3 COLOR REACTIONS FOR THE IDENTIFICATION OF NATURAL MORDANT DYES

THE PRINCIPLE of the reactions described here is that natural mordant dyes form lakes of various colors with tin, aluminum, iron, copper, and uranium. In addition, treatment of the tin lakes with 20% ammonia solution sometimes results in very striking color changes that can lead to the identification of certain dyes, such as orchil or sandalwood. The complete set of six colors is usually unique and provides a rapid and simple means for identifying many natural mordant dyes, provided of course that comparison sets obtained from authentic samples are available.

It is usual to form the tin lake first, since treatment with a strong solution of stannous chloride destroys the color of most acid and direct azo dyes and of acid metal-complex dyes, thus providing an additional indication that a synthetic dye is present.

The stannous chloride reagent is prepared by dissolving stannous chloride dihydrate (1 g) in concentrated hydrochloric acid (1 ml) and diluting with water (4 ml). The procedure is to take a small portion of the fabric that has been cleaned with dilute ammonia solution, boil it in the stannous chloride reagent, allow it to stand in the reagent for 10 minutes, wash it thoroughly with water (until the washings are neutral) and divide it into halves. Drying the treated material for this and the following tests can be assisted by washing it with methanol and then pressing it between filter papers. One half is dried and set aside for comparison purposes. The other half is treated with 20% ammonia solution.

The other lakes are formed similarly by treating four separated portions of the clean sample with 2% aqueous solutions of alum, ferrous sulfate, copper sulfate, and uranyl acetate. As in the formation of the tin lake, the solutions are boiled and then allowed to stand for 10 minutes before the material is washed with water.

The following figures illustrate the various lakes formed by natural dyes of more than local importance that are, or were, used for dyeing textiles. The dyes are grouped by color, since this is the most obvious classification for the analyst.