Principles and practice of agricultural analysis. Volume 2 (of 3), Fertilizers

by Harvey Washington Wiley

Figure. 16.

Gantter’s Nitrogen Apparatus.

It is seen from the above that the nitrate will give, by this treatment, double the volume of nitrogen which it contains. In practice, the two reactions may be secured in one operation by warming the nitrate solution slowly with sulfuric and phosphorous acids and ammonium chlorid. The nitric acid, as it becomes free, gives a part of its oxygen to the phosphorous compound, and the nitrous acid, in a nascent state, is at once reduced by the ammonium chlorid. There are two sources of error which must be guarded against in the work; a portion of the nitrogen may escape reduction to the elementary state, or some of the nitrate may fail to be decomposed. These errors are easily avoided if the reaction be begun slowly, so that the evolution of gas may be gradual. The temperatures at first should, therefore, be kept as low as possible. The development of red fumes, showing the presence of undecomposed nitrogen oxids, shows that the results will be too low. It is necessary, also, to provide for the absorption of the hydrochloric acid which is formed. The reaction is very conveniently conducted in the apparatus shown in [Fig. 16]. The decomposition takes place in the flask A and the mixed gases pass into the absorption bulb C. The delivery-tube is very much expanded, as shown in the figure, so that no soda-lye can enter A during the cooling of the flask. The absorption bulb is connected with A and B by the tubes a and b as shown. The tube d connects the apparatus with the gasvolumeter.[181] The bulb B serves as a pipette for the introduction of the decomposing acid. The operation is conducted as follows: Three cubic centimeters of the nitrate solution, containing no more than 300 milligrams of substance, are placed in the flask A with half a gram each of crystallized ammonium chlorid and phosphorous acid. In the bulb B are placed seven cubic centimeters of sulfuric acid to which has been added one-third its volume of water. Two cubic centimeters of acid are allowed to flow from B into A. The apparatus is brought to a constant temperature by being immersed in a large cylinder, E, containing water at a temperature which can easily be controlled. When this constant temperature has been reached the apparatus is taken from the cooling cylinder which contains also a smaller cylinder, D, nearly filled with water and connected through f′ with the measuring apparatus M. The barometer-tube F is half filled with colored water so that the pressure may be equalized before and after the operation. The flask A is warmed very gently at first, and the nitrogen evolved is conducted into D driving an equivalent volume of water into M. The evolution of the gas must be carefully controlled and the heat at once removed if it become too rapid. The appearance of a red color shows the evolution of oxids of nitrogen rendering the analysis inexact. When the evolution of nitrogen has nearly ceased the lamp is removed and some more sulfuric acid allowed to flow into A from B, after which A is again heated, this time to the boiling-point. All vapors of hydrochloric acid produced are absorbed by the soda-lye in C. The boiling is continued a few minutes, but not long enough to darken the liquid in A. After replacing the apparatus in the cylinder E and bringing both temperature and pressure to the same point as before the beginning of the operation, the volume of nitrogen evolved is determined by measuring the water in M.

The apparatus is first set by using pure potassium or sodium nitrate. Since the temperature and pressure do not vary much within an hour or two the volume of water obtained with a sample of white saltpeter can be compared directly with that given off by the same weight of a pure potassium or sodium nitrate without correction.

Example.—Two hundred and fifty milligrams of potassium nitrate, containing 34.625 milligrams of nitrogen, displaced in a given case sixty cubic centimeters of water; therefore one cubic centimeter of water equals 0.578 milligram of nitrogen. If 289 instead of 250 milligrams be taken then the number of cubic centimeters of water displaced divided by five will give the per cent of nitrogen.

217. Method of Difference.—In the analysis of Chile saltpeter by the direct method a variation of 0.25 per cent in the content of nitrogen is allowed from the dealers’ guaranty. This would allow a total variation in the content of sodium nitrate of 1.52 per cent. Dealers and shippers have always been accustomed to estimate the quantity of sodium nitrate in a sample by difference; i. e., by estimating the constituents not sodium nitrate and subtracting the sum of the results from 100. Chile saltpeter usually contains sodium nitrate, water, insoluble ferruginous matters, sodium chlorid, sodium sulfate, magnesium chlorid, sodium iodate, calcium sulfate and sometimes small quantities of potassium nitrate.

When the total sodium nitrate is to be estimated by difference the following procedure, arranged by Crispo,[182] may be followed:

Water.—Dry ten grams of the finely powdered sample to constant weight at 150°-160°.

Chlorin.—The residue, after drying, is dissolved and the volume made up to one-fourth liter with water and the chlorin determined in one-fifth thereof and calculated as sodium chlorid.