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

by Harvey Washington Wiley

(2) The sulfur dioxid produced by the action of the residual carbon on sulfuric acid exercises a reducing effect on the nitrogenous bodies present:

(3) From the nitrogenous bodies produced by the above reduction ammonia is formed by the action of an oxidizing body:

(4) The ammonia formed is at once fixed by the acid as ammonium sulfate. According to the theory of Asboth the hydrogen which is formed during the action of sulfuric acid on organic matter, when in a nascent state, also aids greatly in the production of ammonia. This idea is based on the fact that with those bodies which afford a deficit of hydrogen the formation of ammonia is imperfect.[151]

181. Preparation of Reagents.—(1) Pure Sulfuric Acid.—As is well known the so-called pure sulfuric acid in the market usually contains ammonia, a fact which compelled Kjeldahl to determine the quantity of nitrogen in the acid in every instance, and to make correction for the same in the analysis. An acid absolutely free from this impurity may, however, easily be prepared by the distillation of the commercial article in a small glass retort holding easily about 400 cubic centimeters. To conduct this operation without danger it is only necessary to arrange the apparatus so that the heavy fluid is heated to boiling, not from the bottom of the retort, but from its sides, and that the upper portion of the body and neck is kept sufficiently warm so that the sulfuric acid fumes are not allowed to condense and flow back into the retort. Both these ends are attained simply by surrounding the retort with a piece of sheet iron, cylinder-shaped beneath, and with an oval upper part, having an opening of about one centimeter in diameter for the neck of the retort. To conduct the distillation a burner is used with an arrangement for spreading the flame. To avoid with certainty all bumping of the sulfuric acid and the resulting danger therefrom, the lamp is so arranged that only the products of combustion go up between the retort and its iron hood, without allowing the flame itself to come into contact with the glass vessel. The retort should be filled about half full, or with 200 cubic centimeters of acid. By this device, without any danger whatever, about one liter of sulfuric acid may be distilled in a day. The retort will stand numerous distillations. Once begun, the distillation takes care of itself; it is necessary to discontinue it when only the bottom of the retort is covered with sulfuric acid, and to fill fresh acid through a funnel when the retort has cooled off. The first twenty cubic centimeters of the distillate going over are collected by themselves and rejected. What comes over later is, as shown by experience, absolutely ammonia-free, and can be used without any correction, for the nitrogen determinations according to Kjeldahl. The acid is kept in a stoppered bottle in a place not reached by ammonia fumes. The ten cubic centimeter pipette used for measuring the quantity of sulfuric acid required for each determination, is fastened in the perforated rubber stopper with which the bottle is kept closed, and is itself closed above by a small rubber tube with a plug of glass wool in it.

(2) Potassium Permanganate.—Crystals of this salt are crushed (not pulverized) with a pestle into small pieces of about one-half millimeter size, which are kept in a long glass tube of about ten millimeters diameter, closed with a stopper.

(3) Ammonia-free Water.—Common distilled water cannot be used in the determination of nitrogen according to Kjeldahl, since it contains ammonia. It may be obtained free from the same by redistillation in a large glass retort with the addition of a few drops of sulfuric acid. All vessels used in the determination are rinsed out beforehand with this water.

(4) Ammonia-free Soda-lye is most conveniently prepared by adding 270 grams of common sodium hydroxid in sticks, little by little, to one liter of distilled water which is kept continually boiling, by means of a small flame, in a good-sized silver dish. The dish is kept covered with a glass plate. Care has to be exercised not to add the alkali too rapidly, nor in too large quantities at a time for in this case the fluid will boil too violently at every addition of the alkali. After the operation is finished the lye is at once siphoned into a glass flask, and when cold, is poured into a glass-stoppered bottle.

(5) One-twentieth Normal Sulfuric Acid is prepared from sulfuric acid and water both absolutely ammonia-free, and is kept in a place where no fumes of ammonia can reach it, in a well-stoppered glass bottle, the stopper being smeared with vaseline.

(6) Sodium Thiosulfate Solution.—This should be of the same strength as the one-twentieth normal sulfuric acid. It is prepared by dissolving the salt in ammonia-free water, and is compared with the acid, to which has been added potassium iodid and iodate, using starch as an indicator, in the manner described above. The solution is kept in a well-stoppered bottle, in the dark. When the salt and water used are perfectly pure, it will keep unchanged for a long time.

(7) Potassium Iodid.—Dissolve five grams of chemically pure potassium iodid in ammonia-free water and make the volume 100 cubic centimeters. Ten cubic centimeters of this solution are used for each determination; keep the solution in the dark and in a well-stoppered bottle.