INDIGO 907 



blue another substance, which has been called indigo-white, ind'lgogcne, or reduced 

 indigo. When an excess of some acid is added to the yellow solution, the indigo- 

 white is precipitated in white or greyish-white flocks, which on filtration and exposure 

 to the atmosphere rapidly become blue, and are re-converted into indigo-blue. 

 Indigo-white is insoluble in water, but slightly soluble in alcohol. It is soluble in 

 caustic alkalis, lime- and baryta-water. The solutions on exposure to oxygen become 

 covered with a pellicle of regenerated indigo-blue. "With an excess of lime it gives 

 an insoluble compound. Its compounds with alumina and metallic oxides, which are 

 insoluble in water, may be obtained by double decomposition. Salts of oxide of 

 copper, when added to its solutions in alkali, convert it immediately into indigo-blue, 

 the oxide of copper being reduced to suboxido. Indigo-blue is also converted into 

 indigo-white, when it is exposed to the action of fermenting or putrefying substances, 

 in the presence of water. Here the decomposing organic matter is the reducing 

 agent, and ammonia, which is usually formed during the process of putrefaction, is 

 the solvent of the indigo-white. If a piece of cotton, wool, or silk be dipped into an 

 alkaline solution of indigo-white, and then exposed to the atmosphere, it acquires a 

 blue colour, which may be made deeper by repeated dippings, and subsequent 

 exposure. It is on this property of indigo-white that the dyeing with indigo 

 depends. 



The true chemical formula of indigo-blue, which was first discovered by Mr. Crum, 

 is C I6 H 5 N0 2 ; and 100 parts contain therefore by calculation 73'28 carbon, 3'81 

 hydrogen, 10*68 nitrogen, and 12 - 23 oxygen. The formula of indigo-white is 

 C' 6 H 6 N0 2 , and it differs therefore from indig6-blue by containing 1 atom more of 

 hydrogen, which is taken up during the so-called reduction of the latter, and lost 

 again by oxidation during its reconversion into indigo-blue. 



Since the value of indigo depends entirely on the quantity of indigo-blue which it 

 contains, it is of great importance to ascertain the exact amount of the latter in any 

 given sample of the article. Before commencing the determination of the indigo- 

 blue, a weighed portion of the indigo ought to be lieated for some hours at 212 F., 

 and then weighed again. The loss in weight which takes place re presents the amount 

 of water contained in the sample. A weighed quantity of the dried indigo is then 

 to be heated over the flame of a lamp until all the organic matter has been burnt 

 away. By weighing the residue which is left the amount of ash or inorganic matter 

 is ascertained. In order, in the next place, to determine the amount of indigo-blue, 

 several methods have been devised by various chemists, none of which however 

 yield very accurate results. Of these methods the following are the principal ones : 



1. A weighed quantity of finely-pounded indigo is rubbed with water in a porcelain 

 mortar. An equal weight of pure lime is then slaked with water, and the hydrate is 

 well mixed with the indigo. The mixture is then poured into a stoppered bottle of known 

 capacity, and the mortar is well rinsed with water, which is added to the rest. The 

 bottle is now heated in a water-bath for several hours, and a quantity of finely-pounded 

 sulphate of iron is added ; the bottle is then filled up with water, the stopper is in- 

 serted, and after the contents have been well shaken the whole is allowed to repose for 

 some hours, until the indigo has become reduced and the sediment has sunk to the 

 bottom. A portion of the clear liquor is then drawn off with a siphon, and the 

 quantity of liquid having been accurately measured, it is mixed with an excess of 

 muriatic acid, and the precipitate, after having been oxidised, is collected on a weighed 

 filter and well washed with water. Lastly, the filter with the indigo-blue is dried 

 at 212 F. and weighed, and the weight of the filter having been subtracted from 

 that of the whole, the weight of the indigo-blue is ascertained. Supposing now that 

 the whole quantity of liquid had been 200 measures, that 50 measures had been 

 drawn off yielding 10 grains of indigo-blue, then the sample contained on the whole 

 40 grains of the latter. For 60 grains of indigo it is necessary to take from 1 Ib. to 

 2 Ibs. of water. 



According to M,r. John Dale of Manchester, who has had great experience in the 

 valuation of indigo for practical purposes, this method, though rather long and tedious, 

 still gives more accurate results than any other. The quantity of indigo-blue in- 

 dicated by it is generally below the actual quantity contained in the sample. Accord- 

 ing to Berzelius, this loss arises from the lime forming an insoluble compound with a 

 portion of the reduced indigo-blue. Mr. Dale, however, is of opinion, that even when 

 every precaution has been taken, a certain loss, proceeding from some hitherto un- 

 ascertained cause, cannot be avoided. When, for instance, pure indigo-blue is treated 

 with lime and copperas in the manner just described, the quantity which is again 

 obtained by precipitation from any portion of the liquid is always less than what it 

 should be by calculation, even when no excess of lime has been employed. 



2. The second method of determining the indigo-blue is performed as follows : 

 About 15 or 20 grains of pure indigo-blue, obtained by precipitation from an indigo 



