Indigo sulphonic acids

Purple, rhombic crystals with coppery luster, or dark-blue powder with a reddish tint, which, on pressure or rubbing, cxliibits a coppery luster. Indigo blue is insoluble in water, alcohol, ether, dilute acids, and alkalies. It dissolves in If) parts of concentrated sulphuric acid, or 5 parts of fuming sulphuric acid, with the formation of indigo sulphonic acids. 

The free acid is obtained by saponification of the ether with an alkali. It forms a crystalline precipitate, sparingly soluble in water. It splits up into indoxyl and carbonic acid on heating. Oxidising agents convert it to indigo blue, W hile on heating with concentrated sulphuric acid indigo sulphonic acid is formed. 

Supplement 1955 3634-3793 Sulphonic acids Indigo-disulphonic acid (indigocarmine), 304. Amines, 308. Keto-ammes Pyramidone, 452. Allan-toin, 474. Murexide, 499. Amino-carboxylic acids Histidine, 513. j Hydrazines, 531. Azo compounds, 535. 1... 

Certain coloured bodies like indigo are neutral, and possess no affinity for textile fibres. Their dyeing is effected by precipitating on the fibre from a solution as in vat dyeing, or by converting into a sulphonic acid, and thereby producing a capability to form salts. 

The commercial dyestuffs known as Patent Blue N and Superfine are lime-salts of the sulphonic acids, obtained by sulphonation and subsequent oxidation of the above leuco-bases. The shades produced are similar to those of indigo extract, and have the advantage of greater fastness and brilliancy (D. R. P. 46384, 48523 Chemikerz. 1889, p. 1702). 

The sodium salts of the sulphonic acids of various indulines come into commerce under the names of Fast Blues, Blackley Blue, Indigo substitute, c. The diflerence of the various marks depends on the shade of the induline sulphonated and the number of sulpho-groups introduced. 

From the derivatives of indigo blue (sulphonic acids, c.), substitution-derivatives of indigo white are obtained on reduction. 

Indoine is formed by reduction of orthonitrophenylpropiolic acid with ferrous sulphate in sulphuric acid solution. It closely resembles indigo, but may be distinguished by the following reactions —It dissolves in cold concentrated sulphuric acid with a blue colour (indigo gives a green solution), and is only converted into a sulphonic acid with difSculty. It is also easily soluble in aniline and in aqueous sulphurous acid, forming blue solutions. 

Indigo red forms a vat and a sulphonic acid, both capable of dyeing. The shades produced in the vat are noteworthy for their extreme fastness to light. 

Two isomers occur within indigo carmine, reflecting different sulfonation points on the molecule disodium 3,3 -dioxo-[d -biindoline]-5,5 -disulfonate and disodium 3,3 -dioxo-[D -biin-doline]-5,7 -disulfonate (also given as 2-(l,3-dihydro-3-oxo-5-sulfo-2H-indol-2-ylidene)-2,3-dihydro-3-oxo-lH-indole-5-sulphonic acid disodium salt Merck Index, 1992). It is prepared by treatment of indigo with sulfuric acid. 

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