Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

VAT DYEING 373
by reduction in alkaline solution and so converted into the alkali
salt of its dihydro compound ; in technical language, indigo is a vat
dye. Primitive peoples have, from remote times, brought about this
conversion biologically, i.e. by means of reducing bacteria. In industry
ferrous hydroxide or zinc dust was used, now replaced by sodium hydrosulphite.
Experiment.-—Grind about 50 mg. of the indigotin prepared, in a
small mortar, with a few drops of water to a fine sludge, wash this
into a small conical flask with a jet of water from a wash-bottle, and
after adding a small excess of alkaline sodium hydrosulphite solution
warm to 30°-40°. Soon a greenish-yellow, then brownish solution,
the vat, is produced. At the surface of this solution there forms,
as a result of contact with the air, a fine blue film, the so-called
" bloom ". Dilute with water to 25-30 c.c, introduce a previously
moistened strip of linen into the solution, and stir with a glass rod
for about a minute. Then withdraw the cloth, press it, and hang it
over two parallel stretched strings or over two thin glass rods.
Already after five minutes the material is coloured deep blue.
Precipitate the dye again from the vat by drawing air through
the solution. This process is also suitable for purifying indigo.
Chemically, conversion into a vat consists in a 1 : 6-addition of
hydrogen and recalls exactly the conversion of quinone into quinol.
Like quinol, " indigo white ", also a dihydric " phenol ", is a weak acid,
the alkali salts of which are coloured intensely yellow.
iO
NH
sC=C*
OH OH
From its alkali salt, which is partially hydrolysed, the large " indigo
white " molecule is adsorbed by the fibre and then, in this finely divided
condition, dehydrogenated by the oxygen of the air, so that the dye now
remains as a fast blue pigment. The oxidation is analogous to that of
indoxyl.
The vat dyes are characterised by quite unusual fastness ; apart
from the true indigoids, the most important members of this group
are found in the anthraquinone series. (Indigoids are dyes containing
ring systems of the type present in indigo connected by a double bond.)
Almost without exception these dyes contain condensed rings possessing
great chemical stability. The blue dye indanthrene, which is obtained