of the new Bengal indigo manufactured from the I. arrecta approximately the same quantity was detected (Perkin, private communication). If indigo mixed with kaempferol is cautiously sublimed the sublimate then contains appreciable quantities of this yellow colouring matter, and it seems likely, therefore, that this is in reality the indigo yellow of Bolley and Crinsoz, but that the indigo experimented with by these authors did not, as they supposed, originate from Bengal. The leaves of the /. sumatrana, the Indian indigo plant, contain but the merest trace of a yellow dyestuff resembling kaempferol, but according to Henry (Gmelin's Handbook of Chem., 1846, 13, 50) the Polygonum tinctorium, or Chinese indigo plant, contains appreciable quantities of a yellow colouring matter.