FOOD-COLORING SUBSTANCES. 9 



once or twice if necessary and making a further examination of the 

 color substance obtained from the dyed wool. This procedure is 

 especially advantageous when dealing with cacao products, since 

 such products give extracts containing much natural color similar m 

 tint to the dyes likely to be present, and special care is necessary to 

 avoid overlooking the dyes. 



It is desirable to obtam as much information as possible from the 

 dyeing test, since the separation with immiscible solvents can not 

 well be followed with the eye when dealmg with dyes of the same 

 shade or with the natural coloring matters, which usually consist of 

 mixtm'es of substances of similar tint but different solubility. 



Attention must be called to the fact that a few rather common 

 dyes are so unstable as to be very easily overlooked when making 

 the dyemg test; for instance, Auramin (No. 425), which is largely 

 used at the present time in European countries for food coloring, is 

 readily decomposed both by acids and alkalies. Naphthol green B 

 also is easily decomposed by acids and not readily dyed on wool 

 from many mixtures. Further, many dyes do not go on wool readily 

 in the presence of certain impurities. In such cases, although 

 getting no positive results by the dyeing test, the analyst should 

 proceed with the separation by immiscible solvents. 



GENERAL STATEMENTS REGARDING THE SEPARATION OF DYE MIXTURES. 



The analyst usually knows something in regard to the coloring 

 matters present m a dye solution before beginning the systematic 

 analysis. The best procedure to be followed will depend on what 

 dyes are probably present; and no set method can equal in value a 

 table of relative solubilities by means of which the distribution con- 

 stants of any given dyes may be compared. It is, of course, advan- 

 tageous in many cases to make gi'oup tests with small portions of 

 the mixture, thus avoiding unnecessary and undesirable additions 

 of reagents to the main solution. 



In carrymg out the fractionations described on pages 11 to 18 any 

 given color will, in general, appear in several washings, but where the 

 maximum amount comes out will be evident from the solubility data ; it 

 being always remembered that these statements apply to solutions 

 of concentration in the neighborhood of 0.01 per cent, and that at 

 wid(!ly different dilutions some variation may be expected. The 

 solubilities of the components of the dye mixture are not likely to be 

 so different as to allow even a quahtative separation by a single 

 shaking out. It is usually necessary to employ more or less system- 

 atic fractionation methods. For example, suppose a mixture is to 

 be Koparalcd, of whirh it is known that one dye, when its amyl alcohol 

 solution is shuk(!n with uw e(iunl volume of acid of a certain concen- 

 tration, distributes itself in cf|ual amount between the two layers, 

 fill47^— Bull. 448-17 2 



