When pure, aniline is a colourless oily liquid, but on exposure rapidly
turns brown. It has strong refracting powers and an agreeable aromatic
smell. It is very poisonous when taken internally; its sulphate is,
however, sometimes used medicinally. It is by the action upon aniline of
certain oxidising agents, that the various colouring matters so well
known as aniline dyes are obtained.
Commercial aniline oil is not, as we have seen, the purest form of
rectified aniline. The aniline oils of commerce are very variable in
character, the principal constituents being pure aniline, para- and
meta-toluidine, xylidines, and cumidines. They are best known to the
colour manufacturer in four qualities--
(_a_) Aniline oil for blue and black.
(_b_) Aniline oil for magenta.
(_c_) Aniline oil for safranine.
(_d_) _Liquid toluidine.
From the first of these, which is almost pure aniline, aniline black is
derived, and a number of organic compounds which are further used for the
production of dyes. The hydrochloride of aniline is important and is
known commercially as "aniline salt."
The distillation and rectification of aniline oil is practised on a
similar principle to the fractional distillation which we have noticed as
being used for the distillation of the naphthas. First, light aniline
oils pass over, followed by others, and finally by the heavy oils, or
"aniline-tailings." It is a matter of great necessity to those engaged in
colour manufacture to apply that quality oil which is best for the
production of the colour required.
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