26 BULLETIN 1390, U. 5.. DEPARTMENT OF AGRICULTURE 
carbon dioxide. ‘Transfer to a 500 c¢. c. volumetric flask and dilute 
to the mark with air-free water. In a 1-liter Erlenmeyer flask dis- 
solve 30 grams of sodium acid tartrate in 200 c. c. of water, boil to 
expel air, and, under carbon dioxide, cool to 85° C. Add 100 «. c. 
of the dye solution and titrate under carbon dioxide at 60° to 70° C. 
Run blanks on 30 grams of the sodium acid tartrate in 300 ec. ec. of 
water, using | mg. of the dye as indicator. Calculate as percentage 
of color acid. 
For naphthol yellow S.—Proceed as directed in the second para- 
pepe of this section, using as an indicator that volume of light green 
I yellowish standardized solution (freshly made) which contains 
10mg. of dye. Runa blank on the tartrate, light green S F yellowish, 
and water. Calculate as percentage of color aa 
DETERMINATION BY PRECIPITATION (28) 
For erythrosine.—Dilute to 100 c. c. that volume of the master 
solution which contains 0.25 gram of dye. Add 5 ce. c. of nitric acid 
(approximately 0.6 N strength) and filter through a weighed Gooch 
crucible. Wash thoroughly with 0.5 per cent nitric acid and finally, 
with not more than 10 c. c. of distilled water. The precipitate should 
not be allowed to cake in the crucible until the washing has been 
completed. Dry te constant weight at 135° C. Calculate as per- 
centage of color acid. 
PURE COAL-TAR DYE 
DETERMINATION BY TITRATION WITH TITANIUM TRICHLORIDE 
For yellow A B and yellow O B.—Dissolve 15 grams of sodium acid 
tartrate in 100 c. c. of water and add 0.1 gram of dye dissolved in 100 
c. c. of 95 per cent alcohol. Titrate with the standard titanium 
trichloride (p. 24) under carbon dioxide, using as an indicator 10 mg. 
of light green S F yellowish from a fresh standardized solution, as 
directed in the color acid determination for naphthol yellow S. Run 
a blank as directed in that determination, including also the 100 ¢. ¢ 
of 95 per cent alcohol. One cubic centimeter of 0.1 N titanium tri- 
chloride is equivalent to 0.006180 gram of yellow A B and to 0.006530 
gram of yellow O B. Calculate as percentage of dye. 
DETERMINATION BY CALCULATION FROM CoLoR AcID DETERMINATION 
For ponceau 8R, amaranth, orange I, naphthol yellow S, tartrazine, 
guinea green B, light green S F yelloursh, and indigotine.-—Multiply 
by the factor 0.956 the percentage of sodium combined in the dye as 
determined for sulphated ash (p. 18), and add the result to the per- 
centage of color acid. Report as percentage of dye. 
For erythrosine.—Multiply by the factor 1.074 the percentage of 
color acid. Report as percentage of dye. 
DETERMINATION BY TITRATION WITH PoTASSIUM PERMANGANATE 
For indigotine.—Dilute to 400 c. c. that volume of the master solu- 
tion which contains 0.04 gram of dye. Add 2 ¢. c. of concentrated 
sulphuric acid and titrate against standard approximately 0.02 N 
potassium permanganate solution. .The end point is shown by the 
production of a clear yellow color. The titer of the standard solution 
