Nitric Acid, HNO₃ = 63.—This acid, which is used in the preparation of many colours, is distinguished by the readiness with which it gives up part of its oxygen, and thus converts metals like antimony and bismuth into oxides, and transforms other compounds into a higher state of oxidation. There are two kinds of nitric acid: ordinary nitric acid, a colourless liquid which is more or less pure; and fuming nitric acid, a yellow or orange-coloured liquid, fuming strongly in the air, which consists of a solution of nitrogen peroxide, NO₂, and nitric oxide, NO, in nitric acid.
| Specific Gravity at 15° C. | Nitric Acid per cent. | Specific Gravity at 15° C. | Nitric Acid per cent. |
|---|---|---|---|
| 1·530 | 100·00 | 1·323 | 55·00 |
| 1·520 | 97·00 | 1·284 | 50·49 |
| 1·509 | 94·00 | 1·251 | 45·00 |
| 1·503 | 92·00 | 1·211 | 40·00 |
| 1·499 | 91·00 | 1·185 | 33·86 |
| 1·495 | 90·00 | 1·157 | 30·00 |
| 1·478 | 85·00 | 1·138 | 25·71 |
| 1·460 | 80·00 | 1·120 | 23·00 |
| 1·442 | 75·00 | 1·089 | 20·00 |
| 1·423 | 69·96 | 1·067 | 15·00 |
| 1·400 | 65·07 | 1·022 | 11·41 |
| 1·346 | 60·00 | 1·010 | 4·00 |
Since the action of nitric acid chiefly depends on its oxidising properties, which are possessed by both kinds, it generally does not matter which is used. The usual impurities are chlorine and sulphuric acid; the presence of the first is shown by silver nitrate solution, of the latter by barium chloride, in each case added after diluting. When the acid is used for oxidations these impurities do not interfere, but nitric acid containing chlorine cannot be used to dissolve silver, because the chlorine would form insoluble silver chloride.
The strength of nitric acid is gauged by its specific gravity as given in the table.
Aqua Regia.—A mixture of 2 parts of hydrochloric acid and 1 part of nitric acid gradually turns orange or yellow and evolves chlorine. This liquid, which can dissolve gold in consequence of the free chlorine it contains (hence its alchemistic name, from gold, the “king of metals”), is used as a very powerful oxidising agent in colour making.
Carbon, C = 12, is the only one of the non-metallic elements to be mentioned here; by itself it forms a group of very important pigments, which we shall describe in detail at a later stage.
Carbonic Acid Gas, CO₂ = 44, is used in the manufacture of white lead, which it precipitates from lead acetate. This is, however, a particular branch of colour making carried on in special works. In describing this manufacture we shall return to the preparation of carbonic acid on a large scale.
Organic Acids.
The organic acids which are important in colour making are acetic, oxalic and tartaric acids.
Acetic Acid, C₂H₄O₂ = 58.—The very dilute form of this substance is known commonly as vinegar, the stronger as pyroligneous acid, and the purest as glacial acetic acid; the latter is, however, scarcely used. Formerly in colour making ordinary vinegar was used, but now pyroligneous acid is almost exclusively employed. This is distinguished by its strong empyreumatic smell, which, however, is without importance in colour making.