Pure potash forms crumbling lumps with a slight yellow or bluish grey tinge, rapidly absorbing moisture from the air, and in time completely liquefying. The yellowish tinge is caused by oxide of iron, the bluish by manganese compounds. The so-called calcined potash has been strongly heated, and thus all organic substances contained in it have been destroyed.
Potashes are in no way pure potassium carbonate; they contain a mixture of all those salts which are found in plants—potassium sulphate and chloride, small quantities of silicic acid, etc. These impurities are rarely harmful, still it is generally necessary to know the percentage of pure potassium carbonate contained in potashes.
Although at present in commerce the strength of potashes is frequently guaranteed, it is still desirable to estimate the strength. It is sufficient for practice to allow a small quantity, say 100 grammes, to stand with an equal quantity of very cold water for some hours, then to filter and pour a similar quantity of water over the residue on the filter. The weight of undissolved substance subtracted from 100 gives with sufficient accuracy the weight of pure potassium carbonate contained in 100 parts of potashes. This method is founded on the fact that potassium carbonate dissolves readily even in cold water, but the other salts with difficulty. This procedure can also be used to obtain pure potassium carbonate from crude potashes; it is only necessary to filter and evaporate to dryness the solution obtained by pouring very cold water on crude potashes.
Potassium Hydroxide (Potassium Hydrate, Caustic Potash), KOH = 56.—The commercial variety consists of very deliquescent white lumps, generally containing a large quantity of impurities. On this account caustic potash, or rather a solution of it, is prepared in the colour works.
With this object 11 parts of potash, contained in a tub with an opening at the bottom, are mixed with 100 parts of cold water. Two hours later, the clear solution is run off into a clean iron pan, in which it is heated to boiling. To the boiling solution is added milk of lime prepared from water and 3·5 parts of quicklime. After the liquid has boiled a few minutes, a small portion is filtered and hydrochloric acid added to the clear filtrate; if no effervescence occurs, then all the potassium carbonate is converted into caustic potash. Should effervescence occur, milk of lime is added until a new portion no longer effervesces on the addition of hydrochloric acid. Then the pan is covered with a well-fitting lid, and the cooled liquid, if not required for immediate use, preserved in well-corked glass bottles.
The strength of a caustic potash solution can be found by means of a hydrometer. The following table shows the relation between the specific gravity of a solution and the percentage of caustic potash it contains:—
| Specific Gravity | Caustic Potash per cent. |
|---|---|
| 1·06 | 4·7 |
| 1·11 | 9·5 |
| 1·15 | 13·0 |
| 1·19 | 16·2 |
| 1·23 | 19·5 |
| 1·28 | 23·4 |
| 1·39 | 32·4 |
| 1·52 | 42·9 |
| 1·60 | 46·7 |
| 1·68 | 51·2 |
Potassium Nitrate (Saltpetre), KNO₃ = 101, consists of large crystals, which quickly dissolve in water. On heating it readily gives up oxygen, and thus finds use as an oxidising agent. In former times, when the colour manufacturer was compelled to make his own materials, saltpetre was of great importance in colour making; at present, when such materials are to be bought at low prices and no colour maker prepares his own, saltpetre is little used.
Potassium Bitartrate, C₄H₅KO₆ = 188.—This salt, known as tartar in large crystals, and as cream of tartar in the form of meal, is occasionally used in colour making. It is little soluble in cold water, but more easily in hot. The hot solution is generally used.
Potassium Bichromate (Bichromate of Potash), K₂Cr₂O⁷ = 295.—This salt is made in special works, by melting chrome iron ore with saltpetre and extracting the mass with water, when a yellow solution of potassium chromate is obtained; to this sulphuric acid is added, which unites with half the potassium, thus leaving potassium bichromate, which is obtained by evaporation of the solution in fine red crystals. These are purified by recrystallisation. At present, in place of the above method, calcium chromate is formed by roasting chrome iron ore with lime; the calcium chromate is then decomposed by a soluble potassium salt, thus forming potassium chromate.