FOOD-COLOEING SUBSTAlS^^CES. 51 



Where diazo compounds are formed, they m.ay be further coupled 

 by the usual method of adding the mixture to an alkaline solution 

 of one of the naphthols, or of a naphthol sulphonic acid. B. C. Hesse 

 has pointed out that the two acid yellows (No. 8 and No. 9) can be 

 distinguished b}^ the use of alpha-naphthol — No. 9 giving in alkaline 

 solution a red compound; No. 8 one which is intensely blue. 



In the test described below the mixture is treated first with nitrous 

 acid, then with hydrazin sulphate. The hydrazin sulphate serves to 

 destroy the excess of nitrous acid, so that the naphthol solution (or 

 an amin if preferred) may be added directly, and the coupling then 

 brought about by addition of alkali. The new dye formed may also 

 be separated readily, if desired, by acidifying and shaking out with a 

 suitable solvent. In the case of only one dye in the table, Safranin 

 (No. 584), does the diazo compomid appear to be reduced or changed 

 by addition of hydrazin sulphate. 



The nitroso compounds formed from Nos. 95 and 88, are decom- 

 posed by the hydrazm salt, the origmal color of the acid solution 

 being restored. 



The test is carried out as follows: The solution of the color at 

 ordinary temperature is made slightly acid by the addition of two or 

 three drops of concentrated hydrochloric acid and one or two drops 

 of 7 per cent sodium nitrite solution are added. With blue or green 

 dyes, where oxidation changes may take place, the mixture may be 

 allowed to stand a few minutes at this stage; but with other colors 

 about 1 cc, or an excess, of 3 per cent hydrazin sulphate solution 

 is added at once. The mixture is allowed to stand one-half or one 

 minute to permit complete destruction of the excess of nitrous acid; 

 then it is divided and a few drops of alpha-naphthol solution are 

 added to one portion. Both portions are then made strongly alka- 

 line with sodium carbonate, the one not containing alpha-naphthol 

 serving as a check to show if any coupling has taken place. 



Tablk ij. — Behavior of colors vjhen treated with sodium nitrite. 

 [" Indicates that no color changes take place other than those produced by the acid or alkali.] 



462. — With fiodium nitrite, blue; then colorless; after making alkaline in the pres- 

 ence of alpha-naphthol, oraii^'O. 



4.34'^, 4'.'}')°, 4'.U')'^. -Attacked very slowly by nitrous acid. 



430. — IJocomes yellow with sodium nitrite. 



491. — IjtiCDiWiH violet with sodium nitrite (rather slowly). 



4^10°, fi02°, 108°. 



8.— With Widiuin nitrite, much paler; after adding alpha-naphthol and excess of 

 Hodium r'arboriale, intcu.Holy blue. 



9.— With sofliiim nitrite, much paler; after adding alpha-naphthol and excess of 

 wxUum carboriute, rod. 



89.— Tied solution becomes yellow with sodium nitrite; on addition of hydrazin 

 HUlphut^, red a^ain. 



692.— With sodium nitrite, slowly oxidized to the yellow isatin derivative. 



V.f.i'^, \0<-/', 107'', 91°. 



398. — With sodium nitrite, brown. 



W'/-', (m^, 188°, 14°. 



