POTAS′SIUM. K. The metallic base of potash. It was discovered, in 1807, by Sir H. Davy, who obtained it by submitting moistened potassium hydrate, under a film of naphtha, to the action of a powerful voltaic current.
It has since been procured by easier methods, of which the following, invented by Brunner, is the best.
Prep. An intimate mixture of carbonate of potassium and charcoal is prepared by calcining, in a covered iron pot, the crude tartar of commerce; when cold, it is rubbed to powder, mixed with 1-10th part of charcoal in small lumps, and quickly transferred into a retort of stout hammered iron; the latter may be one of the iron bottles in which quicksilver is imported, a short and somewhat wide iron tube having been fitted to the aperture; the retort, thus charged, is placed upon its side, in a furnace so constructed that the flame of a very strong fire, preferably fed with dry wood, may wrap round it, and maintain every part of it at a very high and uniform degree of heat. A copper receiver, divided in the centre by a diaphragm, is next connected to the iron pipe, and kept cool by the application of ice, whilst the receiver itself is partly filled with mineral naphtha, to preserve the newly formed potassium as it distils over. The arrangement of the apparatus being completed, the fire is gradually raised until the requisite temperature, which is that of full whiteness, is reached, when decomposition of the alkali by the charcoal commences, carbonic acid gas is abundantly disengaged, and potassium distils over, and falls in large drops into the liquid. To render the product absolutely pure, it is redistilled in an iron or green-glass retort, into which some naphtha has been put, that its vapour may expel the air, and prevent the oxidation of the metal. The pieces of charcoal are introduced for the purpose of absorbing the melted carbonate of potassium and preventing its separation from the finely divided carbonaceous matter. Prod. 3% to 4% of the weight of tartar acted on; 1 lb. yielded 280 gr.
Prop., &c. Pure potassium is a brilliant white metal, with a high lustre; at the common temperature of the air it is soft, and may be easily cut with a knife, but at 32° Fahr. it is brittle and crystalline; it melts completely at 136° Fahr., and in close vessels distils unaltered at a low red heat. Sp. gr. ·865. Its most remarkable property is its affinity for oxygen, which is so great that it takes it from most other substances containing it. Exposed to the air, its surface is instantly tarnished, and quickly becomes covered with a crust of oxide or hydrate. It inflames spontaneously when thrown on water, and burns with a beautiful purple or purple-red flame, yielding a pure alkaline solution. It can only be preserved in naphtha, rock oil, or some other fluid hydrocarbon.
The salts of potassium are all soluble in water, the tartrate, periodate, and fluosilicate being the least so; they are usually colourless, unless the acid be coloured, crystallise readily, and form numerous double compounds. They can be recognised as follows:
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Sulphuretted hydrogen, sulphide of ammonium, and carbonate of ammonium, do not affect them. A solution of tartaric acid, added in excess, to moderately strong neutral or alkaline solutions of potassium salts, gives a quickly subsiding, gritty or crystalline, white precipitate, which is redissolved on heating the liquid, and again separates as it cools; and is also soluble in aqueous solutions containing free alkali, or free mineral acids. Platinic chloride produces, in neutral and acid solutions, a yellow crystalline precipitate. Alkaline solutions require to be first slightly acidulated with hydrochloric acid. The separation of the precipitate here, as well as that produced by tartaric acid, is promoted by violent agitation and friction against the sides of the vessel, and the delicacy of both is increased by the addition of some alcohol. When converted into carbonate by igniting with excess of carbonate of ammonium and alcohol, and treated with sulphuretted hydrogen solution and nitro-prusside of sodium, gives a splendid violet colour, turning through red to green on standing.
Potassium salts give with sodium periodate and hydro-fluosilicic acid white precipitates soluble in much water.
Heated in the inner flame of the blow-pipe on platinum wire, they impart a violet coloration, masked, however, by a mere trace of sodium salts.
Potassium, Acetate of. KC2H3O2. Syn. Acetate of potash, Potassic acetate; Potassæ acetas (B. P., Ph. L., E., D.). Prep. (Ph. L.) Acetic acid, 26 fl. oz.; distilled water, 12 fl. oz.; mix, and add, gradually, carbonate of potassium, 1 lb., or q. s. to saturate the acid; next, filter the solution, and evaporate it by the heat of a sand bath, gradually applied, until the salt is dried.