WHITE INDIGO.

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The sulphurets which aft as reducing agents—as, for instance, realgar, orpi-ment, the protosulphurets of tin and antimony—exert their reducing aftionas a consequence of their tendency to become oxidised by the influence ofalkalies; the sulpharsenic and sulphantimonic acids, and bisulphide of tin,are inert.

On the small scale, the hydrate of protoxide of iron and glucose are fre-quently applied in chemical laboratories for the purpose of preparing indigo-vats. For this purpose the following mixture answers very well:—I part offinely ground indigo,* 2 parts of pure sulphate of protoxide of iron, 3 parts ofslaked lime in fine powder, and 200 parts of water: the dry substances are,when ground, well mixed together, and introduced into a narrow-necked flask,into which should be inserted a tightly-fitting cork ; the water (best tepid) ispoured in after the dry substances have been put into the vessel, which shouldbe completely filled by the liquid, and immediately afterwards closed by the cork.The flask is shaken now and then, and left standing for several hours ; the yellowliquid is a solution of white indigo in lime-water ; the deeper yellow precipitateis a mixture of sulphate of lime and hydrated peroxide of iron. The reactionswhich take place in this experiment may be briefly summarised thus :—A por-tion of the hydrate of lime decomposes the sulphate of iron, forming hydratedprotoxide of iron, which begins to decompose water, immediately the nascenthydrogen reafts upon the indigotin, converting it into white indigo, which inits turn combines with another portion of the lime, and becomes dissolved inwater as a calcic compound—

2 (C 8 H s NO) + H 2 = C i6 H I2 N 2 0 2 .

Indigotin. White indigo.

According to Fritzsche the glucose-vat is prepared as follows:—125 grms.of finely-powdered indigo, along with an equal weight of grape-sugar, areplaced in a flask of about 6 kilos, capacity, filled partly with boiling alcoholat 71 per cent; to this fluid are added 200 grms. of a concentrated alcoholicsolution of caustic soda, and next the flask is entirely filled with boilingalcohol; the flask, well corked, is shaken for a time, and then left standing;the clear supernatant liquid, having been decanted, yields, when stirred up incontaft with air, a precipitate of blue indigotin, in the shape of coppery-looking, small, needle-shaped crystals, these being only obtainable in thehumid way by the process here described.

While it is easy enough to prepare white indigo in alkaline solutions, it isdifficult to obtain that body in a pure and solid state,—a preparation whichrequires great care and dexterous manipulation. White indigo is so readilyoxidisable, when once exposed to air, that the manipulations required for pre-cipitating, washing, filtering, and drying it, cause it to be almost entirelyre-converted into blue indigotin, unless it is completely kept from contaftwith air.

M. Dumas gives the following description of his process devised for pre-paring white indigo :—An indigo-vat, with sulphate of iron as above described,is made with 500 grms. of indigo, in a thoroughly-closed vessel suitable forthe purpose : after having been left standing the clear liquid is, by means of a

* Although somewhat difficult to pulverise, on account of a peculiar toughness of this sub-stance, it is preferable not to purchase it in the powdered state, as it is then far more likely to beadulterated.