REACTION OF COLORS IN AQUEOUS SOLUTION AND WITH 

 CONCENTRATED SULPHURIC ACID. 



While similar tables have been prepared by other workers, consid- 

 erable uncertainty arises in using them, and it has been the aim of 

 the writer in preparing these tables to be somewhat more precise in 

 the description of these reactions. As one means to this end, in Table 

 IV the approximate strength of the color solution used (about 0.01 

 per cent) is indicated by the color of the solution in a test tube three- 

 quarters of an inch in diameter. As before stated, the solutions of 

 natural coloring matter are about ten times stronger than those of the 

 coal-tar dyes, namely. 0.1 and 0.01 per cent, respectively. About 5 cc 

 of color solution, 0.2 gram of zinc dust, and 10 drops of concentrated 

 hydrochloric acid were used for the reduction test, and approximately 

 10 cc of color solution for the other reactions in aqueous solution. 



The dry color test with concentrated sulphuric acid was conducted 

 as follows: 



About 0.01 gram of coal-tar color, or 0.05 gram of natural coloring 

 matter, was dissolved by shaking with 5 cc of concentrated sulphuric 

 acid in a test tube. The solution was diluted with water, 3 to 5 cc 

 at a time, until the volume reached about 20 cc, then more rapidly 

 with constant shaking, noting any changes in the appearance of the 

 solution, until such change seemed merely to affect the depth of the 

 color. 



DISCUSSION OF DETAILS OF MANIPULATION. 



Precautions to be observed in applying the Sostegni and Carpentieri 



method. 



In the case of coal-tar dyes the well-known method of Sostegni 

 and Carpentieri a is adapted : 



If the color is in aqueous solution, slightly acid with hydrochloric 

 acid, the wool can be heated in it directly or after diluting. 



If in alcoholic solution the alcohol should first be driven off by 

 evaporation. 



If in a solid or semisolid substance, the color can generally be 

 extracted by wool after dissolving or suspending the finely divided 

 substance in water and slightly acidifying with hydrochloric acid. 



In some cases, however, it is better to extract the finely divided 

 and dried substance by warming with alcohol or water, made slightly 

 alkaline with ammonia. The alcohol is then evaporated off, keeping 

 up the volume with water, the aqueous solution is made slightly acid 

 with hydrochloric acid and the color extracted by wool. 



For heavy saccharine substances, such as confectionery, it is often 

 best to evaporate as far as possible on the steam bath and then 



a U. S. Dept. Agr., Bureau of Chemistry Bui. 107, revised, p. 190 

 [Cir. 63] 



