FOOD-COLOBING 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 in 
tint to the dyes likely to be present, and special care is necessary to 
avoid overlooking the dyes. 
It is desirable to obtain as much information as possible from the 
dyeing test, since the separation with immiscible solvents can not 
well be followed with the eye when dealing with dyes of the same 
shade or with the natural coloring matters, which usually consist of 
mixtures 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 dyeing 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 in 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 group tests with small portions of 
the mixture, thus avoiding unnecessary and undesirable additions 
of reagents to the main solution. 
In carrying 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 
widely 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 qualitative 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 separated, of which it is known that one dye, when its amyl alcohol 
solution is shaken with an equal volume of acid of a certain concen- 
tration, distributes itself in equal amount between the two layers, 
61147°— Bull. 448—17 2 
