162 



METHODS OF ANALYSIS 



[Chap. 



15 SPECIAL TESTS FOR COAL TAR DYES PERMITTED' UNDER THE FEDERAL 

 FOOD AND DRUGS ACT. 



The dyes, given in 5, are sufficiently characterized in most cases by the solubilities 

 shown in their separation and by the color changes given by acids and alkalies on 

 the dyed fiber. This is especially true with Amaranth, Tartrazine, and Orange I. 

 By treatment with reducing agents such as stannous chlorid, titanous chlorid, zinc 

 dust or sodium hyposulphite in acid solution, Indigo Carmine, Amaranth, Tartra- 

 zine, Ponceau 3R and Orange I are decolorized. With Indigo Carmine the color 

 returns on shaking with air, most readily on warming, or on the addition of oxidiz- 

 ing agents such as ferric chlorid or potassium persulphate. Excess of the reducing 

 agents must of course be avoided. With the last 4 named dyes the color is not re- 

 stored. Dilute solutions of Light Green S F Yellowish, Naphthol Yellow S and 



