The same group reagents may also be applied to the extracts obtained with petroleum ether, benzene and chloroform, in all cases, after the removal of the solvents by evaporation.
ESTIMATION OF NITROGENOUS BODIES
IN AGRICULTURAL PRODUCTS.
382. Total Nitrogen.—Any one of the methods heretofore described for the estimation of total nitrogen in soils or fertilizers is applicable for the same purpose to agricultural products. One among these, however, is so superior in the matter of convenience and certainty, as to make it preferable to any other. The moist combustion of the sample with sulfuric acid with subsequent distillation of the ammonia produced is the process which is to be recommended.[349]
The usual precautions for securing a representative sample should be observed, but no further directions are needed. In all cases hereafter, where the estimation of nitrogen is enjoined, it is understood that the moist combustion process is to be used unless otherwise stated.
383. Estimation of Ammoniacal Nitrogen.—If the distillation of ammonia be accomplished with the aid of magnesia alba or barium carbonate it may be safely conducted on the finely ground materials or, in case of animal bodies, in as fine a state of subdivision as may be conveniently secured. Since the salts of ammonia are easily soluble in water they may be all obtained in aqueous solution, and the distillation of this solution with magnesia gives correct results. Experience has shown that the stronger alkalies, such as sodium and potassium hydroxids, cannot be safely used in the distillation of ammonia from mixtures containing organic nitrogenous materials because of the tendency of these bodies to decomposition, in the circumstances, yielding a portion of their nitrogen as ammonia. Barium carbonate acts with less vigor on non-ammoniacal nitrogenous matters than magnesia, and in some cases, as pointed out further on, may be substituted therefor with advantage. There is no danger of failing to obtain a part of the ammonia on distillation with magnesia provided the latter does not contain more than a trace of carbonate.[350]
When no easily decomposable organic nitrogenous matters are present, the distillation may be conducted with the stronger alkalies in the manner prescribed.[351] All the necessary details of conducting the distillation are found in the preceding volumes of this work.
384. Estimation of Amid Nitrogen.—In bodies containing no ammonia, or from which the ammonia has been removed by the method described in the preceding paragraph, the nitrogen in the amid bodies is converted into ammonia by boiling for about an hour with five per cent sulfuric or hydrochloric acid. The ammonia thus produced is estimated in the usual manner after distillation over magnesia free of carbonate. The free acid is exactly neutralized with sodium or potassium carbonate before the addition of the magnesia. The results are given in terms of asparagin. The reaction which takes place in the decomposition of the amid body is indicated by the following equation:
| Asparagin. | Sulfuric acid. | ||
| 2C₄H₈N₂O₃ + 2H₂O | + | H₂SO₄ | = |
| Aspartic acid. | Ammonium sulfate. | ||
| 2C₄H₇NO₄ | + | (H₄N)₂SO₄. | |
Half of the nitrogen contained in the amid body is thus obtained as ammonia.
It is advisable to calculate all the amid nitrogen in agricultural products as asparagin.