16 BULLETIN 1390, U. S. DEPARTMENT OF AGRICULTURE 
paper. Place a 200 c. c. portion of the filtrate in a 600 c. c. beaker © 
and add enough of a 6 to 7 per cent solution of potassium perman- 
ganate to oxidize the sulphides and produce a faint but permanent — 
pink color. Add about 50 c. c. of water and a slight excess of 10 
per cent silver nitrate solution. (Usually 6 to 8 c. c. is enough to 
precipitate the chloride.) Partially cover the beaker with a watch 
glass and acidify the solution by carefully adding about 12 ¢. c. of 
strong nitric acid. Heat nearly to boiling and then add the saturated 
sulphur dioxide solution slowly and with stirring until the oxides of 
manganese have dissolved, leaving the silver chloride white. Boil 
until the excess of sulphur dioxide is removed, cool, and filter through 
a weighed Gooch crucible. Wash the silver chloride with hot water, 
which is slightly acidified with nitric acid, dry the crucible and its 
contents at 105° to 110° C., cool in a desiccator, and weigh. Cal- 
culate to percentage of sodium chloride. 
For erythrosine.— Dissolve 5 grams of the dye in 400 ec. c. of water 
and transfer the solution to a 500 c. c. volumetric flask. Precipitate 
the color acid by adding a mixture of 2 c. c. of strong nitric acid and 
10 to 20 c. c. of water. Dilute to 500 c. c., mix, and filter through a 
dry paper. Reserve 100 c. c. of the filtrate for possible use in deter- 
mining sodium sulphate in erythrosine (p. 17). Treat 200 c. c. of 
the filtrate with slightly more silver nitrate solution than is required 
to precipitate the halogens present. Add 5c. c. of strong nitric acid 
iA heat to boiling. After cooling, collect the precipitate in a 
weighed Gooch crucible. Wash, dry, and weigh as directed in the 
first paragraph of this section. If a test shows that sodium iodide 
is present, determine its weight as directed on page 30 and subtract 
the weight of silver iodide from the weight of the precipitate. Cal- 
culate the percentage of sodium chloride from the net silver chloride. 
For naphthol yellow S.—Dissolve 5 grams of the dye in 250 ¢. c. of 
water and filter if not clear. Add 5c. c. of strong nitric acid and 
precipitate the chloride by adding a slight excess of 10 per cent silver 
nitrate solution. Boil for a few minutes, cool, and filter through a 
weighed Gooch crucible. Wash, dry, and weigh the precipitate and 
calculate as directed in the first paragraph of this section. 
For ponceau 8R.—Dissolve 5 grams of the dye in 150 ¢. c. of hot 
water, wash into a 250 ¢c. ce. volumetric flask, and add 25 c. ¢c. of a 
10 per cent solution of barium nitrate. Cool the mixture, make up 
to the mark, mix, and filter through a dry paper. Acidify 100 ¢. ¢. 
of the filtrate (representing 2 grams of dye) with strong nitric acid 
and treat with a slight excess of 10 per cent silver nitrate solution. 
Collect the silver chloride on a weighed Gooch crucible. Wash, 
cool in a desiccator, weigh, and calculate as directed in the first 
paragraph of this section. 
For ponceau 3R, amaranth, orange I, naphthol yellow S, tartrazine, 
guinea green B, light green S F yellowish, and indigotine.—In a nickel 
crucible mix 1 gram of the dye with about 15 grams of pure sodium 
peroxide. Place the cover on the crucible and start the fusion by 
introducing a red-hot iron wire ae a hole previously bored in 
the cover. When the reaction is complete allow the crucible to cool. 
(If the fusion is too rapid, repeat the entire procedure, using a larger 
roportion of sodium peroxide.) Wash the cover over a 600 ¢. ¢. 
a cer with 50 to 75 c. c. of water. Place the crucible on its side in — 
the beaker and add water until the crucible is about helf covered. — 
