442

DYEING AND CALICO PRINTING.

syphon, transferred to bottles previously filled with carbonic acid gas, and onthe bottom of which is poured also some pure hydrochloric acid freed fromair, in sufficient quantity to saturate the lime and precipitate the white indigo.Every bottle should be completely filled, stoppered immediately, and nextimmersed in a tub filled with water, and left standing until all the white indigois precipitated. As soon as this is the case the clear supernatant liquid iswithdrawn by means of a syphon, and the precipitate placed on a filter andleft to drain under a bell-jar filled with carbonic acid gas: the precipitate iswashed with cold water, freed from air by long-continued boiling, and cooledin well-stoppered bottles ; the precipitate is next dried over sulphuric acidunder the receiver of a good air-pump. White indigo so prepared is acoherent whitish-grey mass, of silky lustre, void of taste and smell, insolublein water, soluble in alcohol, ether, alkaline solutions, and lime and barytawaters, forming yellow solutions. When in contadt with air the white indigobecomes gradually blue, through and through ; but this change takes place farless slowly than when the compound is moist. Oxidising substances producethe same effedt, and this also applies to a comparatively slight elevation oftemperature, which has the effedt of turning the white substance suddenlyblue : dry distillation decomposes this body, there being formed a small quan-tity of sublimed indigotin, while a bulky charcoal is left. Strong sulphuricacid dissolves white indigo, producing a deep purple liquid ; after addition ofwater a solution of sulphindigotic acid remains.

White indigo is a very weak acid ; it dissolves in and combines with alkalies,alkaline carbonates, and the alkaline earths; but the combination is so feeblethat even the attradtion of textile goods is sufficiently powerful to break it up,and cause the precipitation of the acid on the fibre.

Many of the metallic salts yield, with the alkaline solutions of indigo, heavy,white, sometimes even crystalline, precipitates, which may be considered asconstituting compounds of white indigo with the corresponding metallicoxides. Such are the following:—Salts of magnesia, alumina, zinc, protoxideof iron, cobalt, manganese, lead, silver, protoxide of tin. Salts of copper adtas energetic oxidising substances, converting the white indigo into blueindigotin.

According to Berzelius white indigo forms two compounds with lime, asoluble and an insoluble, the latter containing excess of lime : this fadt is ofpradtical interest, inasmuch as if—in the making up the so-called copperas-vat■ — too much lime is added, a portion of the indigo will precipitate and remaininadtive at the bottom of the vat.

A boiling, concentrated, caustic potassa-Iye, sp. gr. i‘45, containing about45 per cent of dry caustic potassa, dissolves indigo, yielding an orange solution.On the addition of water the liquid becomes at first brownish-yellow, and de-posits, when in contadl with air, indigotin, while isatate of potassa (isatin+ hydrate of potassa) remains in solution. There takes place, in thisinstance, an oxidation as well as a redudtion of indigotin, according to thefollowing formula:—

3C8H5NO + KHO + H 2 0 = C 8 H 6 NK 0 3 -|- C i6 H I2 N 2 0 2 .

Isatate of potassa. White indigo.

The oxidation of this last body gives rise to the formation of indigotin :indigotin fused with caustic potassa gives off hydrogen, and is converted into