The specific gravity is not a criterion of strength to be perfectly relied on, but if it be adopted as a test, the following points must be attended to.
1st. That the temperature of the acid be at or near 60° Fahrenheit; the density of Sulphuric Acid especially is, from its small specific heat, greatly influenced a change of temperature.
2nd. The sample of Nitric Acid must be free from Peroxide of Nitrogen, or only slightly coloured by it. This substance, when present, increases the specific gravity of the acid without adding to its available properties. A yellow sample of Nitric Acid will therefore be somewhat weaker than is indicated by the specific gravity.
3rd. The Oil of Vitriol should yield no solid residue on evaporation. Sulphate of Lead and Bisulphate of Potash are often found in the commercial acid, and add much to its density. Oil of Vitriol containing Sulphate of Lead becomes milky on dilution.
The formula for a definite Nitro-Sulphuric Acid, of the proper strength for making the soluble Pyroxyline, may be stated thus:—
HO NO5, 2 (HO SO3) + 3½ HO
or
| Atoms. | Atomic weight. | |
| Nitric Acid | 1 | 54 |
| Sulphuric Acid | 2 | 80 |
| Water | 6½ | 58 |
| 192 | ||
Having found the percentage of real acid which is present,[31] the following calculation will give the relative weights of the ingredients required to produce the formula:—
| Let |
| a | = | percentage of real | Nitric Acid, | ||||
| b | = | " " | Sulphuric Acid, | ||||||
| then | 5400 a | = | quantity of | Nitric Acid, | |||||
| 8000 b | = | " | Sulphuric Acid, | ||||||
| = | " | Water. | ||||||
