8 BULLETIN 148, U. S. DEPARTMENT OF AGRICULTURE. 
Most other pigments not lakes are compounds with heavy metals 
and a suitable examination according to the ordinary methods of 
inorganic analysis will indicate the nature of the pigment. 1 
SEPARATION AND PURIFICATION OF COLORING SUBSTANCES. 
PRELIMINARY TREATMENT. 
By suitable preliminary treatment the coloring matter should be 
obtained in aqueous or dilute alcoholic solution nearly 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 sirupy 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 directly 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- 
responding solution obtained in the first step of the procedure de- 
scribed on pages 11, 17, and 18. 
Since the coloring 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 RESERVED 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 small piece of wool, such as nuns-veiling, 
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. 2 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 (siuphonated aminoazobenzene, Xo. 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. 
2 See Strohnier, Z. Anal. Chem. 24 (1885), 62-5. Arata, Z. Anal. Cheni. 28 (1889), 639. Winton, Conn. 
Agr. Exp. Sta. Rpt. 2 (1889), 131. Sostegni and rarpentieri, Z. Anal. Chem. 35 (1S96), 397. Tolman, Jour. 
Amer. Chem. Soc. 27 (1905). 25. 
