GRAVIMETRIC DETERMINATION.
It follows from what has been said that there is no direct gravimetric determination. The percentage of nitrogen pentoxide (N2O5) in a comparatively pure nitrate is sometimes determined indirectly in the following way:—Place in a platinum-crucible 4 or 5 grams of powdered and cleaned quartz. Ignite, cool in a desiccator, and weigh with the cover. Mix 1 gram of the dried and powdered salt with the quartz in the crucible by stirring with a stout platinum-wire. Cover the crucible, and heat in a Bunsen-burner flame at scarcely visible redness for half-an-hour. Cool and weigh. The loss in weight gives the amount of nitrogen pentoxide. Sulphates and chlorides in moderate quantity do not interfere. The following is an example of the process:—
| Crucible and sand | 26.6485 grams |
| Nitre taken | 1.0000 " |
| ———— | |
| 27.6485 " | |
| Weight after ignition | 27.1160 " |
| ———— | |
| Loss on ignition | 0.5325 " |
This is equal to 53.25 per cent. of nitrogen pentoxide.
VOLUMETRIC DETERMINATION.
This is based on the oxidising action of nitric acid, or of nitrates in acid solutions on ferrous salts. The pentoxide (N2O5) of the nitrate is reduced to nitric oxide (NO), so that 336 parts of iron peroxidised represent 108 parts of nitric pentoxide as oxidising agent.[112] The quantity of iron peroxidised is determined by taking a known quantity of ferrous salt, oxidizing with a weighed sample of nitrate, and then determining the residual ferrous iron by titration with bichromate or permanganate of potassium solution. The difference between the ferrous iron taken and that found, gives the amount oxidized by the nitrate. The speed with which nitric oxide takes up oxygen from the air, and thus becomes capable of oxidising more iron, renders some precautions necessary; ferrous chloride should, therefore, be used, since it is easier to expel nitric oxide (by boiling) from solutions of a chloride than it is from those of a sulphate. The process is as follows:—Dissolve 2 grams of thin soft iron wire in 50 c.c. of hydrochloric acid in a flask provided with an arrangement for maintaining an atmosphere of carbon dioxide. When the iron has dissolved, allow the solution to cool, and add 0.5 gram of the nitrate. Heat gently for a few minutes, and then boil until the nitric oxide is expelled. An atmosphere of carbon dioxide must be kept up. Dilute with water, and titrate the residual iron with standard solution of bichromate of potassium. The standard "bichromate" is made by dissolving 17.5 grams of the salt (K2Cr2O7) in water, and diluting to 1 litre: 100 c.c. equal 2 grams of iron. Deduct the weight of iron found from the 2 grams originally taken, and multiply by 0.3214. This gives the weight of the pentoxide in the sample. In an example, 0.5 gram of nitre was taken, and 59.4 c.c. of the "bichromate" solution were required. The 59.4 c.c. thus used are equivalent to 1.198 gram of iron. This leaves 0.822 gram as the quantity oxidised by the nitre, which, multiplied by 0.3214, gives 0.2642 gram for the nitrogen pentoxide, or 52.8 per cent.
GASOMETRIC METHOD.
This is based upon the measurement of the nitric oxide evolved on shaking up a weighed quantity of the nitrate with sulphuric acid over mercury in a nitrometer. Each c.c. of nitric oxide obtained, when reduced to normal temperature and pressure, is equivalent to:—
| 0.627 | milligram | of nitrogen. |
| 1.343 | " | of nitric oxide. |
| 2.418 | " | of nitric pentoxide. |
| 2.820 | " | of nitric acid. |
| 3.805 | " | of sodium nitrate. |
| 4.523 | " | of potassium nitrate. |
In working on substances not rich in nitrates, an ordinary nitrometer (fig. 69) is used; but in the assay of sodium nitrate, nitroglycerine, &c., an instrument provided with a bulb having a capacity of 100 c.c. is employed.