Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

304 THBOKY OF DYEING
After the liquid has become clear, filter at the pump, wash the
yellowish-brown precipitate with water, and dry thoroughly, first
on porous plate and then in a vacuum. Then recrystallise from
petrol ether (boiling point .50°-60°) decolorising with a little animal
charcoal. Melting point 98°. Heat a sample with dilute hydrochloric
acid in a test tube.
Further, add 1 g. of dry finely powdered aniline hydrochloride to
5 g. of aniline and heat the mixture in a test tube on the water bath
at 30°, with 2 g. of dry diazoaminobenzene. Continue heating the
frequently stirred mixture for half an hour. Then raise the temperature
to 45° and heat again for half an hour. When now a
sample no longer evolves nitrogen on heating with hydrochloric
acid, dissolve the aniline by adding 24 c.c. of 10 per cent hydrochloric
acid (6 c.c. of concentrated acid and 18 c.c. of water). Eecrystallise
the aminoazobenzene hydrochloride, which remains undissolved, from
100 parts of hot water to which a little hydrochloric acid has been
added. In order to obtain the orange-yellow base decompose the
salt with sodium carbonate.
On the Theory of Dyes.—In carbon compounds absorption in the
visible part of the spectrum, i.e. subjective colour, is conditioned by
the presence of a so-called chromophoric group in the molecule. The
nitroso-group is strongly chromophoric, the nitro-group much less so,
whilst the azo-group is quite considerably chromophoric, but only in
aromatic systems. Azomethane is colourless.
The intensely red azobenzene, however, is no more a dye than is
nitrosobenzene. In order to make it into a dye, another group is
necessary which, in virtue of its chemical nature, confers affinity
towards the fibre and at the same time deepens the colour. The most
important of these groups, which are called auxochromes, are OH and
NH2. We have encountered their auxochromic influence in the simple
cases of o-nitrophenol and the nitranilines.
Wool and silk are protein-like substances and hence are amphoteric.
Accordingly, they can combine with acids as well as with bases. For
this reason wool and silk can be dyed directly by dyes in virtue of their
auxochromic groups.
It is otherwise with cotton, which is almost chemically pure cellulose,
and hence is chemically indifferent in a tinctorial sense. Here combination
with the dye results from the use of mordants which are adsorbed
colloidally on the fibre before dyeing. The mordant can then enter
into chemical union with the dye as a complex compound. For an
important group of acid dyes (p. 335) the mordants are chiefly metallic
hydroxides, namely, those of chromium, aluminium, iron, antimony,
tin, etc., whilst for basic dyes tannin is the usual mordant.