The tannate of
rosaniline (colour principle of Magenta) is a tolerably insoluble lake,
which can be precipitated by Magenta from a solution of tannate of soda,
the Magenta being capable of displacing the soda. But tannic acid,
alone, does not form very fast lakes with Magenta and the other basic
dyestuffs, and so a means of rendering these lakes more insoluble is
needed. It is found that tannic acid and tartar emetic (a tartrate of
antimony and potash) yield a very insoluble compound, a tannate of
antimony. Perchloride of tin, in a similar manner, yields insoluble
tannate of tin with tannic acid. These insoluble compounds, however,
have sufficient acid-affinity left in the combined tannic acid to unite
also with the basic aniline colours, forming very fast or insoluble
colour lakes. This principle is extensively used in practice to fix
basic aniline colours, especially on cotton. We should first soak the
piece of cotton in a solution of tannic acid, and then pass it into a
solution, say, of tartar emetic, when the tannic acid will be firmly
fixed, as tannate of antimony, on the cotton.
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