FOOD-COLORING SUBSTANCES. 51 
Where diazo compounds are formed, they may 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 by 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 compound 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 hydrazin salt, the original 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. 
Table 5. — Behavior of colors when treated with sodium nitrite. 
[° Indicates that no color changes take place other than those produced by the acid or alkali.] 
462. — With sodium nitrite, blue; then colorless; after making alkaline in the pres- 
ence of alpha-naphthol, orange. 
434°, 435°, 436°. — Attacked very slowly by nitrous acid. 
439. — Becomes yellow with sodium nitrite. 
491. — Becomes violet with sodium nitrite (rather slowly). 
440°, 602°, 108°. 
8. — With sodium nitrite, much paler; after adding alpha-naphthol and excess of 
sodium carbonate, intensely blue. 
9. — With sodium nitrite, much paler; after adding alpha-naphthol and excess of 
sodium carbonate, red. 
89. — Red solution becomes yellow with sodium nitrite; on addition of hydrazin 
sulphate, red again.. 
692. — With sodium nitrite, slowly oxidized to the yellow isatin derivative. 
399°, 106°, 107°, 94°. 
398. — With sodium nitrite, brown. 
605°, 604°, 188°, 14°. 
