Formation of colourless reduced indigo-blue, or indigo tine

—Purified indigo-blue is to be treated with copperas and slaked lime, as above described; or the clear wine-yellow supernatant liquor of the cold blue-vat mixture is to be taken, run by a syphon into a matra , a few drops of concentrated acetic or sulphuric acid, deprived of air, are to be poured into it, and the ve el being made quite full, is to be well corked. The reduced indigo soon falls in white flocks, or crystalline scales. They must be edulcorated upon a filter with water deprived of its air by boiling, then pre ed between folds of blotting-paper, and dried under the receiver in vacuo. Indigo-blue may likewise be reduced and di olved by solution of hydro-sulphuret of ammonia; and the colourle indigo tine may be precipitated by muriatic acid. The reduced indigo is sometimes white at the instant of its elimination, sometimes grayish, of a silky lustre, but becomes very readily greenish, blue green, and blue, in the air; in which case it absorbs, according to Berzelius, 4·2 per cent. of oxygen; but according to Liebig, 11·5 per cent. It is void of taste and smell, is insoluble in water; well boiled water free from air is not affected by it, but is turned blue by common water. It di olves in alcohol and ether into a yellow dye; not in dilute acids, but in concentrated sulphuric acid, whereby probably a portion of this is decomposed, and some hyposulphurous acid formed; the colour of this solution is blue. Solutions of the caustic and carbonated alkalis, even the alkaline earths, readily di olve reduced indigo into a wine-yellow liquid; but in contact with air, oxygen is absorbed, and indigo-blue falls, while a purple-coloured froth, pa ing into copper-red, appears upon the surface, just as in the indigo vats of the dyer. The reduced indigo may be combined, by means of complex affinity, with other bases, with the exception of the oxides of copper, zinc, and mercury, which oxidize it. These combinations are white, in part crystallizable, become speedily blue in the air, and afford by sublimation indigo-blue. Berzelius formed with lime a two-fold combination; one easily soluble in water, and another difficultly soluble, of a lemon colour, which contained an exce of lime; this is formed both in the hot and the cold blue vat; in the latter it is occasioned by an overdose of lime. When pure indigo-blue is treated with concentrated sulphuric acid, and particularly with six times its weight of the smoking dry acid, it di olves completely, and several different compounds are produced in the solution. There is first a blue sulphate of indigo; secondly, a similar compound with the resulting hyposulphurous acid; thirdly, a combination of sulphuric acid with the purple of indigo (called Phænicin by Crum), a peculiar substance, generated from indigo-blue. These three compounds are here di olved in an exce of sulphuric acid. The more concentrated the sulphuric acid is, the more blue hyposulphite is formed. The solution in smoking acid, when diluted with water and filtered, affords a considerable precipitate of indigo purple, which that in oil of vitriol does not. The vapour of anhydrous sulphuric acid combines with indigo-blue into a purple fluid. In order to obtain from the dark blue solution each of these blue acids in a pure state, we must dilute it with forty times its weight of water, and immerse in the filtered liquor, well washed wool or flannel, with which the blue acids combine, while most of the sulphuric acid and some other foreign substances remain free in the liquor. The wool must be then scoured with water containing about half a per cent. of carbonate of ammonia, or potash, which neutralizes both of the blue acids, and produces a blue compound. This being evaporated to dryne at the temperature of 140° F., alcohol of 0·833 is to be poured upon the residuum, which di olves the blue hyposulphite, but leaves the blue sulphate undi olved. From either salt, by precipitating with acetate of lead, by acting upon the precipitate with sulphuretted hydrogen water, and evaporation, either of the two blue acids may be obtained. They may be both evaporated to dryne , especially the blue sulphate of indigo; they both become somewhat moist in the air, they are very soluble in water, and the blue sulphate also in alcohol; they have a not unpleasant smell, and an acid astringent taste. From these habitudes, particularly in reference to the bases, it appears that indigo-blue does not comport itself like a saline base towards the acids, but rather like an acid, since it enters into the salts, just as the empyreumatic oil of vinegar and oil of turpentine do into resin soaps. The blue pigment of both acids is reduced by zinc or iron without the disengagement of hydrogen gas; as also by sulphuretted hydrogen, tepid pro to chloride of tin, while the liquor becomes yellow. Indigo-blue sulphate of potash, or ceruleo-sulphate of potash , may be obtained by extracting the blue colour from the wool by water containing 1 per cent. of carbonate of potash, evaporating nearly to dryne , treating the extract with alcohol to remove the indigo-blue hyposulphite , then with acetic acid and alcohol to remove any exce of carbonate of potash. It is found in commerce under the name of precipitated indigo, indigo paste, blue carmine, and soluble indigo. To prepare it economically, indigo is to be di olved in ten times its weight of concentrated sulphuric acid; the solution after twenty-four hours is to be diluted with ten times its weight of water, filtered, and imperfectly saturated with carbonate of potash; whereby a blue powder falls down; for the resulting sulphate of potash throws down the ceruleo-sulphate, while the hyposulphite of potash remains di olved. It is a dark blue copper shining powder, soluble in 140 parts of cold water, and in much le of boiling water. It is made use of as a dye, and to give starch a blue tint. When mixed with starch into cakes, it is sold under the name of blue for washerwomen. Ceruleo-sulphate of ammonia may be formed in the same way. It is much more soluble in water. Ceruleo-sulphate of lime is obtained by saturating the above dilute acid with chalk, filtering to separate the undyed gypsum, and washing with water till the purple colour be extracted. This liquor evaporated and decomposed by alcohol, affords a bluish flocky precipitate, which is more soluble in water than common gypsum, and dries up in a purple-blue film. Ceruleo-sulphate of alumina may be obtained by double affinity; it is dark blue while moist, but becomes black-blue by drying, and is soluble in water. The blue present in all these salts of ceruline is destroyed by sunshine, becomes greenish-gray by caustic alkalis; and turns immediately yellow-brown by alkaline earths. But when the solution is very dilute, the colour becomes first green, then yellow. The carbonates of alkalis do not produce these changes. Nitric acid decomposes the colour quickly. Mr. Crum considers ceruline to be a combination of indigo-blue with water. Phenicine , or indigo-purple combined with sulphuric acid, is obtained when the solution of indigo-blue in concentrated sulphuric acid, has been diluted for a few hours with water, and then filtered. It seems to be an intermediate body into which the indigo-blue pa es, before it becomes soluble ceruline . Hence it occurs in greater quantity soon after digesting the indigo with the acid, than afterwards. It is dark blue, di olves gradually in water, affords after evaporation a blue residuum, of the same appearance as the above blue acids. When a salt is added to it a purple precipitate ensues, which is a compound of indigo-purple, sulphuric acid, and the base of the salt. Indigo-purple is reduced by bodies having a strong attraction for oxygen, if a free alkali or alkaline earth be present, and its solution is yellow, but it becomes blue in the atmosphere. According to Mr. Crum, Phenicine contains half as much combined water as ceruline. The table which I published in 1830 (as given above ) shows very clearly how much the real quality and value of indigo differ from its reputed value and price, as estimated from external characters by the brokers. Various test or proof proce es of this drug have been proposed. That with chlorine water is performed as follows. It is known that chlorine destroys the blue of indigo, but not the indigo-red or indigo-brown, which by the resulting muriatic acid is thrown down from the sulphuric solution in flocks, and the chlorine acts in the same way on the gliadine or gluten of the indigo. Pure indigo-blue is to be di olved in 10 or 12 parts of concentrated sulphuric acid, and the solution is to be diluted with a given weight of water, as, for example, 1000 parts for 1 of indigo-blue. If we then put that volume of liquor into a graduated gla tube, and add to it chlorine water of a certain strength till its blue colour be destroyed by becoming first green and then red-brown, we can infer the quantity of colour from the quantity of chlorine water expended to produce the effect. The quantity of real indigo-blue cannot, however, be estimated with any accuracy in this way, because the other colouring matters in the drug act also upon the chlorine; and, indeed, the indigo itself soon changes, when di olved in sulphuric acid, even out of acce of light, while the chlorine water itself is very susceptible of alteration. Perhaps a better appreciation might be made by avoiding the sulphuric acid altogether, and adding the finely-powdered indigo to a definite volume of the chlorine water till its colour ceased to be destroyed, just as pru ian-blue is decoloured by solution of potash in making the ferro-cyanide. Another mode, and one susceptible of great precision, is to convert 10 or 100 grains of indigo finely powdered into its deoxidized state, as in the blue vat by the proper quantity of slaked lime and solution of green sulphate; then to precipitate the indigo, collect and weigh it. The indigo should be ground upon a muller along with the quicklime, the levigated mixture should be diluted with water, and added to the solution of the copperas. This exact analytical proce requires much nicety in the operator, and can hardly be practised by the broker, merchant, or manufacturer.