8 BULLETIN 448, U. S. DEPARTMENT OF AGRICULTURE. 



Most other pigments not lakes are compounds with heavy metals 

 and a suitable exammation according to the ordinary methods of 

 inorganic analysis will indicate the nature of the pigment.^ 



SEPARATION AND PUSIFICATION OF COLORING SUBSTANCES. 



PRELIMINARY TREATMENT. 



By suitable preliminary treatment the coloring matter should be 

 obtained in aqueous or dilute alcoholic solution nearl}^ free from 

 acids, alkalies, or large quantities of salt. (Concerning the oil- 

 soluble dyes see pages 6 and 7.) The alcohol content of the solution 

 should not exceed 10 per cent. Usually it is better to remove 

 excessive alcohol (by evaporation) than to add water; but if the 

 liquid contains so much sugar as to be sirup}^ it should be diluted. 

 If the evaporation causes a separation of coloring substance, the 

 sediment should not be removed before the treatment with immisci- 

 ble solvents. When the color has been extracted dhectly from solid 

 products by acid amyl alcohol, this may be shaken out with salt 

 solution, dilute hydrochloric acid, or water, as directed for the cor- 

 respondmg solution obtained in the first step of the procedure de- 

 scribed on pages 11, 17, and 18. 



Since the colormg substances of flowers and fruits are, generally 

 speaking, rather unstable, especially in the presence of alkalies, it is 

 well to divide the solution containing the colors into two portions, 

 one portion to be examined for the natural colors, the other for 

 coal-tar dyes. 



TREATMENT OF SOLUTION RESER^TED FOR TESTING FOR COAL-TAR DYES. 



If coal-tar dyes are not known to be present, a preliminary test 

 may be made by warming a smaU piece of wool, such as nun's-veilmg, 

 or some white woolen yarn with some of the solution; first neutral^ 

 then, if no dyeing takes place, made acid with a few drops of hydro- 

 chloric acid.^ If the wool is dyed in either case, the main portion 

 of the solution reserved for dyes is treated as indicated on page 9. 



Acid Yellow (sulphonated aminoazobenzene. No. 8) is sometimes 

 more easily separated from mixtures by dyeing on wool than by the use 

 of solvents; hence if the test wool is dyed yellow it may be stripped 

 with dilute ammonia and this solution tested for Acid Yellow by 

 diazotization, etc., as described on page 51. 



In the presence of very large amounts of natural coloring matter, 

 it may be advisable occasionally to make the dyeing test with a 

 comparatively large portion of the solution, stripping and redyeing 



1 For pigments in tea compare Read, V. S. Treasury Decision No. 32322; Knight, J. Ind. Eng. Chem. 

 6 ("1914), 909. 



= See Strohmer, Z. Anal. Ctiem. 24 (188.5), 62.5. Arata, Z. Anal. Chem. 38 (1889), 639. Winton, Conn. 

 Agr. Exp. Sta. Rpt. 2 (1889), 131. Sostegni and Caxpentieri, Z. Anal. Chem. 35 (1896), 397. Tohnan, Jour. 



Amer. Chem. Soc, 37 (1905). 25. 



