32 BULLETIN 1390. U. S. DEPARTMENT OF AGRICULTURE 
DETERMINATION 4 
Connect the apparatus as shown in Figure 4 and place 10 grams of 
the dye, a few glass beads, and about 250 c. c. of carbon-dioxide-free 
water in the flask /. Pass cold water through the outer jacket of 
condenser C. Insert the stopper in the dropping funnel, attach the 
air pump at K, and draw air through the apparatus for about 
20 minutes. 
Toward the end of the 20-minute period, close the stopcock of the 
dropping funnel, pour into the dropping funnel a mixture of 4 ¢. ¢. of 
concentrated sulphuric acid and 10 to 15 ¢. ¢. of distilled water free 
from carbon dioxide, and reinsert the stopper. Disconnect suction 
and at A connect the U-tube containing the standard barium 
hydroxide. Reconnect the suction beyond the U-tube and draw air 
through the entire train for five minutes. If the barium hydroxide 
remains clear the train may be considered to be free from uncombined 
earbon dioxide. 
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Fic.4.—Apparatus for determination of sodium carbonate in erythrosine 
Place 50 c. c. of standard approximately 0.1 N barium hydroxide 
in the glass tube A. Draw air through the entire train for a few — 
minutes and then discontinue the suction. Open the stopcock 
and the stopper of the dropping funnel so that the acid may flow into 
the dye solution, taking care to close the stopcock immediately when 
only a few drops remain in the funnel. Reconnect the air pump, 
heat the dye solution rapidly to boiling, and continue to draw air 
through the apparatus, slowly at first, until all the carbon dioxide has ~ 
been carried over into the standard barium hydroxide. When the — 
evolution of carbon dioxide has ceased, disconnect the flask B, raise 
the tube A, and let the beads slide into the flask. Wash the tube © 
quickly with a little carbon-dioxide-free water, close the flask with a — 
stopper and bunsen valve or other device to prevent access of carbon — 
dioxide, and heat to boiling to make the precipitate crystalline and — 
less soluble. Finally cool and titrate carefully with standard — 
hydrochloric acid, usmg phenolphthalein as an indicator. Calculate — 
as percentage of sodium carbonate. é 
