Experiment No. 13

Testing for Coal Tar Dyes

Dilute 20 to 30 cc. of the material to 100 cc.; boil for 10 minutes with 10 cc. of a 10 per cent solution of potassium bisulphate and a piece of white woolen cloth which has previously been boiled in a 0.1 per cent solution of NaOH and thoroughly washed in water. Remove the cloth from the solution, wash in boiling water, and dry between pieces of filter paper. A bright red indicates coal tar dye. If the coloring matter is entirely from fruit, the woolen cloth will be either uncolored or will have a faint pink or brown color which is changed to green or yellow by ammonia and is not restored by washing. This is the Arata test.

(Adapted, Winston, Conn. Experiment Station Report.)

1. Describe Arata's wool test for coal tar dyes. 2. What is the appearance of the woolen cloth when the coloring matter is entirely from fruit? 3. What effect has NH₄OH upon the color? 4. Why is NaOH used? 5. Why may not cotton cloth be used instead of woolen? 6. What can you say of the use of coal tar dyes in foods?