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

by Harvey Washington Wiley

(5) Combustion tubes about forty centimeters long and twelve millimeters internal diameter, drawn out to a closed point at one end.

(6) Large-bulbed U tubes with glass stop-cock, or Will’s tubes with four bulbs.

Manipulation.—The substance to be analyzed should be powdered finely enough to pass through a sieve of one millimeter mesh; from seven-tenths to one and four-tenths grams, according to the amount of nitrogen present, are taken for the determination. Into the closed end of the combustion tube, put a small loose plug of asbestos, and upon it about four centimeters of fine soda-lime. In a porcelain dish or mortar, mix the substance to be analyzed, thoroughly but quickly, with enough fine soda-lime to fill about sixteen centimeters of the tube, or about forty times as much soda-lime as substance, and put the mixture into the combustion tube as quickly as possible by means of a wide-necked funnel, rinsing out the dish and funnel with a little more fine soda-lime, which is to be put in on top of the mixture. Fill the rest of the tube to within about five centimeters of the end with granulated soda-lime, making it as compact as possible by tapping the tube gently while held in a nearly upright position during the filling. The layer of granulated soda-lime should not be less than twelve centimeters long. Lastly, put in a plug of asbestos about two centimeters long, pressed rather tightly, and wipe out the end of the tube to free it from adhering soda-lime.

Connect the tube by means of a well-fitting rubber stopper or cork with the U tube or Will’s bulbs, containing ten cubic centimeters of standard acid, and adjust it in the combustion furnace so that the end of the tube projects about four centimeters from the furnace, supporting the U tube or Will’s bulb suitably. Heat the portion of the tube containing the granulated soda-lime to a moderate redness, and when this is attained extend the heat gradually through the portion containing the substance, so as to keep up a moderate and regular flow of gases through the bulbs, maintaining the heat of the first part until the whole tube is heated uniformly to the same degree. Continue the combustion until gases have ceased bubbling through the acid in the bulbs, and the mixture of substance and soda-lime has become white, or nearly so, which shows that the combustion is finished. The process should occupy about three-quarters of an hour, or not more than one hour. Remove the heat, and when the tube has cooled below redness break off the closed tip and aspirate air slowly through the apparatus for two or three minutes to bring all the ammonia into the acid. Disconnect the tube, wash the acid into a beaker or flask, and titrate with the standard alkali.

During the combustion the end of the tube projecting from the furnace must be kept heated sufficiently to prevent the condensation of moisture, yet not enough to char the stopper. The heat may be regulated by a shield of tin slipped over the projecting end of the combustion tube.

It is found very advantageous to attach a bunsen valve to the exit tube, allowing the evolved gases to pass out freely, but preventing a violent sucking back in case of a sudden condensation of steam in the bulbs.

170. The Official French Method.—The French chemists prefer to drive out the traces of ammonia remaining in the combustion tube by means of the gases arising from the decomposition of oxalic acid.[140] The operation is conducted by mixing about one gram of oxalic acid with enough of dry granular soda-lime to form a layer of four centimeters in length at the bottom of the tube. The rest of the tube is then charged substantially as directed above. At the end of the combustion the oxalic acid is decomposed by heat furnishing sufficient hydrogen to remove from the tube all traces of ammonia which it may contain. The French chemists employ, for titration, either normal acids and alkalies or some decimal thereof or else an acid of such strength as to have each cubic centimeter thereof correspond to ten milligrams of nitrogen, thus making the computation of results exceedingly simple. Such an acid is secured when one liter thereof contains thirty-five grams of pure monohydric sulfuric acid or forty-five grams of pure crystallized oxalic acid. The corresponding alkaline reagent should contain, in each liter, forty grams of pure potassium hydroxid.

171. The Hydrogen Method.—Thibault and Wagner recommend that the combustion with soda-lime be conducted in an atmosphere of hydrogen,[141] and Loges replaces this by common illuminating gas freed from ammonia by conducting it through a tube filled with glass balls moistened with dilute sulfuric acid.[142]

In these cases the combustion tube is left open at both ends and the materials under the tube confined to the proper position by asbestos plugs. The gases used act in a merely mechanical manner and their use affords so few advantages over the method of aspirating air at the end of the combustion as to render it unadvisable.

172. Coloration of the Product.—It often happens, especially in the combustion of animal products, such as tankage and fish scrap, that the acid securing the ammonia is deeply colored by the condensation of some of the other products of combustion. This coloration interferes in a very serious way with the delicacy of the indicator used to determine the end of the reaction. In this case the liquid may be mixed with an alkali and distilled and the ammonia secured in a fresh portion of the standard acid as in the moist combustion process to be hereafter described.