22 BULLETIN 139, U. S. DEPARTMENT OF AGRICULTURE 
the 20 per cent potassium iodide solution. Heat to about 90° C., 
add 3 drops of the stannous chloride solution, and keep the solution 
at 90° C. for 10 minutes. Cool the generator and its contents, first 
in air and then to about 5° C.in a pan containing water and ice. 
Add about 15 grams of the stick zine and quickly connect the entire 
apparatus. Keep the generator in ice water for 15 minutes. Remove 
from the bath and let the evolution of gas continue 
for an hourlonger. Finally, place the generator bottle 
on a hot plate and heat cautiously for about 30 minutes. 
Remove the sensitized paper and compare the stain 
with similar stains produced under like conditions with 
known quantities of arsenic, using portions of the 
standard arsenic solution containing 0.001, 0.002, 0.005, 
0.010, 0.015, 0.025, and 0.030 mg. of arsenious oxide 
(As.O;). Conduct a blank test on the reagents alone. 
The blank should not exceed 0.001 mg. Calculate as 
parts per million of arsenious oxide. 
For erythrosine.— Dissolve 18 grams of the dye in 425 
c. c. of water and add 5c. c. of the bromine water and 
20 c. ec. of dilute hydrochloric acid (14+9). Mux, 
filter, and treat 250 c. c. of the filtrate (corresponding 
to 10 grams of dye) with the ammonium hydroxide, 
using enough to make it slightly alkaline (usually 
about 5c. c¢.). Complete the determination and cal- 
culation as directed in the first paragraph of this sec- 
tion, beginning with “Then add 20 c. c. of the sodium 
phosphate solution. ”’ 
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DETERMINATION AFTER TREATMENT Wi1TH Nitric AcID 
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For ponceau 3R, amaranth, erythrosine, orange I, 
naphthol yellow S, tartrazine, and vndigotine.—Place 12.5 
grams of the powdered dye in a 600 ¢. c. pyrex beaker, 
add 25 c. c. of the strong nitric acid, and, if the dye 
tends to clot or cake, stir the mixture thoroughly. 
Boil for about five minutes, add 150 to 200 c. c. of 
water, and dilute the mixture to 250 c.c. in a graduated 
cylinder. Pour the mixture back into the beaker. 
Allow it to stand for a few minutes and then filter 
through an 18 cm. paper. Treat 200c.c. of the filtrate 
(corresponding to 10 grams of dye) with the strong 
: ammonium hydroxide, using a measured quantity suf- 
Fic.1--Apparatus ficient to make the solution slightly alkaline (about 
for determination 95¢.¢.). Complete the determination and calculation 
as directed in the first paragraph of this section, begin- 
ning with “Then add 20c. ¢. of the sodium phosphate solution.” 
For yellow A B and yellow O B.—In a 400 c. c. -beaker mix thor- 
oughly 12.5 grams of the powdered dye with 200 c. c. of nitric acid 
(1+9) and heat to boiling for 5 to 10 minutes. Allow to cool, add 
25 c. c. of the strong ammonium hydroxide, dilute to 250 ¢c. c. ina 
graduated cylinder, and filter. Determine the arsenic in a 200 c. ¢. 
portion of the filtrate (corresponding to 10 grams of dye) and calculate 
as directed in the first paragraph of this section, beginning with 
“Then add 20 ec. é. of the sodium phosphate solution.” 
