PERMITTED COAL-TAR FOOD DYES 25 
- yolume against 10 c. c. of 0.1 N ferric ammonium sulphate solution, 
using 5 grams of pure ammonium thiocyanate as an indicator. Make 
the solution approximately 0.1 N and place it in a container provided 
with a hydrogen atmosphere (fig. 2). Take special care to have all 
joints airtight and cover the stoppers (preferably counter sunk) with 
suitable wax. Let stand for two days for absorption of residual 
oxygen. Standardize against 20 c. c. of 0.1 N ferric ammonium 
sulphate, using 5 grams of ammonium thiocyanate as indicator and 
titrating in a 500 c. c. Erlenmeyer flask, with a protecting stream of 
carbon dioxide. Preserve in an atmosphere of hydrogen. 
Indicator —For many dyes the titanium trichloride titration end 
point is indicated by a sharp decolorization. For some dyes the 
change is so gradual that an excess of titanium trichloride (not more 
than 0.3 c. c. of approximately 0.1 N solution) is required and 
a suitable standard solution of some other dye must be used for the 
back titration, methylene blue serving well for this purpose. In 
other cases it is better to use an indicator which is reduced after the 
original dye has reacted with the titanium trichloride. Thus a 
known quantity of light green S F yellowish is used as an indicator 
for the naphthol yellow S titration. 
TABLE 12.—Quantities of color acids equivalent to 1 c. c. of standard titanium 
trichloride solution 
Color acid 
Color acid 
Molecular | equivalent |! Molecular | equivalent 
Dye weight of toic.c. Dye | weight of | tolc.e. 
color acid -0O.1N coloracid | O.1N 
TiCl; | TiCls 
| } | 
Gram | Gram 
Ponceau $R_____-_______- 450. 4 0. 01126 | Partravine- 28!) At) ) 468. 3 0. 01171 
U: sae 2s eee 538. 4 .01346 |} Guinea green B__________- 668. 5 . 03342 
DUT ee Se eee 328.3 . 008207 || Light green S F yellowish_| 748.7 . 03743 
Naphthol yellow S-----__ 314. 2 | - 002618 | Indigotine-_-_---_---------- | 422.3 | . 02112 
DETERMINATION BY TITRATION WITH TITANIUM TRICHLORIDE 
For amaranth.—Into a 500 c. c. Erlenmeyer flask pipet that volume 
of the master solution which corresponds to 20 c. c. of 0.1 N titanium 
trichloride solution (Table 12). Add water, if necessary, to bring 
the volume to 50 c. c. and acidify with 1.5 c. c. of strong hydrochloric 
acid. Pass in astream of carbon dioxide and heat to vigorous boiling. 
Then, without delay, titrate with the standardized titanium tri- 
chloride solution, keeping the carbon dioxide flow continuous to the 
end. No indicator is required; the end point is shown by the disap- 
pearance of the characteristic color of the dye. Calculate as per- 
centage of color acid. 
For ponceau 3R, orange I, tartrazine, and indigotine.—Proceed as 
directed in the foregoing paragraph, substituting 15 grams of sodium 
acid tartrate for hydrochloric acid and bringing the volume to 100 
c.c. First completely dissolve the tartrate by boiling; then add the 
dye solution and boil, again under carbon dioxide. Run blanks on 
15 grams of the acid tartrate, using 100 c. c. of water and, as indicator, 
1 mg. of the dye concerned. Calculate as percentage of color acid. 
For guinea green B and light green S F yellowish.—Dissolve 5 grams 
of the dye in 400 c. c. of water previously boiled and cooled under 
