XXVIII. METHODS FOR THE DETECTION AND DETERMINATION 

 OF COLORING MATTER. PROVISIONAL. 



1. General Discussion. 



In the manufacture of coal-tar dyes many become contaminated with poison- 

 ous metals, such as arsenic, copper, zinc, tin, and lead. There is always the 

 possibility of the presence of arsenic, as sulphuric acid is used at one stage or 

 another in the preparation of nearly every dye. 



Some colors have metallic atoms in their molecule, such as malachite green, 

 which is a double chlorid of zinc in combination with the organic group. 



Many vegetable colors are sold as lakes of tin or alum. Other colors are 

 known to have a toxic action, such as picric acid and naphthol yellow. 



2. Detection of Artificial Colors by Dyeing Wool.o 

 (a) METHOD OF SOSTEGNI AND CABPENTIEBI.& 



Dissolve from 10 to 20 grams of the sample in 100 cc of water, filter if neces- 

 sary, acidify with from 2 to 4 cc of a 10 per cent solution of hydrochloric acid. 

 In this solution immerse a piece of woolen cloth, which has been washed in a 

 very dilute solution of boiling potassium hydroxid and then washed in water, and 

 boil for from 5 to 10 minutes. Remove the cloth, thoroughly wash it in water, 

 and boil in a very dilute hydrochloric acid solution. After washing out the acid 

 dissolve the color in a solution of ammonium hydroxid (1:50). With some of 

 the dyes solution takes place quite readily while with others it is necessary to 

 boil for some time. Take the wool out, add a slight excess of hydrochloric acid 

 to the solution, immerse another piece of wool, and boil it again. 



With vegetable coloring matter this second dyeing gives practically no color, 

 and there is no danger of mistaking a fruit color for one of coal-tar origin. 

 It is absolutely necessary that the second dyeing should be made, as some of the 

 coal-tar dyes <? will dye a dirty orange in the first acid bath which might be 

 <>:isil\ passed for vegetable color, but after solution in alkaline bath the second 

 acid bath dyes a bright pink. 



The method of Sostegnl and Carpentlerl and Arata's method for coal-tar colors :uv 

 not reliable In the presence of archil, archil derivatives, and sulphonated Indigo, as these 

 substances give dyeing reactions not to be distinguished from coal-tar colors. It is, how- 

 ever, comparatively easy to detect archil. A red color turning purple with dilute ammo 

 nium hydroxid, reduced by zinc and hydrochloric acid and easily reoxidizing in the air, 

 is either archil or a closely related color. (Tolman, J. Amer. Chem. Soc., 1905, 27: 25.) 

 Archil can be extracted from an ammonlacal solution by amyl alcohol, but the sulphonated 

 archil colors now on the market do not respond to the test. Indigo Is used in many green 

 and violet colors and can be recognized as described by Allen. (Com. Org. Anal., 8: (1) 

 625-541. 



Zts. anal. Chcm., 1896, 35: HOT. 



r V. S. I )ipi. Agr., Bureau of Chemistry, Bui. G6, p. 24. 



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