173. General Considerations.—(1) Preparation of Sample.—In the soda-lime method it is of great importance that the organic substances be in a fine state of subdivision so as to admit of intimate mixture with the alkali. In cases where fragments of hoof, horn, hair, or similar substances are to be prepared for combustion it is advisable to first decompose them by heating with a small quantity of sulfuric acid. The excess of acid may be neutralized with marble dust and the resulting mixture dried, rubbed to a fine powder, and mixed with the soda-lime in the usual way. Care must be taken not to lose any of the ammonia from the sulfate which may be formed in mixing with the soda-lime in filling the tube.
(2) Purity of Soda-Lime.—The soda-lime employed must be entirely free of nitrogenous compounds and some blank combustions should be made in proof of its purity.
(3) Temperature.—The temperature of the combustion should not be allowed to exceed low redness. At very high temperatures there would be danger of decomposing the ammonia.
(4) Aspiration of Air.—Before aspiring a current of air through the tube to remove the last traces of ammonia the gas should be put out under the furnace and the tube be allowed to cool below redness to avoid any danger of acting on the nitrogen in the air.
174. The Ruffle Soda-Lime Method.—Many attempts have been made to adapt the soda-lime method to the determination of nitric nitrogen. Of these the process devised by Ruffle is the only one which has proved successful.[143] The method is founded on the action of sulfurous vapors on the nitrogen oxids produced during the combustion whereby sulfuric acid is formed and the nascent nitrogen is joined with hydrogen to form ammonia. By this process all the nitrogen contained in the sample, even if in the nitric form, is finally obtained as ammonia. In the original method the reagents employed were a mixture of sodium thiosulfate and soda-lime and a mixture of charcoal, sulfur, and granulated soda-lime. Subsequently the official chemists substituted sugar for the charcoal.[144] The method was used for a long time by the official chemists and came into general favor until displaced by the simpler and cheaper processes of the moist combustion method adapted to nitric nitrogen. As finally modified and used by the official chemists the process was conducted as described below.
175. The Official Ruffle Method.—[145]Reagents.—(1) Standard solutions and indicator the same as for the kjeldahl method.
(2) A mixture of equal parts by weight of fine-slaked lime and finely powdered sodium thiosulfate dried at 100°:
(3) A mixture of equal parts of weight of finely powdered granulated sugar and flowers of sulfur:
(4) Granulated soda-lime, as described under the soda-lime method:
(5) Combustion tubes of hard Bohemian glass seventy centimeters long and one and three-tenths centimeters in diameter: