FOOD-COLORING SUBSTANCES. 35 
IDENTIFICATION OF COLORING SUBSTANCES. 
COAL-TAR DYES. 
The coloring matters are usually obtained by the fractionation dis- 
solved in various aqueous or organic solvents, but free from non- 
volatile substances. Neutral solutions suitable for certain tests are 
most easily obtained by evaporating to dryness and taking up the 
residue with water or other suitable solvent. 
It is not intended to discuss here the innumerable tests' that 
may be used for the individual dyes. There are described a limited 
number only of general procedures which are quickly and easily 
carried out, and the chemistry of which is for the most part under- 
stood. These tests are sufficient for identification in ordinary cases. 1 
For ready comparision of colors of similar solubility, it is con- 
venient to have tables of properties in which the arrangement is based 
on the solubility. The familiar reactions of reduction and of color 
changes with reagents applied to the dyed fiber are given in this 
order in Tables 2 and 3, these -tests being made as follows: 
Reactions on dyed fiber. 2 — Small pieces or shreds of the dyed wool 
are distributed on the porcelain plate and are thoroughly moistened 
with the reagents. The fiber must be dry or nearly so, and must have 
been dyed in a fairly pure solution of the color, since colorless organic 
impurities may easily obscure the reaction. 
Reduction and subsequent reoxidation. 3 — The neutral color solution 
is treated with a few particles of powdered sodium hydrosulphite, 
conveniently dropped in from a small spatula. If no color change is 
seen at once, the mixture is warmed somewhat and more reagent 
added, carefully avoiding excess, however. If reduction, shown by 
disappearance of the color, takes place, the solution is thoroughly 
shaken with air, and should this not bring back the dye, it is warmed 
and allowed to stand a few minutes. Finally, if remaining practically 
colorless, a little powdered potassium persulphate is dropped in. A 
slight yellowish or brownish tint produced by air or especially by the 
potassium persulphate is disregarded. 
In regard to the tests on the dyed wool, it may be said that the dye- 
ings obtained from colors in food products are necessarily variable in 
depth and usually paler than those used as a basis for standards. 
With the very small amounts of color available, it is impossible to 
1 For the identification of the simpler azo dyes by reduction, separation of the reduction products and 
characterization of these by coupling with diazo compounds, by condensation with nitrosodimethylani- 
lin, and by diazotization, see especially Witt, Ber. Chem. Ges. 19 (1886), 1719, and 21 (1S88), 3468. 
Properties of the various amins, aminophenols, and their sulphonic acids are summarized by Heumann 
(Friedliinder, Schultz), Die Anilinfarben. Braunschweig, 1888-1906. 
2 The tables are made to correspond as nearly as possible with those of IT. S. Dept. Agr., Bur. Chem. 
Cir. No. 63, 0.5 per cent dyeings and reagents of similar concentration being used. 
3 Hydrosulphite and persulphate are the reagents advocated by Green, Yoeman, and Jones, J. Soc. 
Dyers & Colorists9 (1905), 236; also Green, Identification of Dye Stuffs, Leeds (1913). 
