Acids.
In colour making many acids are used for the solution of metals, the production of precipitates, for oxidations and so forth. Commercial acids, especially inorganic acids, generally contain not inconsiderable quantities of impurities which are injurious in the manufacture of many colours.
Hydrochloric Acid, HCl = 36·5.—The commercial acid (muriatic acid, spirits of salt) generally contains large quantities of iron, which colour it yellow—fortunately, in many cases, this is not a disadvantage, and also at times the iron can be removed from solutions made in the acid. Another impurity is sulphuric acid. This can be detected by diluting and adding barium chloride; if sulphuric acid be present, a white precipitate, or, at least, a cloudiness, appears.
Ordinary hydrochloric acid is a solution of hydrochloric acid gas in water. The strongest acid contains 42·85 per cent. of the gas, and has the specific gravity 1·21. The following table gives the strengths of acids of various specific gravities:—
| Specific Gravity. | Hydrochloric Acid per cent. | Specific Gravity. | Hydrochloric Acid per cent. |
|---|---|---|---|
| 1·21 | 42·85 | 1·10 | 20·20 |
| 1·20 | 40·80 | 1·09 | 18·75 |
| 1·19 | 38·88 | 1·08 | 16·71 |
| 1·18 | 36·36 | 1·07 | 15·49 |
| 1·17 | 34·34 | 1·06 | 13·86 |
| 1·16 | 32·32 | 1·05 | 11·49 |
| 1·15 | 30·30 | 1·04 | 8·97 |
| 1·14 | 28·28 | 1·03 | 6·93 |
| 1·13 | 26·26 | 1·02 | 4·89 |
| 1·12 | 24·24 | 1·01 | 2·03 |
| 1·11 | 22·22 | —— | —— |
Sulphuretted Hydrogen, H₂S = 34.—This is a gas of acid properties smelling like rotten eggs; it precipitates the sulphur compounds of many metals when led into the acid solution of the corresponding salt. This substance is seldom required in colour works, so that it is convenient to have an apparatus which permits of the preparation of any required quantity. [Fig. 3] represents an apparatus devised by the author, which is well adapted for the preparation of sulphuretted hydrogen. It consists of a small, wooden tub, on whose upper edge lies a thick paper ring, so that the lid may be pressed down air-tight by the screws B. Through the lid pass a tap-funnel, T, a movable screw, S, and a tube, R, to carry away the gas. On the screw S hangs a basket, K, by a handle; this is filled with pieces of iron sulphide as large as nuts. The tub is filled to about one-third of its height with a mixture of 9 parts of water and 1 part of sulphuric acid.
Fig. 3.
When sulphuretted hydrogen is required the basket is lowered by the screw, S, until it dips in the liquid; according as the basket dips more or less into the liquid a fast or slow current of the gas is obtained. When the gas is no longer required the basket is raised out of the liquid, and the evolution of gas at once ceases. The funnel, T, serves for the introduction of the liquid, the tap, H, for drawing off the iron sulphate solution, which can be used with advantage for the preparation of fine iron colours. The apparatus should not be opened so long as sulphide of iron remains in the basket.
Sulphuric Acid comes into the market in two different forms: oil acid; both are used in colour making. Oil of Vitriol, H₂SO₄ = 98, is a colourless, oily liquid of high specific gravity; it is generally tolerably pure, and contains, as a rule, only a small quantity of lead, the presence of which is indicated by a turbidity on largely diluting the acid. The amount of pure sulphuric acid in the liquid is practically determined by taking the specific gravity. The table indicates the relation between the specific gravity and the content of sulphuric acid.
| Specific Gravity. | Sulphuric Acid per cent. | Specific Gravity. | Sulphuric Acid per cent. |
|---|---|---|---|
| 1·8485 | 100 | 1·8043 | 89 |
| 1·8475 | 99 | 1·7962 | 88 |
| 1·8460 | 98 | 1·7870 | 87 |
| 1·8439 | 97 | 1·7774 | 86 |
| 1·8410 | 96 | 1·7673 | 85 |
| 1·8376 | 95 | 1·7570 | 84 |
| 1·8336 | 94 | 1·7465 | 83 |
| 1·8290 | 93 | 1·7360 | 82 |
| 1·8233 | 92 | 1·7245 | 81 |
| 1·8179 | 91 | 1·7120 | 80 |
| 1·8115 | 90 | —— | —— |
Nordhausen Sulphuric Acid, H₂S₂O₇ = H₂SO₄ + SO₃, is generally a yellowish brown liquid, which gives off white fumes in the air. It contains varying quantities of sulphur trioxide dissolved in sulphuric acid. It often contains selenium, which separates as a red powder when the acid is diluted. The presence of this impurity does not interfere with the use of the acid for dissolving indigo, the only purpose for which it is required in the colour factory.
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.