To avoid any danger whatever of decomposing organic nitrogenous compounds, the ammonia may be determined in the cold by treatment with soda-lye, under a bell-jar containing some set sulfuric acid. The operation must be allowed to continue for many days. Even at the end of a long time it will be found that some ammonia is still escaping. It may therefore be finally inferred that all the nitrogen as ammonia is not obtained by this process, or that even magnesia may gradually convert other nitrogenous compounds into ammonia. In this connection the methods of determining ammonia in soils 406, 407, and 408 of volume one may be consulted.
201. Method of Boussingault.—The official French method is essentially the original method of Boussingault with slight modifications. It is conducted as follows:[169] In case the sample is ammonium sulfate about half a gram is placed in a flask of half a liter capacity, together with 300 cubic centimeters of distilled water and two grams of caustic magnesia. The flask is connected with a condenser of glass or metal which ends in a tube drawn out to a point and dipping beneath the set acid in the receiver in the usual way. The acid is colored with litmus or lacmoid tincture. The distillation is continued until about 100 cubic centimeters have gone over. The receiver is then removed with the usual precautions and the residual acid titrated. Suppose twenty cubic centimeters of normal acid have been employed and twelve and a half cubic centimeters of normal alkali be necessary to neutralize the excess of the acid. Then the nitrogen is found by the following equations: 20.0 - 12.5 = 7.5 and 7.5 × 0.014 = 0.105 gram = weight of nitrogen found. Then 0.105 × 100 ÷ 5 = 21.00 = per cent of nitrogen found.
The distilling apparatus of Aubin is preferred by the French chemists, an apparatus so arranged with a reflux partial condenser, that nearly all the aqueous vapor is returned in a condensed state to the flask while the ammonia, on account of its great volatility, is carried over into the receiver. To avoid the regurgitation which might be caused by the concentrated ammonia gas coming in contact with the acid the separable part of the condensing tube is expanded into a bulb large enough to hold all the acid which lies above its mouth. By the means of this apparatus the ammonia is all collected in the standard acid without greatly increasing its volume and the titration is thus rendered sharper. The employment of caustic magnesia has the advantage of not decomposing any organic matters or cyanids that may be present.
If the sample under examination hold part of its ammonia as ammonium magnesium phosphate it will be necessary first to treat it with sulfuric acid in order to set the ammonia free and then to use enough of the magnesium oxid to neutralize the excess of the sulfuric acid and still supply the two grams necessary for the distillation. When the sample contains a considerable quantity of organic matter it sometimes tends to become frothy towards the end of the distillation. This trouble can be avoided by introducing into the flask one or two grams of paraffin.
Where carbon dioxid is given off during the distillation the contents of the receiver must be boiled before titration, or else lacmoid must be used as an indicator instead of litmus.
202. Determination of Thiocyanates in Ammoniacal Fertilizers.—The extended use of ammonium sulfate as a fertilizer renders it important to determine the actual constituents which may be present in samples of this material. The following bodies have been found in commercial ammonium sulfates: Sulfuric acid, chlorin, ammonia, thiocyanic acid, potash, soda, lime and iron oxid. These are found in the soluble portions. In the insoluble portions have been found silica, sulfuric acid, lime, magnesia and iron oxid. A sample of commercial ammonium sulfate analyzed by Jumeau contained the following substances:[170]
| Per cent. | |
| Moisture | 10.5109 |
| Ammonium sulfate | 67.8453 |
| Ammonium thiocyanate | 9.3935 |
| Sodium sulfate | 9.2429 |
| Potassium sulfate | 0.9774 |
| Calcium sulfate | 0.6800 |
| Iron thiocyanate | 0.5000 |
| Magnesium chlorid | traces |
| Silica | 0.0830 |
| Undetermined | 0.7670 |
The determination of the thiocyanic acid in the thiocyanate is generally made by the oxidation of the sulfur to sulfuric acid and its subsequent weighing in the form of barium sulfate. Jumeau has modified the method by determining the amount of the thiocyanate by means of a titrated liquid. The method is practiced as follows:
A solution of ammonium thiocyanate is prepared, containing eight grams of this salt per liter, and its exact content of thiocyanate is rigorously determined by titration with silver nitrate or by the weight of the barium sulfate produced after the oxidation of the sulfur. Ten cubic centimeters of the titrated liquor are taken and diluted with water to about 100 cubic centimeters and ten cubic centimeters of pure sulfuric acid added. Afterward, drop by drop, a solution of potassium permanganate is added, containing about ten grams of that salt per liter. The permanganate is instantly decolorized. There is an evolution of hydrocyanic acid as the thiocyanate passes to the state of sulfuric acid. A single drop in excess gives to the mixture the well-known rose coloration of the permanganate solution which persists for several hours. The number of cubic centimeters necessary to produce the persistent rose tint is noted and the same operation is carried on with from one-half to one gram of the unknown product which is to be assayed. A simple proportion indicates the content of the thiocyanate in the unknown body. The process is of great exactitude and permits the rapid determination of thiocyanic acid in the presence of chlorids, cyanids, etc., which remain without action upon the permanganate. In case chlorids and cyanids are absent the thiocyanate can be determined directly by silver nitrate either by weighing the precipitate or by the process of Volhardt based upon the precipitation of the silver by thiocyanate in the presence of a ferric salt. The end of the reaction is indicated by the red coloration which the liquid shows when the thiocyanate is in excess.
203. Separation of Albuminoid from Amid and Other Forms of Nitrogen in Organic Fertilizers.—It may be of interest to the dealer, farmer, and analyst, to discriminate between the albuminoid and other nitrogen in fertilizers, such as oil-cakes. The final value of the nitrogen for plant nourishment is not greatly different, but the immediate availability for nitrification is a matter of some importance. The most convenient process in such a case is the copper hydroxid separation process as improved by Stutzer.[171] The process is conveniently carried out in accordance with the method prescribed by the official chemists.[172]