444

DYEING AND CALICO PRINTING.

purple remains, and may be employed to dye wool in an acidulated bath.The aqueous solutions of sulphopurpuric acid are blue, and yield whenneutralised with either an alkaline carbonate or acetate a purple flocculentprecipitate; the ensuing alkaline salts are red in the dry state and blue inaqueous solution. Their solubility in water is not very great, i part of thepotassa salt requiring ioo parts of water; even very dilute solutions of sulpho-purpuric acid are precipitated by the salts of lime, magnesia, alumina,iron, tin, and copper. Reducing agents, such as protochloride of tin, albumen,&c., bleach this acid, but its colour is restored by exposure to air. Concen-trated sulphuric acid converts sulphopurpuric acid into sulphindigotic acid,which latter is in its turn re-converted into sulphopurpuric acid by digestionwith indigo. By long-continued boiling with water the colour of sulpho-purpuric acid is converted into a deep blue; hot alkaline lyes turn it green,but in the cold alkalies are without adtion upon it, though they cause sulph-indigotic acid to become yellow.

Sulphindigotic acid, C8H 5 N0,S0 3 , is readily obtained by the aftion of sul-phuric acid either upon indigotin or crude indigo. When ordinary concentratedsulphuric acid is employed the best proportion is from 15 to 20 parts of the acidto 1 of indigo; the reaction is rendered more complete by the aid of a tem-perature of 40° t0 50°; when fuming sulphuric acid (so-called Nordhausen) isused the proportion of acid is from 6 to 8 parts; the blue liquid (which, iffuming acid is used, inclines more to a purple) is poured into from thirty toforty times its bulk of cold water; any sulphopurpuric acid present is therebythrown down, and after having been removed by filtration, there is obtaineda deep blue liquid, containing sulphindigotic, and a certain quantity ofhyposulphindigotic, acid. These acids are separated from each other, accordingto Berzelius, by immersing white woollen worsted yarn in the liquid; thedye is attra&ed by the wool, which is next digested for some time with a weaksolution of carbonate of ammonia, whereby the coloured acids are re-dissolved.This solution is evaporated to dryness at 50° on a water-bath, and the dryresidue exhausted with alcohol at 83 per cent strength, which dissolves thehyposulphindigotate of ammonia ; the insoluble residue is taken up withwater, and precipitated with acetate of lead; the sulphindigotate of leadhaving been colle&ed on a filter and washed, is suspended in water, and de-composed by a current of sulphuretted hydrogen gas. The sulphide of leadbeing removed by filtration, the filtrate is at first colourless, but becomes blueby contact with air. On being evaporated at 50° sulphindigotic acid remainsbehind as an amorphous deep blue, hygroscopic mass, which has an acid andsimultaneously astringent taste, and a pleasant^very characteristic smell; thisacid is very soluble in water and alcohol. The salts of this acid are blue,amorphous, and exhibit in the dry state very strongly the coppery lustre ofgood indigo. These salts are prepared either by the direct reaction of theacid and a base, or by the reaction of the acid with a carbonate or acetate,while the sulphindigotates, which are insoluble, or only very slightly soluble,may, of course, be prepared by double decomposition. The alkaline salts ofthis acid are very sparingly soluble in liquids which contain neutral salts insolution, as, for instance, chlorides of potassium and sodium, sulphate of soda,and, consequently, the alkaline sulphindigotate is precipitated as a flocculentmatter when to any of its aqueous solutions a solution of any neutral salt is