460

DYEING AND CALICO PRINTING.

indigotin = 59 c.c. of sulphuric indigo ; 5g c.c. contain therefore o - 046 grm.of indigotin ; how much is contained in 1000 c.c. ? or how many times x is .contained in 1 grm. of x = Jg = the weight of indigotin contained in 1 grm.of indigo. This process is deficient in the following particulars :—-The solutionof bleaching-powder changes, and therefore has to be tested afresh, and itsvalue in relation to indigotin estimated whenever a new series of experimentsis commenced—say at one or two days interval; it is also impossible to guardcompletely against loss of chlorine at the moment of mixing the hypochloritesolution with the acid indigo or indigotin solution. In consequence of thesedefedts this process has been improved upon by substituting for the use ofchlorine other more manageable oxidating matters, as, for instance, perman-ganate of potassa, by itself; a mixture of chlorate of potassa and hydrochloricacid ; and bichromate of potassa mixed with the last-named acid, as proposedand carried out by the late Dr. Penny. This method has met with generalapprobation, especially at the hands of the Mulhouse and other Alsatianmanufacturers, who unanimously speak in favour of this process, based uponthe following fails :—When very finely pulverised indigo is aCted upon by verystrong fuming sulphuric acid, the foreign organic matters it contains are car-bonised, and remain behind upon the filter, after the addition of water, whilethe indigotin becomes soluble ; when to this solution first hydrochloric acid isadded, and next to the previously thoroughly warmed fluid bichromate ofpotassa, the sulphindigotic acid is decomposed and converted into sulphisaticacid. When, therefore, a titrated solution of bichromate of potassa is em-ployed, it is plain that the amount of indigotin may be calculated from thevolume of the bichromate solution used to destroy the blue colour.

It has been found, by experience, that in order to bleach a given weight ofindigotin, in the shape of sulphindigotic acid, the quantity of bichromate re-quired is precisely that which is necessary to convert the indigotin intosulphisatic acid: the readtion is very precise, and as long as any indigotinremains the bichromate only attacks it: 3 molecules or 393 parts of indigotinrequire 1 molecule or 2gy2 parts of bichromate, or, percentically, 100 parts ofindigotin require 75'6 parts of bichromate. This process is executed in thefollowing manner :—The sulphuric acid solution of indigo is made with 1 grm.of that substance, and further proceeded with as above stated in Schlumberger’smethod; the titrated bichromate solution is to contain 7-66 grms. of the puresalt to the litre : 1 c.c. of this liquid corresponds to 10 c.c. of a solution of1 grm. of pure indigotin to the litre, equal to o-oi grm. of indigotin, so thatevery c.c. of this fluid is equal to 1 centigrm. of indigotin. Take 100 c.c. ofthe diluted solution of indigo in sulphuric acid, boil it gently in a porcelainbasin, add 10 c.c. of hydrochloric acid, and add next with the burette the bi-chromate solution, taking care to cease adding the very moment the greenishhue disappears and makes place for the orange-yellow colour. M. Meyer, inthe paper above quoted, while testing by this method the various kinds ofindigo, found that the end of the operation is not so readily seen as is gene-rally stated, because the colour of the sulphisatic acid is rather more a deepyellowish-brown than orange-yellow ; the original colour of the indigo solutionbeing deep blue, the disappearance of that colouration is, according to him,somewhat obscured from view. It is, at all events, always desirable to maketwo, if not three, estimations of the same sample: the number of tenths ofcubic centimetres of the chromic solution consumed gives diredtly the