FOOD-COLOEIl^G SUBSTANCES. 35 



roENTIFICATION OF COLORING SUBSTANCES. 



COAL-TAR DYES. 



The coloring mPttters are usually obtained by the fractionation dis- 

 solved in various aqueous or organic solvents, but free from non- 

 volatile substances. Neutral solutions suitable for certain tests are 

 most easily obtained by evaporating to drjmess and taking up the 

 residue with water or other suitable solvent. 



It is not intended to discuss here the mnumerable tests that 

 may be used for the individual dyes. There are described a limited 

 number only of general procedures which are quickly and easily 

 carried out, and the chemistry of which is for the most part under- 

 stood. These tests are sufficient for identification in ordinary cases.^ 



For ready comparision of colors of similar solubility, it is con- 

 venient to have tables of properties in which the arrangement is based 

 on the solubiUty. The familiar reactions of reduction and of color 

 changes with reagents apphed to the dyed fiber are givan in this 

 order in Tables 2 and 3, these tests being made as follows: 



Reactions on dyed Jiher.^ — Small pieces or shreds of the dyed wool 

 are distributed on the porcelain plate and are thoroughly moistened 

 with the reagents. The fiber must be dry or nearly so, and must have 

 been dyed in a f ahly pure solution of the color, since colorless organic 

 impurities may easily obscure the reaction. 



Reduction and subsequent reoxidation.^ — The neutral color solution 

 is treated with a few particles of powdered sodium hydrosulphite, 

 conveniently dropped in from a small spatula. If no color change is 

 seen at once, the mixture is warmed somewhat and more reagent 

 added, carefully avoiding excess, however. If reduction, shown by 

 disappearance of the color, takes place, the solution is thoroughly 

 shaken with air, and should this not bring back the dye, it is warmed 

 and allowed to stand a few minutes. Finally, if remaining practically 

 colorless, a httle powdered potassium persulphate is dropped in. A 

 slight yellowish or broAvnish tint produced by air or especially by the 

 potassium persulphate is disregarded. 



In regard to the tests on the dyed wool, it may be said that the dye- 

 ings obtained from colors in food products are necessarily variable in 

 depth and usually puhr than those used as a basis for standards. 

 With the very smaU amounts of color available, it is impossible to 



' Fortho Irlentifinatlon of tho simplorazoflyos by reduction, separalion of (ho rcfluntion products and 

 characterization of those ijy wjuplinK with diazo compounds, l)y condcnsiition with nitrosodimothylani- 

 lin, and by diazotizalion, ne,(: especially Witt, IJor. f'hom. Ocs. 1«> (IRS(l), 1719, and 21 (ISHH;, :j4(i8. 

 Prop«?rtlr.s of the various amins, arninophonols, and thoirsulpliouic acids arosummarizod by lleumann 

 (l-'riedlilndcr, Schultz;, Dio Anilinfurben. IJraunsc.hwciK, I. SHH 11)0(1. 



'The tahle:i are made to corr(;s))ond as nearly as possible with those of U. S. Dopt. A^r., liur. Chom. 

 CIr. \o. 0.'{, 0.5 f)er cent, dyeintjs and reagents of similar confent ration heint,' used. 



' Uydrrwulphlte and persulphate are tho roaRenls advocalwl by (iroen, Yoeman, and .Tones, J. See. 

 DyerH* ColorialsO (iwr>), ZW>; al.so Uroon, Identification of Dyo Stufl's, iiOods (1913). 



