PERMITTED COAL-TAR FOOD DYES 21 
for an hour in a 5 per cent solution of mercuric bromide in 95 per 
cent alcohol, squeeze out the excess solution, and dry on glass rods. 
(In the preparation care must be taken to protect the paper from 
contact Laer 4 the table.) The ends of the strips should be cut off 
before using. 
Potassium iodide solution.—Dissolve 20 grams of potassium iodide 
in distilled water and dilute to 100 c.c. 
Stannous chloride solution.—Dissolve 40 grams of arsenic-free 
stannous chloride crystals in enough strong hydrochloric acid to 
make 100 e¢. c. 
Standard arsenic solution.—Dissolve 1 gram of arsenious oxide in 
25 c. c. of 20 per cent sodium hydroxide solution, neutralize with 
dilute sulphuric acid, add, in excess, 10 c. c. of concentrated sulphuric 
acid, and dilute to 1 liter with air-free water. One cubic centimeter 
of this solution contains 1 mg. of arsenious oxide (As,O;). Dilute 
20 c. c. of this solution to 1 liter. Diluting 50 c. ¢. of this second 
solution to 1 liter gives a final solution which contains 0.001 mg. of 
arsenious oxide (As,O;) per cubic centimeter. This is used to prepare 
the standard stains. The dilute solutions must be prepared im- 
mediately before use. 
APPARATUS 
Connect the generator (fig. 1), a 2-ounce wide-mouth bottle, by a 
perforated rubber stopper to a glass tube, 1 cm. in diameter and 6 
cm. long, containing a piece of the lead acetate paper rolled into a 
spiral. Connect this tube by a perforated rubber stopper with a 
similar tube filled with the lead acetate cotton, squeezed to remove 
excess of the solution. The cotton in all determinations must be 
uniformly moist in order to obtain comparable stains. Connect the 
second tube by a perforated rubber stopper to a narrow glass tube, 
3 mm. in internal diameter and 12 cm. long, containing a strip of the 
mercuric bromide paper. Rubber stoppers used for connections 
must be free from white coating. 
DETERMINATION BY DIRECT PRECIPITATION 
For amaranth, naphthol yellow S, tartrazine, guinea green B, and 
light green S F yelloursh.—Dissolve 10 grams of the dye in 250 ¢. c. 
of water and add 10 c. c. of the bromine water. Make the mixture 
alkaline with 1 to 2 c. c. of the strong ammonium hydroxide. Then 
add 20 ¢. c. of the sodium phosphate solution and a slight excess of the 
magnesia mixture. The quantity of magnesia mixture used must 
be from 1 to 5c. ¢c. in excess of that required to completely precipitate 
the phosphate, as ascertained previously by experiment. Add the 
magnesia mixture slowly, stirring the solution well during the addi- 
tion. Add 10c.c. of the ammonium hydroxide and allow the mix- 
ture to stand for atleast 30 minutes. Filter through an 18 cm. paper 
and wash with dilute ammonium hydroxide (1+9) until practically 
all dye is removed. Then wash with about 5c. c. of water. Allow 
the filter containing the washed precipitate to drain for 15 to 30 
minutes to remove most of the adhering liquid. Finally dissolve 
the magnesium ammonium phosphate and arsenate by pouring 
40 c. c. of 10 per cent hydrochloric acid over the filter in small por- 
tions and letting it drain into the generator bottle. Add 4c. c. of 
