10 BULLETIN 448, U. S. DEPAETMENT OF AGEICULTURE. 



wliile 94.1 per cent of the other color, under the same conditions, 

 remains in the aqueous layer. If such a mixture in water solution is 

 brought to the given acidity and is shaken out successively with three 

 portions of amy] alcohol, each ecjual to one-fourth its volume, calcu- 

 lation shows that if the distribution ratios remain constant there will 

 be present in the acid solution after the third shaking twenty-seven 

 sixty-fourths or 42 per cent of the first dye, one sixty-fourth or 1.6 

 per cent of the second. Conversely the first amyl alcohol portion 

 after two washings with portions of acid of concentration similar to 

 that of the original solution, will contain 42 per cent and 1.6 per cent 

 of the second and first dyes, respectively. Obviously, for a practi- 

 cable quantitative separation, somewhat greater differences in solu- 

 bihty must exist; but it is usually sufficient to separate, in fairly pure 

 condition, a portion of each of the colors that seem to be present, in 

 order to characterize completely the components of the mixture.* 



Emulsions occasionally cause trouble when the analyst is working 

 with impure mixtures. These are most effectively broken by a good 

 centrifuge. Should a solid stratum form between the two layers, it 

 should be broken with a glass rod, the tube replaced in the cen- 

 trifuge and whirled again. Heatmg tends to promote rapid separation, 

 but the relative solubilities vary somewhat in hot mixtures. Strong 

 acid solutions show much less tendency to emulsif}^ than neutral or 

 alkaline ones, 



Tlie final separation of mixtures of dyes of rather similar solu- 

 bility will usually be made by selecting some pair of solvents in which 

 they show a decided difference. Mixtures of dyes of practically 

 identical solubility can, in most cases, be separated satisfactorily by 

 chemical means or by precipitation reactions. Since the fractiona- 

 tion will have removed all except a few dyes belonging to a known 

 group, suitable chemical methods may usually be chosen without 

 difiiculty. 



T^Tiile the scheme described is not intended to be applied to rela- 

 tively concentrated solutions, in practice in the examination of col- 

 ored food products, such are seldom or never obtained. The chief 

 concern of the analyst here will be to avoid, as far as possible, the 

 dilution of the color by the use of unnecessarily large portions of 

 organic solvents and washing liquids. Only in working with prod- 

 ucts sold for use as coloring matters are solutions likely to be made 

 too concentrated to be adapted to the schem.e of separation. 



As the common food dyes are, for the most part, salts of polybasio 

 acids, the equilibrium conditions are obviously quite complex and 

 concern not only the relative solubilities and dissociation constants of 

 the free color acid, but of the various acid salts and the sodium salt as 



1 For procedure and calculation as to quantitative fractionations, no coloring matter being rejected, 

 see J. Ind. Eng. Chem. 5 (1913), 26. 



