Indigo, Sul′phate of

Syn. Sulphindylic acid , Sulph indigotic a. , Saxony blue , Soluble indigo . Prep. By gradually adding indigo (in fine powder), 1 part, to fuming sulphuric acid (Nordhausen sulphuric acid), 5 parts, or oil of vitriol, 8 parts contained in a stone-ware ve el placed in a tub of very cold water, to prevent the mixture heating; the ingredients are stirred together with a gla rod at short intervals until the solution is complete, after which the whole is allowed to repose for about 48 hours, by which time it becomes a homogeneous pasty ma of an intense blue colour, which in a dull light appears nearly black. Obs. In this state it forms ‘ Barth’s blue ,’ or the ‘ CHEMIC BLUE ’ or ‘ INDIGO COMPOSITION ’ of the dyer. Diluted with about twice its weight of soft water, it is converted into the ‘ Saxony blue ’ or ‘ LIQUID BLUE ’ of the shops, also used for dyeing. When commercial sulphate of indigo is diffused through a large quantity of water, nearly boiling, and wool (old white flannel rags, .) is macerated in it for some time, the latter absorbs the whole of the sulphate and is dyed blue, whilst the liquor a umes a greenish-blue colour. Wool, so prepared, when well rinsed in cold water, and boiled for some minutes in a large quantity of that liquid containing 1% or 2% of carbonate of pota a, or a quantity equal to about 1-3rd that of the indigo originally employed, gives up its blue colour, and becomes of a dull brown. The liquid is now a rich blue-coloured solution of sulphindylate of pota a, from which the salt may be obtained by cautious evaporation. This compound is prepared on the large scale, by diluting sulphate of indigo with about 12 times its weight of soft water, and imperfectly saturating the solution with carbonate of pota a; the sulphindylate falls down as a dark-blue coppery-looking powder, soluble in 140 parts of cold water and in about 90 parts of boiling water. This substance is kept both in the moist and dry state, and is known in commerce under the respective names of ‘ DISTILLED INDIGO ,’ ‘ PRECIPITATED INDIGO ,’ ‘ SOLUBLE INDIGO ,’ ‘ INDIGO PASTE ,’ ‘ BLUE CARMINE ,’ ‘ DISTILLED BLUE ,’ ‘ SOLUBLE BLUE ,’ . It is extensively used in dyeing; and when mixed with starch, whilst in the moist state, and made into cakes or knobs, it constitutes the finest variety of the ‘ BLUE ’ used by laundre es for tinging linen. The ammonia and soda salts may be prepared in the same way as the pota a salt, by substituting the carbonates of those bases for carbonate of pota a. The ammonia salt is very soluble.