The Estimation of Indigo in Dyed Cotton.

Posselt's Textile Journal 7, 1909

*Paper read before the Society of Dyers and Colorists.

By Prof. Edmund Knecht, Ph.D.

The estimation of the amount of indigo present in a dyed yarn or fabric is of interest to the dyer from several points of view. As is well known, it is not possible, in indigo dyeing, to dye with a certain percentage of dyestuff, as is done with most other colors, the indigo dyer merely aiming at getting the desired shade, not being concerned with the actual percentage of indigo on the fibre. According as the fabric is dyed through, or is only surface dyed, it is manifest that the same shade may be obtained with different amounts of indigo, and this is one of the many points which might be studied with a rapid and reliable method of estimating the dyestuff on the fibre. But it is chiefly from the point of view of being able, especially in large works, to ascertain whether waste or loss of dyestuff is taking place which might be located and remedied that such a method would be of use.

That some importance is attached to the subject is shown by the fact that several methods have already been worked out with this object in view. One of the earliest of these was that suggested in 1886, by A. Renard, who extracted the indigo from the dyed fabric by means of hydrosulphite of soda, and precipitated the indigo from the solution by blowing air through. The same principle has been adopted by others, but the method is not satisfactory because it is necessary to extract several times to get all the indigo out, and the oxidation of the reduced liquor and the subsequent filtration require much time.

The nitrobenzene process of B. W. Gerland consists in extracting the weighed sample in a specially devised extractor with boiling nitrobenzene, but it does not seem to have been largely introduced, and according to his own statement it takes a considerable time to get the indigotine into a sufficiently pure state for weighing. The substitution of aniline oil for nitrobenzene in extracting indigo from dyed fabrics has been shown to bring about a not inconsiderable loss in dyestuff, and any method based on the use of this solvent is consequently of little value.

Extraction with glacial acetic acid, as suggested by Brylinski, and weighing the indigotine precipitated by pouring the acetic acid solution into water, is the method most generally adopted at the present time. I have recently had occasion to test the accuracy of the method, and though I find this highly satisfactory, the time required for extracting, especially in dark shades, is so great as to constitute a distinct drawback to the process.

Möhlau and Zimmermann's method consists in extracting the dyed fabric on a water bath with acetic acid containing some conc. sulphuric acid. A sulphate of indigotine is thus produced which dissolves in the acetic acid. By pouring the solution in the mixed acids into water the sulphate dissociates, and the indigotine comes out as a crystalline precipitate. A drawback to this method is that a correction has to be made, since the indigotine precipitated contains impurities from the cellulose.

The idea of dissolving the weighed sample of dyed cotton straight away in cold concentrated sulphuric acid, and then sulphonating with or without the further addition of fuming acid, has no doubt occurred to several interested in this question. That such a method cannot be employed, is however, at once evident when the experiment is tried, for although some of the indigotine is sulphonated and goes into solution, a very large proportion is destroyed, and with light shades I have on more than one occasion noticed that every trace of color was destroyed.

The new method which I wish to bring to your notice is based on two well-known facts, viz.,
(1) That cellulose readily and completely dissolves in sulphuric acid of 80 per cent. at from 95 deg. to 105 deg. F. and
(2) That according to the patents of the Badische Anilin and Soda Fabrik, and of Geigy and Co., indigotine is also soluble in sulphuric acid of this strength, forming a sulphate which, when poured into water dissociates with separation of the whole of the indigotine as such. According to Binz and Kufferath this sulphate has the composition C₁₆H₁₀N₂O₂₂H₂SO₄. If the precipitated indigotine is filtered off on a Gooch crucible through silica or asbestos, and dried, it can be sulphonated and then estimated by any of the ordinary volumetric processes.

In order to test the process, a quantity of pure crystallised indigotine was prepared which was shown by analysis to contain 99.5 per cent. of the dyestuff.

Five grammes cotton yarn and 0.2 grm. of this indigotine were then dissolved together in 8o per cent. H₂SO₄, and the estimation carried out as indicated before, the titration being done by titanous chloride. A sharp end reaction was obtained, and the amount of indigotine found was 01984 grm. in place of 0.1990.

The experiment was repeated with 5 gyms. cotton and 02 grm. indigotine, giving 0.1979 grm. in place of 0,1990.

It was thus evident that in principle the method is correct, and sufficiently accurate for practical purposes. Although there is a slight loss this is somewhat less than that observed in Brylinski's acetic acid method tested with the same materials, but in either case the result would not be affected except in the second place of decimals.

The following is the modus operandi which we have hitherto adopted: Four grammes of the sample of dyed cloth are cut up into small pieces and placed in a porcelain beaker; 25cc. of sulphuric acid of 80% strength (148 deg. Tw.) are now added, and the whole is stirred slowly, the temperature being maintained at about 104 deg. F. In a very short time the cloth begins to dissolve, and after about ten minutes' stirring all has gone into solution. The contents of the beaker are now diluted to about 120CC. with water, and the precipitated indigotine is filtered off through a Gooch crucible containing sand and asbestos as the filtering medium. Diluting the acid solution is better than pouring it into water, since in the latter case the indigotine comes out in such a fine state of division that it is difficult to filter. The Gooch is then put in the oven to dry at 230 to 250 deg. F., and is then placed in a weighing bottle with a small amount of conc. H₂SO₄, when it is heated for an hour in a water oven. The sulphonated indigotine is then taken up with water, and titrated either by means of titanous chloride or by Rawson's permanganate process.

The method was tried against that of Brylinski on a medium shade of indigo dyed on bleached cloth, with the following results:
New method (TiCl₃ titration) ... 1.43%
Acetic acid method (gravimetric) ... 1.48 "
Acetic acid method (titr. with KMnO₄ after sulphonating) ... 1.47"

Since much of the indigo dyed material which comes into the market is either topped or bottomed with other dyestuffs, this circumstance must not be lost sight of in carrying out the test. The colors used for topping are usually either methyl violet or safranine, but these can easily be detected by means of acetic acid, and if present will remain in solution in the sulphuric acid on diluting with water.

Of colors used for bottoming, manganese bronze is the only one likely to give trouble, for if it is present I find that it destroys, during the first operation, a considerable proportion of the indigotine contained in the fibre. If on testing the ash of the fabric with nitre and fusion mixture it is found to be present, it must be removed before proceeding further by treatment with bisulphite and acid.

The sulphide colors which are at present largely used for bottoming indigo do not appear to give any trouble when the final titration is effected with TiCl₃, being apparently destroyed either by the first or second treatment with sulphuric acid. In one experiment which was carried out 2.5grms. cloth dyed with Eclipse Brown were dissolved along with 0.2grm. crystallised indigotine in 80% H₂SO₄, and further treated as described before. On titrating, 98.9% of the indigotine taken was accounted for. Pyrogene Brown, Thion Yellow 2 G, Immedial Brown used up no TiCl₃. Katigen Indigo (1 grm.) used up a very small amount, but not sufficient to influence the result beyond the second place of decimals.

ANALYSIS OF COMMERCIAL INDIGOS: A method similar to the one which I have just described was suggested some twelve years ago by B. W. Gerland. The acid which he used in the first treatment was, however, somewhat weaker, while the temperature was higher; the principle was, however, the same. At that time no artificial indigo was on the market, and it seemed therefore of interest to try the process on this product, and to compare the results obtained by those furnished by the methods of Möhlau and Zimmermann and of Bloxam. The following results were obtained on three different commercial samples of the artificial product. The percentages indicate the amounts of indigotine found.

	Möhlau. Per Cent.	Bloxam (TlCl3) Per Cent.	H2SO4 Method. (TlCl3) Per Cent.
Artificial indigo powder I	94,80	94,33	94,73
Artificial indigo powder II	91,80	91,70	91,48
Artificial indigo paste	20,30	20,57	20,60

The indigo powder No. 1 contained 4,21%, No. 2 4,30%, and the paste 97,38% of moisture at 230 deg. F. It would thus appear that the process is also applicable to the analysis on artificial indigos.

Some further results on natural indigos are appended below, given simply to show that with thesee the process works equally well. In all cases the titrations were done with TiCl₃.

	Grossmann. Per Cent.	Bloxam Per Cent.	H2SO4 Method. Per Cent.
Natural Indigo I	48,22	48,57	48,70
Natural Indigo I	55,97	-	55,45
Fig Indigo	12,74	Not satisfactory	12,77