Pyrolusite and hydrochloric acid are now generally used for the preparation of chlorine, because the solution of manganese chloride, left at the end of the operation, is valuable.
If all the chlorine made in one operation is not at once required for the manufacture of a colour, it can be utilised by sending it into a box filled with slaked lime, which is converted into chloride of lime or bleaching powder. The liquid run away from the apparatus at the conclusion of the operation contains manganese and sodium sulphates, or manganese chloride as the case may be, and can be used for the preparation of manganese pigments.
Ammonia, NH₃ = 17.—Ammonia is obtained from chemical works in the form of a strong solution of ammonia gas in water, which is generally very pure. The density of an aqueous solution of ammonia is the smaller the more ammonia it contains, and thus the strength of a solution of ammonia can easily be formed by means of the hydrometer. The following table shows the percentage of ammonia, NH₃, in a liquid of known specific gravity at the temperature of 14° C.:—
| Specific Gravity. | Ammonia per cent. | Specific Gravity. | Ammonia per cent. |
|---|---|---|---|
| 0·885 | 36·00 | 0·953 | 11·50 |
| 0·886 | 35·00 | 0·955 | 11·00 |
| 0·889 | 34·00 | 0·957 | 10·50 |
| 0·891 | 33·00 | 0·959 | 10·00 |
| 0·893 | 32·50 | 0·961 | 9·50 |
| 0·895 | 32·00 | 0·963 | 9·00 |
| 0·897 | 31·30 | 0·965 | 8·50 |
| 0·905 | 25·39 | 0·970 | 7·50 |
| 0·925 | 19·54 | 0·972 | 7·00 |
| 0·932 | 17·52 | 0·974 | 6·50 |
| 0·947 | 13·46 | 0·976 | 6·00 |
| 0·951 | 12·00 | 0·978 | 5·50 |
The Hydrometer.—In the above table the percentage content of the ammonia solution is given according to its specific gravity, that is, according to the ratio between the weight of any volume of the liquid and the weight of an equal volume of water. According to scientific principles, only those hydrometers should be used which are graduated in specific gravities. In spite of all exertions in this direction, manufacturers have not yet been induced to use such instruments in every case. Hydrometers, with quite arbitrary scales, such as those of Baumé and Twaddell, are frequently found in works. These hydrometers generally only show that a liquid is of so many degrees on the particular scale, and the manufacturer in using them is restricted to the following out of a certain recipe which requires the use of a liquid of a certain strength which is expressed in degrees Baumé, etc. He does not learn by this how many per cent. of the particular substance are dissolved in the water when the liquid has a certain hydrometric strength.
For the sake of uniformity, it is urgently to be desired that all manufacturers who use the hydrometer to estimate the content of a liquid in ammonia, potash, soda, hydrochloric, sulphuric, nitric acids, etc., should employ simple specific gravities. This is desirable, because the percentage strength of a solution, corresponding to the specific gravity, can be at once accurately found from tables. On these grounds, in the present work, we have restricted ourselves to tables showing simply the specific gravities of solutions and the corresponding composition.
Sal Ammoniac or Ammonium Chloride, NH₄Cl = 53·5.—This substance comes into commerce in the form of a white crystalline meal, more rarely in the form of sugar loaves (crystallised sal ammoniac) or of flat cakes (sublimed sal ammoniac). It is usually very pure, since impure forms, generally containing much iron, are difficult of sale. At a particular temperature sal ammoniac is volatile; it is used in certain mixtures in order to prevent the temperature, on heating, from rising beyond a certain point. Like ammonia, it is more used in dyeing.
Ammonium Sulphide, NH₄HS.—This compound is obtained by leading sulphuretted hydrogen into ammonia solution so long as it is dissolved, and a test portion of the liquid still gives a white precipitate with a solution of magnesium sulphate. Ammonium sulphide decomposes by long standing in the air, sulphur being separated. It gives precipitates with the salts of certain metals, for example, iron, cobalt, manganese, zinc, nickel. These precipitates, which consist of the sulphides of the metals, are not formed by sulphuretted hydrogen in acid solutions.
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.