Alkalis.
The compounds of the alkali metals, potassium and sodium, play a considerable part in colour making. Formerly the potassium compounds were in general use, but the sodium compounds are at present obtainable at a much lower price, and in most cases they can be used equally well. Thus, in colour making, sodium compounds are chiefly employed. The cyanogen compounds are an exception; their potassium compounds are used exclusively.
Potassium Compounds.—The potassium compounds which are chiefly used in colour making are potassium carbonate (potashes, pearl-ash), potassium hydroxide (caustic potash), potassium nitrate (saltpetre), potassium tartrate (tartar), and potassium ferrocyanide and ferricyanide (yellow and red prussiate of potash). The cyanogen compounds have peculiar properties. We shall describe them separately after the potassium and sodium compounds.
Potassium Carbonate (carbonate of potash), K₂CO₃ = 138, is known commercially as potashes, a name derived from its former method of preparation by heating the ashes of plants in pots. At present potashes are prepared in large quantities from other sources.
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.
Potassium bichromate is unaltered in air; it dissolves easily in water, and is of great importance in the preparation of many colours, in particular chromium oxide and the lead pigments. The commercial salt generally contains potassium sulphate, with which at times it is intentionally adulterated. The adulteration is detected by dissolving in water, adding half the volume of pure hydrochloric acid, and cautiously and carefully dropping in spirits of wine. A rapid action takes place, which is only assisted by warming when necessary. The red liquid changes to emerald green. If barium chloride be now added, a white precipitate is obtained in the presence of potassium sulphate.
Potassium Sodium Chromate, KNaCrO₄ = 279, is also used in colour making. Its solution is made by adding soda to a solution of potassium bichromate so long as an effervescence of carbonic acid occurs, and until the liquid turns red litmus paper blue; the solution of the double salt is yellow.
Chrome Alum, KCr(SO₄)₂.12H₂O = 499.—This salt occurs in commerce as beautiful violet crystals. It is obtained as a by-product in the manufacture of aniline and anthracene dyes, and may often be bought at lower prices than other chromium salts; 100 parts of water dissolve approximately 20 parts of chrome alum.
Potassium Ferrocyanide, K₄Fe(CN)₆.3H₂O = 422.—The potassium iron cyanogen compounds are made in special works, particularly in the neighbourhood of large towns, by melting potashes with nitrogenous organic substances and iron, washing out the mass and purifying the salt so obtained by recrystallisation. Potassium ferrocyanide (yellow prussiate of potash) forms large transparent crystals of a peculiar soft nature, which dissolve readily in water. It often contains considerable quantities of potassium sulphate, up to 5 per cent., and it is to be noted that the impurity is much the cheaper of the two salts. When barium chloride is added to a solution of the salt, a white precipitate forms if sulphate be present.
The behaviour of yellow prussiate towards iron salts is noteworthy. With ferrous salts, for example green vitriol (copperas), it gives a white precipitate which gradually turns blue in the air; with ferric salts, for example ferric chloride (“nitrate of iron”), it at once gives a blue precipitate.
Potassium Ferricyanide (Red Prussiate of Potash), K₃Fe(CN)₆ = 329, is obtained by passing chlorine through a solution of yellow prussiate until the liquid smells strongly of chlorine and no longer gives a precipitate with a solution of a ferric salt. The solution then contains potassium ferricyanide and chloride. The former is obtained by evaporating and allowing to crystallise.
Pure potassium ferricyanide forms beautiful dark red crystals, which readily dissolve in water. The solution gives a blue precipitate with ferrous salts, but only a brown colouration and no precipitate with ferric salts. Both yellow and red prussiate are used in the preparation of several much used colours, for Prussian and Chinese blues, and several others. All cyanogen compounds, with the exception of yellow prussiate, are extremely poisonous. The following table gives the solubility of potassium ferricyanide at different temperatures:—
| 100 Parts of Water dissolve Parts of Salt. | Temperature. | Specific Gravity of Solution. |
|---|---|---|
| °C. | ||
| 33 | 4.44 | 1.151 |
| 36 | 10.00 | 1.164 |
| 40.8 | 15.50 | 1.178 |
| 58.8 | 37.80 | 1.225 |
| 77.5 | 100.00 | 1.250 |
| 82.6 | 104.40 | 1.265 |
Sodium Salts.—In chemical properties the sodium salts are very similar to the potassium salts, and, being cheaper, they are generally used in place of the latter.
Sodium Carbonate (Soda Crystals), Na₂CO₃.10H₂O = 286, is made in enormous quantities in great works and in a very pure state. It forms large transparent crystals, which effloresce in the air, losing a large quantity of water, and so falling to a white powder. Although this property does not interfere with the use of soda, since it is generally used in solution, yet efflorescence should be as far as possible avoided by keeping the salt in well-closed packages, because effloresced soda dissolves more slowly than crystallised, since it has to combine with water before it can enter into solution.
In the retail trade a form of soda is found which is adulterated with very large quantities of Glauber’s salt. This is recognised by the different form of the crystals. Manufacturers sell soda stating its strength. The colour maker should only buy with this guarantee.
Sodium Hydroxide (Sodium Hydrate, Caustic Soda), NaOH = 40, comes into commerce in the form of hard masses, the use of which would be very convenient for the colour maker if it were not often very impure. Thus it is better to prepare a solution oneself, which is accomplished in the manner given above for caustic potash.
Caustic soda and caustic potash have similar properties; they have a corrosive action on the skin, readily unite with carbonic acid from the air, and separate the heavy metals from their solutions in the form of hydrated oxides.
The strength of caustic soda solutions is given in the following table:—
| Specific Gravity | Caustic Soda per cent. | Specific Gravity | Caustic Soda per cent. |
|---|---|---|---|
| 2·00 | 77·8 | 1·40 | 29·0 |
| 1·85 | 63·6 | 1·36 | 26·0 |
| 1·72 | 53·8 | 1·32 | 23·0 |
| 1·63 | 46·6 | 1·29 | 19·0 |
| 1·56 | 41·2 | 1·23 | 16·0 |
| 1·50 | 36·8 | 1·18 | 13·0 |
| 1·47 | 34·0 | 1·12 | 9·0 |
| 1·44 | 31·0 | 1·06 | 4·7 |
Besides soda and caustic soda, few soda salts are used in colour making. However, sodium nitrate (Chili saltpetre), NaNO₃, is frequently used instead of ordinary saltpetre, from which it differs in being deliquescent.
Sodium Thiosulphate (Hyposulphite), Na₂S₂O₃.5H₂O = 248, is a common article of commerce, being much used by photographers; it is used in a few cases in colour making. It forms large crystals with a somewhat bitter taste, permanent in air and readily soluble in water.
Sodium Chloride (Common Salt), NaCl = 58·5, which has a little application in colour making, is sufficiently pure in the form in which it is generally used for household purposes.