For instance, the nitrogen which finally appears in the proteids of a plant has entered its organism chiefly as nitric acid, and the nitric acid which is found in a vegetable product is therefore a representative of the quantity of unabsorbed nitrogen present in the tissues at the moment when the vital activity of the plant is arrested. In some instances, it is found that the absorption of nitrates by vegetable tissues takes place in far larger quantities than is necessary for their nutrition, and in these cases the excess of nitrates accumulates, sometimes to a remarkable extent. In a case cited in the reports of the Kansas Agricultural Experiment Station, where Indian corn was grown on ground which had been used for a hog pen, the quantity of potassium nitrate found in the dried stalks was somewhat remarkable. When one of the stalks was cut in two and tapped lightly upon a table, crystals of potassium nitrate were easily obtained in the form of fine powder. On splitting the cornstalk the crystals in the pith could be seen without the aid of a microscope. On igniting a piece of the dried stalk it burned rapidly with deflagration. The percentage of potassium nitrate in the dried material was 18.8. Cattle eating this fodder were poisoned.[341]
In preserved meat products large quantities of oxidized nitrogen are often found, and these come from the use of potassium nitrate as a preserving and coloring agent. Ammonia is rarely found in vegetable tissues in greater quantities than mere traces, but may often exist in weighable amounts in animal products.
Amid nitrogen is found rather constantly associated with proteid matters in vegetable products. Asparagin and glutamin are instances of amid bodies of frequent occurrence. Betain and cholin are found in cottonseed.
The occurrence of nitrogen, in the form of alkaloids, is of interest to agricultural chemists in this country, chiefly from its presence as nicotin in tobacco and from a toxicological point of view, but in other localities the production of alkaloids, as for instance in opium, tea and coffee, is a staple agricultural industry. The methods of separating and determining these forms of nitrogen will be given further on. This description can evidently not include an extended compilation of the methods of separating and determining alkaloidal bodies, with the exception of those with which the agricultural analyst will be called upon frequently to deal, viz., nicotin and caffein and nitrogenous bases such as betain and cholin.
QUALITIVE TESTS FOR
NITROGENOUS BODIES.
374. Nitric Acid.—Any nitric acid or nitrate which an agricultural product may contain may be leached out by treating the fine-ground material with cold water. From vegetable matters this extract is evaporated to a small bulk, filtered, if necessary, and tested for nitric acid by the usual treatment with ferrous sulfate and sulfuric acid. In the case of vegetable substances there will not usually be enough of organic matter to interfere with the delicacy of the reaction, but in animal extracts this may occur. Colored extracts should be decolorized with animal char (bone-black) before they are subjected to examination. It is not well to attempt to remove the organic matters, but, since they are more insoluble in water than the nitrates, the solution containing both may be evaporated to dryness and treated with a quantity of cold water insufficient for complete solution. The nitrates will be found in the solution obtained in a larger proportionate quantity than before.
375. Amid Nitrogen.—One or more atoms of the hydrogen in ammonia may be replaced by acid or basic bodies (alcohol radicles). In the former cases amids, in the latter amins result. In the ratio of displacement there are formed primary, secondary, and tertiary bodies determined by the number of hydrogen atoms replaced. The primary amids are the only ones of these bodies that are of interest in this connection.
The amids are easily decomposed, even on heating with water and the more readily with acids and alkalies, the amido radicle being converted into ammonia. A type of these reactions is given below.
CH₃.CO.NH₂ + H₂O = CH₂.CO.OH + H₃N.
On boiling an amid with hydrochloric acid, the ammonia is procured as chlorid whence it is easily expelled by heating with an alkali. In a body free of ammonia, an amid is easily detected by subjecting the substance containing it to the action of hot hydrochloric acid, filtering, neutralizing the free acid with sodium hydroxid, adding an excess thereof and distilling into an acid.[342] In case the quantity of ammonia produced is very small it may be detected by the nessler reagent.[343] Amids are soluble in a fresh, well washed preparation of cupric hydrate suspended in water. The hydrate also passes into solution forming a liquid of a deep blue color.