INDIGO.

459

is calculated, it being known that I grm. of this material absorbs 45 c.c. ofoxygen gas while being converted from white into blue indigotin. Thismethod is utterly unfit for the commercial laboratory, and still more for theworkshop, warehouse, or dye-works.

Another series of methods are based upon solution in sulphuric acid : forthis purpose, however, only the strongest fuming acid, sp. gr. i'8g to 1-92, isavailable ; but what is then quantitatively determined is not so much indigotinas the sulphindigotic acid, and this portion of the process varies in differentmethods. Sometimes the assay is a colorimetrical estimation, the liquid, ofcourse, having been diluted to a certain invariable bulk; ora weighed quantityof wool is dyed with the sulphindigotic acid.

Another group of methods for the quantitative estimation of indigotinpreviously converted into sulphindigotic acid depends upon the use of anoxidising material which can be used in the shape of a titrated liquid ; for in-stance, solution of chlorine, of bleaching-powder (hypochlorite of lime),of bichromate and permanganate of potassa, &c. The end of the operation isreadily enough seen, since it coincides with the total destruction of the bluecolour of the substance. If the normal liquid can be preserved withoutundergoing any change, it is only required to determine exactly the quantityof indigotin converted into sulphindigotic acid which can be decolourised by1 c.c. of the oxidising liquid; but it is undoubtedly better as advised byM. H. Schlumberger, that whenever an assay is made, the experimenter shouldoperate upon equal quantities of pure indigotin and of the sample to be valuedat the same time.

Berthollet first suggested the use of chlorine: this method has been im-proved upon by M. H. Schlumberger, and is executed as follows:—Weigh offexactly 1 grm. of each of the samples of indigo to be tested, and previouslydried at 100°, and very finely pulverised ; weigh off also 1 grm. of pure indigo-tin (easily obtainable, in dye and calico-printing works, from the vats, and tobe washed previous to use, first with acidulated water and next with purewater); the substance is put in small porcelain capsules, and thoroughlymixed with 12 grms. of strongest fuming sulphuric acid ; the capsules arecovered with glass plates, and kept standing for at least twenty-four hours ata temperature of from 20° to 22 0 ; the liquid is diluted with water so as to filla litre measure : take of that fluid 5g c.c. by means of a graduated pipette,pour it into a suitable glass vessel, and add next, gradually, by means of aburette, a weak solution of hypochlorite of lime, at a strength of 1° Baume(sp. gr. 1-007), ' n quantities of about 2-5 c.c. at a time ; cease adding as soonas the blue colour is entirely destroyed : in order to estimate the excess ofchlorine which may have been used, there is added, drop by drop, from aburette, a solution of sulphindigotic acid : the same solution, as above men-tioned, may do, until a yellow-greenish hue reappears. Suppose that 5 c.c. ofthe bleaching-powder solution have sufficed to decolourise exadtly 5g c.c. of sul-phate of indigo (viz., 50 c.c. primitively taken, and g c.c. added to restore thegreenish-yellow colour). It is, however, advisable not to rely upon one expe-riment, but to repeat it to judge more corredtly. Suppose, further, that 46 c.c.of the indigotin taken—treated, of course, as has been mentioned for theindigo—is also equal to 5 c.c. of the chlorine solution, then the followingsimple proportion will serve to calculate the results:—46 c.c. of sulphuric