The following table gives the tension of aqueous vapors in millimeters of a mercurial column for each degree of temperature from zero to forty.
| Temperature. | Tension of vapor in millimeters. | Temperature. | Tension of vapor in millimeters. |
|---|---|---|---|
| 0° | 4.525 | 21° | 18.505 |
| 1° | 4.867 | 22° | 19.675 |
| 2° | 5.231 | 23° | 20.909 |
| 3° | 5.619 | 24° | 22.211 |
| 4° | 6.032 | 25° | 23.582 |
| 5° | 6.471 | 26° | 25.026 |
| 6° | 6.939 | 27° | 26.547 |
| 7° | 7.436 | 28° | 28.148 |
| 8° | 7.964 | 29° | 29.832 |
| 9° | 8.525 | 30° | 31.602 |
| 10° | 9.126 | 31° | 33.464 |
| 11° | 9.751 | 32° | 35.419 |
| 12° | 10.421 | 33° | 37.473 |
| 13° | 11.130 | 34° | 39.630 |
| 14° | 11.882 | 35° | 41.893 |
| 15° | 12.677 | 36° | 44.268 |
| 16° | 13.519 | 37° | 46.758 |
| 17° | 14.409 | 38° | 49.368 |
| 18° | 15.351 | 39° | 52.103 |
| 19° | 16.345 | 40° | 54.969 |
| 20° | 17.396 |
When a gas is in contact with water the aqueous vapor is diffused throughout the mass, and the pressure to which the mixture is subjected, is partly neutralized by the tension of the water vapor. The real pressure to which the gas, whose volume is to be determined is subjected, is therefore diminished by that tension. If for instance a gas in contact with water show a volume of thirty-five cubic centimeters at 22° and 740 millimeters barometric pressure its volume is really greater than if it were perfectly dry. How much greater can be determined by inspecting the table, for at 22° the tension of water vapor is 19.675 millimeters of mercury. The real pressure to which the volume of gas is subjected is therefore 740 - 19.675 = 720.325 millimeters.
If, therefore, in the example given, the nitrogen were in contact with water, the calculation would proceed as follows:
| V′ = | 32.18 × 720.325 | = 30.5 |
| 760 |
and 30.5 × 1.25456 = 38.26.
Hence, 38.26 milligrams of nitrogen correspond to 7.65 per cent, when half a gram of substance is taken for the combustion.
165. Aqueous Tension in Solutions of Potassium Hydroxid.—Even in strong solutions of potassium hydroxid the tension of aqueous vapor is not destroyed, but is reduced to a minimum, which is negligible in the calculation of the percentage by weight of the nitrogen in a sample of fertilizer. When dilute solutions of a caustic alkali are used however, the neglect of the tension of the aqueous vapor may cause an error of some magnitude. In such cases the strength of the solution should be known and correction made according to the following table:[137]
| Millimeters tension of aqueous vapor for KOH solutions of | |||||
|---|---|---|---|---|---|
| Temperature. | 9.09 per cent. | 16.66 per cent. | 23.08 per cent. | 28.57 per cent. | 32.89 per cent. |
| 10°.00 | 8.62 | 8.01 | 7.31 | 6.50 | 5.62 |
| 11°.00 | 9.21 | 8.56 | 7.82 | 6.95 | 6.01 |
| 12°.10 | 9.90 | 9.21 | 8.41 | 7.47 | 6.46 |
| 13°.00 | 10.50 | 9.77 | 8.92 | 7.93 | 6.86 |
| 13°.95 | 11.17 | 10.39 | 9.49 | 8.44 | 7.30 |
| 15°.15 | 12.06 | 11.22 | 10.25 | 9.11 | 7.86 |
| 16°.00 | 12.74 | 11.85 | 10.82 | 9.62 | 8.33 |
| 17°.00 | 13.57 | 12.63 | 11.54 | 10.26 | 8.88 |
| 18°.00 | 14.46 | 13.45 | 12.29 | 10.93 | 9.47 |
| 19°.00 | 15.39 | 14.33 | 13.09 | 11.65 | 10.09 |
| 20°.00 | 16.38 | 15.25 | 13.93 | 12.40 | 10.75 |
| 21°.00 | 17.42 | 16.22 | 14.82 | 13.20 | 11.44 |
| 21°.82 | 18.32 | 17.06 | 15.59 | 13.88 | 12.04 |
| 23°.00 | 19.68 | 18.32 | 16.75 | 14.92 | 12.94 |
| 24°.00 | 20.92 | 19.47 | 17.80 | 15.86 | 13.76 |
| 25°.00 | 22.19 | 20.67 | 18.91 | 16.85 | 14.62 |
| 26°.00 | 23.55 | 21.94 | 20.07 | 17.89 | 15.53 |
| 26°.98 | 24.95 | 23.25 | 21.27 | 18.96 | 16.46 |
| 27°.93 | 26.38 | 24.59 | 22.51 | 20.07 | 17.45 |
| 29°.00 | 28.08 | 26.18 | 23.96 | 21.38 | 18.59 |
| 30°.00 | 29.76 | 27.74 | 25.40 | 22.67 | 19.72 |
| 31°.00 | 31.51 | 29.38 | 26.91 | 24.03 | 20.91 |
| 32°.13 | 33.61 | 31.34 | 28.72 | 25.65 | 22.34 |
| 33°.00 | 35.30 | 32.93 | 30.18 | 26.97 | 23.50 |
| 34°.00 | 37.34 | 34.84 | 31.94 | 28.56 | 24.89 |
166. Use of Volumetric Method.—For practical purposes it may be said that the volumetric determination of nitrogen in fertilizer analysis has gone entirely out of use. For control and comparison it is still occasionally practiced but it has had to give way to the more speedy and fully as accurate processes of moist combustion with sulfuric acid which have come into general use in the last decade. The student and analyst however should not fail to master its details and become skilled in its use. There are certain nitrogenous substances such as the alkaloids which are quite refractory when subjected to moist combustion. While such bodies may not occur in fertilizers it is well to have at hand a means of accurately determining their nitrogen content.