908 INDIGO 



vat, and the same quantity of the indigo to bo tested, which must l>o previously ground 

 to a fine powder, are weighed off, and each of them is treated with about 12 times its 

 weight of concentrated sulphuric acid in a flask or porcelain basin. After being 

 heated at a temperature of 120 to 140 I 7 , for about 24 hours, and occasionally well 

 agitated, the two liquids are mixed with water, so that the volume of the two shall 

 be exactly equal. Two equal measures of a weak solution of hypochlorito of lime 

 are then taken, and to the first is added a quantity of the solution of pure indigo. 

 The chlorine liberated by the excess of sulphuric acid in the solution destroys the 

 blue colour of the indigo-sulphuric acid. More of the solution must be added until 

 the liquid begins to acquire a greenish tinge, and the number of measures necessary 

 for the purpose is noted. The same experiment is then made with the solution of 

 crude indigo. The quantity of indigo-blue in the latter is of course in inverse ratio 

 to the number of measures which are requisite in order to take up the whole of the 

 chlorine which is liberated. If, for example, the same quantity of hypochlorite of 

 lime decolourises 167 measures of the solution of pure indigo-blue and 204 measures 

 of the solution of crude indigo, then the quantity of indigo-blue contained in 100 parts 

 of the latter is given by the following proportion : 204 : 1 67 : : 100 : x = 8T8. 



A number of samples of indigo may bo tested in this manner at the same time. 

 Care must be taken to prepare a fresh solution of indigo-blue for every series of trials, 

 since this solution undergoes a change on standing, which renders it quite inapplicable 

 as a standard of comparison. It is necessary also to pay great attention at the 

 moment when the greenish colour indicating an excess of the sulphate of indigo 

 begins to appear, for it will often be found that this colour disappears after standing a 

 few minutes, and a fresh quantity of the blue solution must then be added cautiously, 

 until the greenish tinge becomes permanent, even after standing for some time. 

 Modifications of this process have been introduced by various chemists by the use of 

 permanganate of potash, chlorate of potash, or bichromate of potash, in the place of 

 hypochlorite of lime ; but as the principle on which the process depends is in each 

 case identical and the modus operandi is almost the same, it will be unnecessary to enter 

 into any minute description of these modifications. The whole method is, however, 

 open to serious objections, and the results which it affords cannot at all be depended 

 on. In the first place, it is difficult to institute a strict comparison between the dif- 

 ferent shades of colour resulting from the decomposition of the sulphate of indigo in 

 different cases, since the pure green tinge observed when an excess of the pure 

 sulphate has been added to the decomposing agent, gives place to a dirty olive or 

 brownish-green, when a solution of crude indigo is employed, in consequence of the 

 impurities contained in the latter. Secondly, it is almost impossible to avoid the 

 formation of a certain quantity of sulphurous acid during the action of concentratpd 

 sulphuric acid on crude indigo. This sulphurous acid during the following operation 

 becomes oxidised before the blue sulphate is destroyed, and hence the percentage of 

 indigo-blue is apparently raised. In employing this method, it is common to find 

 more than 80 per cent, of indigo-blue in a good sample of indigo, whereas the best 

 qualities seldom contain above 60 per cent., and average qualities between 40 and 50 

 per cent. This method may show a percentage of 70 indigo-blue, when the method 

 first described indicates between 50 and 60. 



t3. The third method of estimating the indigo-blue is performed in the following 

 manner: Equal weights of the samples to be tested are treated with equal quantities 

 of concentrated sulphuric acid in the manner above described, and the solutions are 

 then diluted with water and introduced into graduated glass cylinders, water being 

 added to each xmtil they all exhibit exactly the same shade of colour. The richer the 

 sample is in indigo-blue, the greater will be the quantity of water necessary for this 

 purpose, the number of measures of water required in each case indicating the relative 

 amount. The great objection to this method consists in the circumstance, that the 

 different kinds of indigo do not give the same shade of blue when their solutions in 

 sulphuric acid are diluted with water, some exhibiting a pure blue colour, others a 

 blue with a greenish, or purplish tinge. It therefore becomes difficult to institute an 

 exact comparison between them. 



Employment of Indigo in Dyeing. As indigo-blue is insoluble in water, and as it can 

 penetrate the fibres of wool, cotton, silk, and flax only when in a state of solution, the 

 dyer must study to bring it into this condition in the most complete and economical 

 manner. This is effected either by exposing it to the concurrent action of alkalis 

 and of bodies which have an affinity for oxygen superior to its own, such as n-rtain 

 metals and metallic oxides, or by mixing it M-ith fi-rnu-ntiii^ matters, or finally, by 

 dissolving it in a strong acid, such as the sulphuric. The first method is that which 

 is employed in the 



Copperas or Common blue Vat. Before being used the indigo must be broken into 

 small pieces, the size of nuts, moistened with hot water, and then left for a day ; after 



