2 BULLETIN 448, U. S. DEPAETMEXT OP AGKICULTUEE. 



throsin, Orange I, Napththol yellow S, Tartrazin, Light green S. F. 

 yellowish; and Indigo disuKoacid — 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 bulletm 

 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 Httle is known regarding the stabihty 

 of many of them with the common reagents. Although the most 

 important natural colors have been included in the tables, Httle 

 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.^ 



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 

 alkahes 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 weU 

 and are to be preferred because of their greater suitabiHty 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. 



