Element Potassium K, Alkali Metal
|While the knowledge of some of the potassium compounds can be followed back almost to the most remote monuments of culture, the characterisation of the potassium compounds as derivatives of a special element was first effected towards the end of the eighteenth century by Marggraf. On account of the preparation of potassium carbonate from cream of tartar, which is deposited in the barrels in the fermentation of wine, that compound received the name of vegetable alkali, in contradistinction to mineral alkali, sodium carbonate or soda. Although potassium hydroxide or caustic potash could not be decomposed, it was long felt that it was no simple substance; but the actual proof that a metallic element formed the basis of the potassium compounds was first given in 1807 by H. Davy, who decomposed potassium hydroxide by an electric current derived from a voltaic battery, which had just then been invented. |
After it had been obtained in this way, the method of preparing it by purely chemical means was soon discovered, a method which was for long the only one employed. The most important of these reactions is the heating of potassium carbonate with charcoal; carbon monoxide and metallic potassium are formed, the latter of which volatilises and is condensed under rock oil - K2CO3 + 2C = 2K + 3CO. Quite recently the electrical method of preparing it has been again adopted, since the necessary electrical energy can now be cheaply generated in any desired amount.
Potassium is a silver-white metal which melts at 62°, and which, even at the room temperature, is so soft that it can be kneaded and easily cut with a knife. At 720° it volatilises; the vapour is blue- green in colour. The colour can be rendered visible by heating the metal in a glass tube which is filled with a gas or vapour free from oxygen; the phenomenon, however, is visible only for a moment, since the potassium vapour quickly attacks the glass, which thereby becomes covered with a black coating of liberated silicon.
Potassium combines with very great readiness with oxygen, so that it decomposes almost all substances which contain that element. In the air, therefore, under the joint action of the water vapour, it immediately becomes tarnished, owing to the formation of a layer of hydroxide, and its metallic lustre can be observed only immediately after a fresh surface has been made. If it is enclosed in a tube which is exhausted or filled with hydrogen, and then fused, the metallic surface can thus be rendered visible and permanently preserved.
On account of this property, potassium must be kept in such a way that oxygen has no access to it. In large quantities it is preserved in a soldered tin; smaller quantities are kept under rock oil, since this liquid does not contain oxygen. It, however, absorbs gaseous oxygen, and the potassium kept under rock oil soon becomes covered with a grey-brown crust which, however, only slowly becomes thicker and protects the metal fairly well.
It is very remarkable that in dry oxygen potassium is not (i.e. is only very slowly) oxidised, whereas the smallest amount of water immediately produces a rapid reaction. Such behaviour, however, in spite of its great generality, must not be regarded as universal, for instances of oxidation processes have been proved (e.g. the combination of nitric oxide with oxygen) where the reaction takes place with undiminished velocity, even between the very carefully dried substances.
Concerning the determination of the combining weight of potassium, the essential points have already been given under chlorine. It amounts to K = 39.15.
|Potassium is present in many rocks in the form of silicates, such as orthoclase, K2O,Al2O3,6SiO2; mica, K2O,3Al2O3,4SiO2; and leucite, K2O,Al2O3,4SiO2. It is a constituent of the waters of mineral springs and of the ocean. The deposits at Stassfurt in North Germany, formed by evaporation of large volumes of land-locked sea-water, contain enormous quantities of carnallite, a double chloride of potassium and magnesium of the formula KCl,MgCl2,6H2O; kainite, a mixture of potassium and magnesium sulphates with potassium and sodium chlorides; and sylvine or potassium chloride. From these and other similar deposits in the same locality most of the potassium salts of commerce were obtained prior to August 1914. The outbreak of the great European War stopped the supply, and the ensuing dearth of potassium salts gave a valuable stimulus to research on their production from orthoclase, which contains about 2.4 per cent, of K2O. Potassium salts are a constituent of the soil, and are present in large quantities in plants. In these vegetable products the metal is usually combined with organic acids such as oxalic, tartaric, and malic.|