2 BULLETIN 448, U. S. DEPARTMENT OF AGRICULTURE. 
throsin, Orange I, Napththol yellow S, Tartrazin, Light green S. F. 
yellowish, and Indigo disulfoacid — and among nonpermitted dyes of 
the oxy-monazo colors. An entirely different scheme might be pref- 
erable if, for instance, the difficultly soluble benzidin dyes were the 
most common colors. 
The method for the separation of colors described in this bulletin 
is based mainly upon the employment of immiscible solvents. By 
this means most mixtures of the commonly occurring coal-tar dyes 
may be separated with relative ease. In dealing with the natural 
coloring substances the analyst is hampered by the lack of any 
exact knowledge concerning many of them, by the difficulty of 
obtaining pure preparations free from accompanying colored sub- 
stances, by the lack of good methods for their quantitative esti- 
mation, and by the fact that little is known regarding the stability 
of many of them with the common reagents. Although the most 
important natural colors have been included in the tables, little 
attempt has been made to indicate means of separation other than 
by the methods more suitable for synthetic dyes. 
Of the colors used in developing the methods described in this 
paper about 40 of the commonest were synthesized and purified in 
the laboratory. The physical and chemical properties of the others 
indicated their identity and proved that the samples were of suffi- 
cient purity for practical purposes. 1 
GENERAL STATEMENTS CONCERNING REAGENTS USED IN COLOR 
ANALYSIS. 
The examination of food-coloring matters requires the frequent 
employment of some reagents that are not so often used in other 
kinds of chemical work. In addition to the ordinary acids and 
alkalies it is convenient for many purposes to have solutions of 
hydrochloric acid and of sodium hydroxid of accurately known 
strengths. Five-normal and tenth-normal hydrochloric acid and 
tenth-normal sodium hydroxid may be kept in stock bottles pro- 
vided with attached burettes and guard tubes. Of the dilute solu- 
tions, eighth-normal is the most convenient concentration for use 
in separations, but tenth-normal solutions serve the purpose well 
and are to be preferred because of their greater suitability for titra- 
tions. A standard solution of five-normal sodium hydroxid is also 
needed and a portion may be kept in a 500-cc bottle, through the 
rubber stopper of which passes a graduated 10-cc pipette which is 
capped or closed above when not in use. 
1 The coal-tar dyes have been designated in this bulletin by the numbers given in the tables in A 
Systematic Survey of the Organic Coloring Matters; by A. G. Green. Founded on the German of Drs. G. 
Schultz and P. Julius, second edition, London and New York, 1904. On page 56 have been given the 
corresponding numbers used by G. Schultz, Farbstofftabellen, Berlin, 1911-1914, and by Mulliken, A 
Method for the Identification of Pure Organic Compounds, vol. 3, New York, 1910. 
