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.
Although some indication of the indigo-yielding capacity of the plant can He obtained by ordinary steeping experiments, this method was found by Rawson ("Cultivation and Manufacture of Indigo," loc. cit.) to possess several drawbacks, and numerous experiments were therefore carried out by him on the quantitative formation of indigo from the leaf extract by the simultaneous action of acids and oxidising agents. As regards the latter, ferric chloride, potassium chlorate, and hydrogen peroxide were tried, but persulphuric acid gave much the best results.
Persulphate Method.
20 grams of leaves are extracted for two minutes with 250 c.c. of boiling water, the solution is strained through muslin, and the residues squeezed and washed with boiling water. The solution is treated with 5 c.c. of 20 per cent, hydrochloric acid, and 40 c.c. of a 5 per cent, solution of ammonium persulphate. The persulphate is not added all at once; at first 2 c.c. are added, after half an hour 2 c.c. more, and again 2 c.c. after another half an hour. After two hours the remainder of the ammonium persulphate is added, and when the mixture has stood for a further period of an hour, the indigo is collected and estimated by permanganate in the usual manner. Bergtheil and Briggs (J. Soc. Chem. Ind., 1906, 734) point out, however, that this process of Rawson's requires modification, as the addition of the reagents at such a high temperature involves a loss of indigotin. The main features of a modification of the process devised by these latter authors are the addition of acid to the cooled extract, and a determination of the course of the reaction, after addition of small amounts of persulphate, by filtration of a portion of the mixture and the addition to the filtrate of a trace of the oxidising agent.
Orchardson, Wood, and Bloxam (ibid.., 1907, 40; cf. also Bloxam and Leake, "Research Work on Indigo," Dalsingh, Serai, 1905), who employ sulphuric acid and persulphate, arrived independently at the same conclusion. To 200 c.c. of the leaf extracts these authors add 100 c.c. of a mixture of equal parts of 2 per cent, ammonium persulphate, and 4 per cent, sulphuric acid, and the mixture is kept at 60 for one hour. A comparison of their methods with that of Bergtheil and Briggs indicated an identical result in each case, and an increase of 20 25 per cent of pure colouring matter in comparison with that yielded by Rawson's original process.
The Isatin Method.
Beyerinck (Proc. K. Akad. Wetensch., 1899, 120), in discussing indican, suggested the possibility that by warming its solution with isatin and acid a quantitative yield of indirubin might be produced. Orchardson, Wood, and Bloxam (loc. cit.) have employed this reaction for the estimation of the leaf, and have devised the following method for this purpose:
250 c.c. of extract, equivalent to 5 grams of the leaf, is treated with 0,1 gram of isatin, and the mixture boiled for five minutes to expel air, carbon dioxide being passed through the flask. 20 c.c. of hydrochloric acid is then added by means of a tap funnel, and the whole kept boiling for thirty minutes. The precipitate is collected on a tared filter, washed with hot 1 per cent, soda to remove brown compounds, then with 4 per cent, acetic acid and dried. An aliquot portion of the crystalline product is sulphonated, and analysed by the titanous chloride method, adopting the modifications employed by Bloxam (loc. cit.). The indirubin thus obtained is usually almost pure (98,5 per cent.), so that for an approximate estimation the latter part of the process is unnecessary. Gaunt, Thomas, and Bloxam (ibid., 1907, 26, 56) have examined the process in greater detail, and point out that by its employment pure indican gives quantitative figures (cf. also Perkin and Bloxam, Chem. Soc. Trans., 1907, 91, 90). On the other hand, this method gives considerably higher figures, both with pure indican (15 per cent.) and the leaf extract (25 percent.), than those which are obtained by the persulphate process (Orchardson, Wood, and Bloxam; and Gaunt, Thomas, and Bloxam, loc. cit.). The unsatisfactory figures in the latter cases arise from a further oxidation of the indigo by the persulphate. That this isatin method does not appear to be affected by other plant constituents was shown by the successful estimation of indican, purposely added to an extract of the leaves of the Tephrosia purpurea (Pers.), a plant in which this glucoside is absent.
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