Properties of Indigo

—It po e es a dark blue colour, pa ing into violet-purple, is void of taste and smell, dull, but by rubbing with a smooth hard body, it a umes the lustre and hue of copper. It occurs sometimes le and sometimes more dense apparently than water, which circumstance depends upon its freedom from foreign impurities, as well as upon the treatment of its paste in the boiling, pre ing, and drying operations. It is insoluble in water, cold alcohol, ether, muriatic acid, dilute sulphuric acid, cold ethereous and fat oils; but boiling alcohol and oils di olve a little of it, which they deposit on cooling. Creosote has the property of di olving indigo. Indigo is a mixture of several dye-stuffs, and other substances. Berzelius found in it a matter resembling vegetable gluten or gliadine, a brown, red, and blue pigment, besides oxide of iron, clay, lime, magnesia, and silica. 1. Indigo gluten or gliadine is di olved along with the calcareous and magnesian salts by acids. If the powder be treated with dilute sulphuric acid, if the solution be saturated with carbonate of lime, evaporated to dryne , and its residuum treated with alcohol; the solution thus formed leaves, after being evaporated, a yellow transparent extract, easily soluble in water, more difficultly in acid liquids; showing that acids extract only a portion of the gliadine from the indigo. It yields, by dry distillation, much ammonia, a fetid oil, and comports itself in other respects like vegetable gluten. 2. Indigo-brown , occurs in combination with lime, as also with vegetable acid in considerable quantity, and more abundantly in the coarser sorts of indigo than in the finer. Indigo purified by acids is to be treated with hot strong caustic lye, which di olves the indigo-brown; the liquid part of the mixture pa es with difficulty through the filter, is black-brown, opaque, and holds some indigo-blue in solution, or diffused in fine powder. The alkali being neutralized with acetic acid, the liquor is to be evaporated, and alcohol poured on the residuum, whereby the alkaline acetate is di olved out from the brown. This pigment is a dark brown, almost black, but is not yet entirely deprived of the other constituents of indigo. It is nearly tastele , is combustible, affords, by dry distillation, ammonia and fetid oil, forms with acids combinations hardly soluble in water, with alkalis soluble ones, but with earths hardly soluble. Lime po e es the property of precipitating the indigo-brown completely from its alkaline solution. Chlorine occasions a pale yellow brownish precipitate, which consists of indigo brown and muriatic acid, but causes no further change. By drying, it becomes again dark coloured. Indigo-brown seems to exist also in woad. 3. Indigo-red , or more properly red resin of indigo. This may be obtained by boiling alcohol of sp. grav. 0·830 upon some indigo which has been previously treated with acids and alkalis; for the red substance is hardly soluble in cold alcohol. The solution is dark red, opaque, and leaves, by distillation, the indigo-red in the form of a black-brown powder, or a glistening varnish, slightly soluble in alcohol and ether. Alkalis do not di olve it, but concentrated sulphuric acid forms with it a dark yellow dye, from which water causes no precipitation; wool extracts the colour from the acid solution, and becomes of a dirty brown hue. Chlorine does not seem capable of destroying the colour for though it makes it yellow, it becomes as dark as ever on being dried. Indigo-red melts with heat, burns with a bright flame, affords, when heated in vacuo, first a white crystalline sublimate, and then unchanged indigo-red. That white matter is changed by nitric acid into indigo-red. 4. Indigo-blue , or pure indigo remains, after treating the indigo of commerce with dilute acid, alkalis, and alcohol; it retains, however, still traces of the matters thereby extracted, along with some earthy substances. In order to procure indigo-blue in its utmost purity, we must deoxidize the above blue residuum, thus form colourle indigo, which again acquires a blue colour from the air, and constitutes the pure pigment. For this purpose the above moist indigo is to be mixed with slaked lime, green sulphate of iron, and hot water in an air-tight matra . The indigo when deoxidized by protoxide of iron being soluble in lime-water, the clear yellow solution is to be poured off, and exposed to the air. The indigo absorbs oxygen, and becomes again blue. By digestion with dilute muriatic acid the foreign matters are di olved, and may then be washed away with distilled water, from the absolute indigo. The indigo-blue obtained in this manner has a cast of purple red, displaying the characteristic copper lustre in a high degree, but in powder, it is blue. It is void of taste and smell, is by my experiments of specific gravity 1·50, affords at 554° Fahr. a purple vapour, and sublimes in shining purple scales, or slender needles in an apparatus open to the air, whereby, however, much of it is destroyed. Some carbon remains after the sublimation. A quick heat produces most sublimate. These needles contain a brown oily matter, which may be di olved out by means of hot alcohol. Their specific gravity is 1·35, according to Mr. Crum. The sublimate from common indigo does not contain any oil, but some indigo-red and the above white crystalline matter. According to Mr. Crum, indigo-blue consists of carbon, 73·22; oxygen, 12·60; azote, 11·26; hydrogen, 2·92; while according to Dumas, crystallized indigo consists of carbon, 73·26; oxygen, 10·43; azote, 13·81; and hydrogen, 2·50: precipitated indigo consists of carbon, 74·81; oxygen, 7·88; azote, 13·98; and hydrogen, 3·33: sublimed indigo, of carbon, 71·71; oxygen, 12·18; azote, 13·45; hydrogen, 2·66. My own analysis afforded—carbon, 71·37; oxygen, 14·25; azote, 10·00; hydrogen, 4·33. In another analysis of Dumas, 3·93 parts of hydrogen were obtained. Hence we must infer that considerable differences exist in the composition of indigo in its purest state. Reagents act upon it much as upon common indigo. Chlorine, iodine, and bromine convert it into a reddish brown soluble substance. Concentrated sulphuric acid, especially the smoking or anhydrous of Nordhausen, di olves indigo-blue with the disengagement of heat, but it makes it suffer some modification; for though it retains an intense dark blue colour, it has become soluble in water, and may be blanched by light, which does not happen with indigo itself. Nitric acid destroys indigo-blue, forms indigotic (carb azotic) acid, carbonic acid, artificial resin, and bitter principle. Indigo-blue may be reduced by substances oxidized, with the co-operation of alkalis or alkaline earths; for example, by such substances as have a strong affinity for oxygen, and are imperfectly saturated with this principle, as the sulphurous and phosphorous acids and their salts, the protoxides of iron and manganese, the protoxide salts of tin, and the corresponding compounds of chlorine, as the proto-chlorides of tin and iron; and the solution of the former in potash. When in these circumstances, in the presence of alkali, a deoxidation or reduction of the indigo-blue takes place, the other bodies get oxidized by absorption of the oxygen of the indigo-blue; the protoxides become peroxides, and the acids in ous become acids in ic , . Several metallic sulphurets also reduce the indigo-blue in the same predicament, as the sulphurets of pota ium, of calcium, of antimony, and of arsenic (orpiment). A similar influence is exercised by fermenting vegetable substances, such as woad, madder, bran, raw sugar (mola es), starch, syrup, in consequence of the formation of carbonic and acetic acids, by absorption of the oxygen of the indigo-blue, for acetic acid and acetic salts are found in the liquor of the warm blue vat, in which indigo has been reduced by means of woad, madder, and bran.