Figure. 12.
Distillation Apparatus of
Halle Agricultural Laboratory.
The acid used should contain 38.1 grams of sulfuric acid of 1.845 specific gravity in a liter; and it should be set by titration with chemically pure sodium carbonate. For this purpose seven-tenths gram of sodium carbonate is heated in a platinum crucible for two hours over a small flame, weighed, and placed in an erlenmeyer together with twenty cubic centimeters of the sulfuric acid, care being taken to avoid loss from the vigorous evolution of carbon dioxid. After boiling for ten minutes all the carbon dioxid is removed from solution. After cooling, the excess of acid is determined by titration with a standardized barium hydroxid solution, using rosolic acid as indicator.
The solution of barium hydroxid is made as follows: Digest, with warm water, 260 grams of caustic baryta, Ba(OH)₂, until it is nearly all dissolved, filter, and make up to a volume of ten liters and keep in a flask free of carbon dioxid. A solution of barium hydroxid is to be preferred to the corresponding sodium compound for titration. If traces of carbonate be formed in the two liquids, the sodium salt will remain in solution while the barium compound will settle at the bottom of the flask.
The Indicator.—The indicator used to determine the end of the reaction is made by dissolving one gram of rosolic acid in fifty cubic centimeters of alcohol. From one to two drops are enough for each titration. The color reaction is less definite as the quantity of ammonia in the liquid increases. When the titration solutions have been prepared as above described it is found to require about ninety of the barium hydroxid to neutralize twenty cubic centimeters of the sulfuric acid.
By direct titration with sodium carbonate it is ascertained how many grams of nitrogen the twenty cubic centimeters of sulfuric acid represent.
Example.—Suppose the weight of the dried sodium carbonate prepared as above directed is 0.6989 gram.
| ½Na₂CO₃, | ½N₂ | ||||
| Then 0.6989 | : | 53 | = x | : | 14 |
Whence x = 0.184615 gram of nitrogen.
Suppose further that twenty cubic centimeters of sulfuric acid solution require ninety-four cubic centimeters of barium hydroxid for complete saturation and after treatment with the above amount of sodium carbonate, ten and a half cubic centimeters of the barium solution to neutralize the remaining acid.