This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
The French official tests for the presence or absence of foreign colours in wine are as follows: -
Fifty c.c. of the wine are made slightly alkaline with ammonia, and shaken with about 15 c.c. of colourless amyl alcohol. The latter should remain uncoloured. If so, it is decanted, filtered, and acidified with acetic acid; it should still remain uncoloured.
Treat the wine with a 10 per cent. solution of mercury acetate until there is no further change in colour, then make the mixture alkaline with slight excess of magnesia. Boil, and filter; the liquid made slightly acid with dilute sulphuric acid should remain uncoloured.
Fifty c.c. of the wine are taken in a porcelain dish (7-8 cm. diameter), one or two drops of dilute sulphuric acid added, and a tuft of white wool. Boil for exactly five minutes, adding boiling water as the liquid evaporates. Withdraw the wool, and wash it in water. It should be scarcely tinted, of dull rose colour. Placed in ammoniacal water, it should assume a dull green tinge.
These tests readily show whether or not a wine contains aniline colours. The identification of the particular colours, if found to be present, is often a troublesome task, and not usually worth while attempting. Added vegetable colours are especially difficult to detect, by reason of their similarity to those naturally present in the wine.