FOOD^OLORING SUBSTANCES. 21 



fairly pure condition b}^ the fractionation and the solutions obtained 

 will be ready for identification tests. Obviously, no essential differ- 

 ence exists between these and the so-called coal-tar colors ; as a class, 

 however, they show much less tendency to dye wool than do the 

 common synthetic colors, and m addition are in many cases so sensi= 

 tive to alkahes as to be completely destroyed in the double-dyeing 

 test; i. e., by dyeing, stripping the fiber with dilute ammonia, acidi- 

 fying, and dyeing again. The preliminary dyeing with wool described 

 on page 8 serves fairly well in practice as a fii'st indication of 

 the course to be followed; but when for some reason the results 

 obtained are not decisive, the general treatment for coal-tar colors 

 with immiscible solvents should be carried out with consideration of 

 solubihties of the coloring matters described in the footnotes in the 

 tables of solubilities. In the dyeing test not only are certain S}ti- 

 thetic dyes, especially Aiu-amin, mistable under the conditions of 

 treatment and likely to be destroyed ; but, on the other hand. Archil, and 

 to a lesser extent other natural colors, may give well-marked dyeings. 



The crude products constituting the commercial natural coloring 

 matters in most cases are mixtures containing several closely related 

 chemical individuals. These may have different solubilities, but 

 usually they contain the same chromophore groups, and are of closely 

 similar shade. In practice, the analyst will scarcely attempt a full 

 separation, but having identified the coloring matter in one fraction, 

 can judge as to the likelihood of the other substances present being 

 derived from the same soiu'ce. 



The natural colors as a class do not contain strongly acidic groups, 

 and their distribution ratios between immiscible solvents do not show 

 wide variations with the acidity of the latter, at least not within 

 convenient limits. The coloring matters of cochineal and tm'meric 

 give less trouble than the others, partly because they are less hete- 

 rogeneous. 



When coal-tar dyes are absent, and it is desired to fractionate with 

 immiscible solvents, it is best to -begin extraction with neutral solu- 

 tions; perhaps first using ether (petroleum ether is better when 

 chlorophyll and the accompanying leaf colors are to be separated). 

 The final extraction may be made with amyl alcohol from acid solu- 

 tion, but it is of no advantage to have the acidity high, not, perhaps, 

 above thirty-s(!cond normal. The anthocyans wliich constitute tlic 

 coloring matters of the common red fruits and flowers are glucosids, 

 and are extract(!d from acid solution only in very small amountby amyl 

 alcohol and similar solvents. Tlieir ncutrad solutions may be treated 

 with excess of lead acetate solution (normal salt) when practically 

 aU of the glucosid will b(; precij)itat(!d. Tliis mixture may be cen- 

 trifugcid and the precipitate waslicd in tlie centrifuge tubes with 

 several portions of water until sugar and similar soluble substances 



