Indigo Brown. / 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.

An important impurity of natural indigo is the so-called indigo brown, a product isolated and cursorily examined by both Chevreul (Gmelin, Handbook of Chem., 1859, J 3> 48) and Berzelius (ibid.). In order to isolate this substance, the latter chemist digested indigo with boiling dilute sulphuric acid to remove indigo gluten, and subsequently with potassium hydroxide to dissolve the brown. The alkaline liquid was neutralised with acetic acid, evaporated to dryness, the residue digested with alcohol, and the solution evaporated. Thus obtained the indigo brown consisted of a dark-coloured resin, soluble in alkaline solutions. According to Schunck (Phil. Mag., 1855, [iv.], 10, 74, and ibid., 1858, 15, 127) the indihumin, C₁₀H₉O₃N, produced in conjunction with other brown amorphous products by the action of dilute acids on his indican is, perhaps, identical with indigo brown.

Perkin and Bloxam (Chem. Soc. Trans., 1907, 91, 279) extracted Bengal indigo, which had been already digested with boiling dilute hydrochloric acid to remove the gluten, with boiling pyridine. In addition to a little indirubin the product contained three substances: (a) the main constituent, C₁₆H₁₂O₃N₂ (?), insoluble in alcohol and acetic acid, (b) C₂₄H₂₂O₅N₃ (?), soluble in acetic acid, and (c) C₁₆H₁₄O₄N₂ (?), soluble in alcohol. These compounds, the molecular weight of which is uncertain, consist of brown amorphous powders, closely resembling one another in general property, and are readily reduced by zinc- dust in alkaline solution with formation of pale brown liquids. When digested with boiling 50 per cent, potassium hydroxide solution they give some quantity of anthranilic arid, a point which indicates that they are derived from indoxyl. At the same time a brown resinous substance is also produced, and this studied in the case of the main constituent (a) C₁₆H₁₂O₃N₂ contained C = 71,39; H = 4,05; N = 7,94. Natural indigo further contains a small quantity of a brown substance, insoluble in pyridine, but soluble in boiling dilute alkali (Chem. Soc. Trans., 1910, 97, 1473) and is distinguished from the compounds above enumerated by the fact that it is not susceptible to sulphonation (with 96 per cent, sulphuric acid) or conversion by this means into a product soluble in water. In the analytical method described by Rawson (J. Soc. Chem. Ind., 1899, 18, 251) this brown material, at least in part, consists of the impurity which is carried down by a precipitation of barium sulphate in the liquid. There is now considerable evidence in favour of the view that the constituents of indigo brown are derived from indoxyl during the manufacture of indigo from the plant. The fact that indican itself, when boiled with dilute acids, produces the very similar indoxyl brown, and the isolation of brown substances, although in trifling amount, from indigo prepared by the hydrolysis of pure indican in the laboratory, harmonises with this suggestion.

Beyerinck (Proc. Roy. Akad. Scien. Amsterdam, 1899, 120) observed that the disappearance of indoxyl in a dyeing woad (Isatis tinctoria) leaf is accompanied by the appearance of brown substances. Again, he states that "strong acids, just as alkalis... favour the formation of indigo from indoxyl, but then part of this substance constantly changes into a brownish-black matter". It has also been noted by Thomas, Perk in, and Bloxam that the disappearance of indican in the leaves of the I. sumatrana on keeping is accompanied by the formation of brown extractive matter. Rawson, again (Report on the Cultivation and Manufacture of Indigo, Mozzufferpore, 1904), says, "The blower... by quickly getting rid of CO₂ gas... prevents decomposition of a portion of the colouring matter into worthless brown substances, which takes place to a greater extent under other conditions". All indigos, moreover, appear to contain indigo brown, so that this property is irrespective of their origin, which may have been due to such distinct plants as the Indigoferæ, the Polygonum tinctorium, or the Lonchocarpus cyanescens of West Africa. Finally, it has been shown by Perkin (loc. cit.) that among the decomposition products of commercial indoxylic acid which has been kept for a long time, a brown compound exists, which has a very similar percentage composition, and is probably identical with the main constituent of indigo brown. It is quite possible that indoxylic acid is produced during the fermentation process (Perkin), (cf. Ranking, loc. cit.), and may be concerned in the formation of this product. The percentage of indigo brown soluble in pyridine in natural indigos is very variable, and appears to depend upon the details of manufacture. Analyses made by Bloxam and Perkin (Chem. Soc. Trans., 1910, 97, 1472) gave the following result:
Java indigo, new process, hot water.... 5,4
Java indigo, new process, cold water.... 5,2
Java indigo, ordinary process.... 4,15
Covetry process indigo.... 8,7
New bengal indigo.... 9,60

Except in the case of the Coventry process indigo, these samples had all been derived from the I. arrecta. The average amount of crude indigo brown containing, however, some mineral matter in numerous samples of Bengal indigo prepared from the I. sumatrana was 14 per cent. (Chem. Soc. Trans., 1907, 297).

Indigo brown dissolved in alkaline hydrosulphite solution does not colour cotton fabrics, though woollen material dyed with natural indigo and stripped by the method of Green, Gardner, Frank, and Lloyd (loc. cit.) frequently possess a light brown tint which is probably due to the presence of this compound. The amount, however, remaining in this way on the fibre is not sufficient to materially strengthen the colour effect, and the frequently asserted superiority of the natural over the artificial variety of indigo can hardly therefore be accounted for in this manner.