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.
It is well known that indirubin is more resistant to oxidation and reduction than indigotin, properties which also apply to the sulphonic acids of these colouring matters. When dealing, therefore, with sulphonated mixtures of these substances and employing either potassium permanganate, titanium chloride, or sodium hydrosulphite, the indigotin sulphonic acid is to some extent preferentially attacked, so that towards the end of the operation the colouring matter consists entirely of indirubin sulphonic acid. In the case of the first named reagent, however, Koppeschaar (Zeitsch. anal. Chem., 1899, 38, i) finds that it is not possible to obtain trustworthy analytical figures with indigos in which some quantity of indirubin is present, although Rawson (loc. cit.) considers that the indirubin may be approximately estimated in this manner. Bloxam and Perkin (Chem. Soc. Trans., 1910, 97, 1462), however, support the view of Koppeschaar. The latter authors, who also experimented with titanous chloride, show that this reagent behaves in an identical manner towards both indigotin and indirubin sulphonic acids, but although the former is somewhat preferentially attacked, it is not possible in this way to differentiate as to the amount of each of the sulphonated colouring matters which may be present in a mixture of the two. On the other hand, according to Knecht, Rawson, and Löwenthal ("A Manual of Dyeing," 821) indirubin present in mixtures of the two colouring matters may be approximately estimated by the hydrosulphite method.
For analysis of indigos rich in indirubin, processes of extraction based on the greater solubility of the latter have been usually employed.
Extraction with Ether (Rawson, loc. cit.).
From 0,1 to 0,25 gram of the sample is boiled with about 150 c.c. of ether for half an hour. When cold the solution is made up to 200 c.c. with ether, mixed with 10 c.c. of water and well shaken. The suspended particles of indigotin settle immediately and a clear solution of indirubin is obtained. A measured quantity of the solution is withdrawn, and compared in a colorimeter with a standard solution of indirubin.
Extraction with Acetic Acid (Koppeschaar, loc. cit.).
The indigo is extracted with glacial acetic acid, and the solution, which contains a mixture of indirubin and indigo brown, is treated with caustic soda. The indirubin, which is thus precipitated, is collected, redissolved in acetic acid, and estimated by comparison with a standard solution of the pure colouring matter.
Extraction with Acetone (Gardner and Denton, J. Soc. Dyers, 1901, 170).
0,2 gram of the indigo is digested for half an hour with 100 c.c. of boiling acetone. After cooling the solution is made up to 100 c.c. with acetone, and then to 200 c.c. with 10 per cent, salt solution, and well shaken. The precipitate of indigotin, indigo brown, and other impurities is removed by filtration, and the indirubin solution estimated colorimetrically with a standard solution of indirubin prepared with acetone and salt solution in a similar way.
Extraction with Pyridine.
Bloxam and Perkin (Chem. Soc. Trans., 1900, 97, 1460) find, as the result of experiments on mixtures of indigotin and indirubin, that neither commercial ether nor acetone are reliable solvents for the complete extraction of indirubin, and that their action, especially in the former case, is chiefly due to the presence of alcohol. Whereas acetic acid is efficient in this respect, and Koppeschaar's process gives approximately good results, pyridine is a much better solvent, and a method for the complete analysis of indigos containing indirubin based on the application of this liquid is described by these authors.
The indigo (0,25-1 gram) evenly incorporated with purified sand (20-30 grams) is introduced into a thin-walled glass tube, termed the "container," closed at one end by means of cotton cloth, on which has been placed a layer of asbestos and purified sand or of sand alone. Sufficient sand is then added to form a layer on the surface of the indigo mixture, which is then covered with asbestos, and the container is now placed in a Soxhlet tube and extracted with boiling pyridine. The extract is distilled down to a small bulk, the residue treated with boiling water and again distilled, and this operation is repeated until the last traces of pyridine have disappeared. The precipitate, which consists of indirubin together with a little indigotin and indigo brown, is collected, freed from the latter by means of dilute alkali, and the residue is sulphonated with 5 c.c. of sulphuric acid at 100°. The product is dissolved in water, filtered, and the amounts of indigotin and indirubin present ascertained by means of the Duboscq tintometer.
The residue in the container is percolated with water, followed with boiling dilute hydrochloric acid to remove indigo gluten, and is now introduced into a beaker and dried. The colouring matter present is sulphonated with 20 c.c. of sulphuric acid in the usual way, the product after dilution is filtered, and the solution of the indigotin sulphonic acid is estimated with permanganate, employing the directions given by Bloxam (loc. cit.). Analyses of mixtures of pure indigotin and indirubin, and also of commercial indigos, are given in the paper, and it is also pointed out that by this method an approximate estimation of the indigo brown present in the latter can be carried out.
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