2nd. Baryta water is added to the wine under examination until the mixture acquires a greenish hue, after which it is shaken with acetic ether or amylic alcohol. If the wine be pure, the upper layer remains colourless, even after acidulation with acetic acid; whereas, in presence of basic coal-tar dyes, such as fuchsine, amidobenzole, safranine, chrysoidine, chrysaniline, etc., characteristic colorations will be obtained.
3rd. A few c.c. of the sample are made alkaline by the addition of dilute potassium hydroxide, some mercuric acetate added, and the mixture agitated and filtered. With pure wines, the filtrate is colourless; in the presence of acid coal-tar derivatives, it is red or yellow.
The general character of the artificial dye contained in the wine having been ascertained by the foregoing tests its more precise nature is determined as follows:—
In case the foreign colouring is basic, the supernatant layer obtained in the second test is separated, and divided into two portions; one portion being evaporated with pure woollen yarn, the other with filaments of silk. The dyed threads are then subjected to the following tests:—
(a) Rose-aniline or safranine affords a red coloration; safranine usually attaches itself only on silk.
(b) Soluble aniline violet produces coloured threads which become green upon treatment with hydrochloric acid, the primitive colour reappearing upon dilution with water.
(c) Mauve-aniline gives a colour which turns blue upon addition of the acid.
(d) Chysotoluidine causes a coloration which is only slightly affected by the acid, but which is discharged upon boiling with zinc powder; upon protracted exposure to the air the colour reappears.
(e) Chrysoidine and Amidonitrobenzole produce yellow colours, the former turning poppy-red if treated with sulphuric acid, the latter, scarlet. A general characteristic of dyes, similar to rose-aniline, is that they are decolorised by treatment with sodium bisulphite.
If the presence of an acid coal-tar dye is indicated by the third preliminary test, the following special methods of procedure are employed:—