Potassium

Potassium (sym. K, equiv. 39) is one of the alkaline metals. The letter K is selected as its symbol, as being the first letter of Kali, the Arabic word for potash, the letter P being already taken as the symbol for phosphorus. The following are the chief characters of this metal. It is of a bluish-white colour, and presents a strong metallic lustre. It melts at 146.5° (62.5° C.), and at a red heat is converted into vapour. Its affinity for oxygen is so great that on exposure to moist air it immediately becomes covered with a film of oxide, and hence it must be kept below the surface of naphtha. When heated it burns with a violet flame. Its intense affinity for oxygen is well shown by throwing it into water, on which, from its low specific gravity, it floats. The metal abstracts oxygen from the water, and forms oxide of potassium (potash); while the liberated hydrogen carries off a small portion of the volatilised potassium, and, taking fire from the heat evolved by the energetic chemical action, burns with a brilliant violet flame. The experiment is a very beautiful one, the burning metal swimming about rapidly on the water, and finally disappearing with an explosion of steam, when the globule of melted potash becomes sufficiently cool to come in contact with the water.

Potassium does not occur in the native state, and can only be obtained by the reduction of its oxide, potash. In 1807 Davy prepared it by decomposing its hydrated oxide (potash) by means of a voltaic current, but this process is not applicable on the large scale. It is now usually manufactured by distilling a mixture of carbonate of potash and charcoal in an iron retort.

If proper proportions are taken, the mixture is wholly converted into carbonic oxide and potassium, as is shown in the equation:

Carbonate of Potash.      Carbonic Oxide.

Potassium forms two compounds with oxygen, viz. a protoxide, $K\_2O$, which constitutes potash, and is strongly basic, and a peroxide, $K\_2O\_4$. Of these the former is the only important one.

Potash can be procured in the anhydrous form by heating thin slices of the metal in air perfectly free from moisture or carbonic acid. It is white, very deliquescent, and caustic. When moistened with water it becomes incandescent, and the water cannot be expelled by any degree of heat. A far more important substance is the Hydrate of Potash or Caustic Potash ($KOH\; =\; K\_2O\; \backslash cdot\; H\_2O$). This is commonly prepared by dissolving carbonate of potash in ten times its weight of water, and gradually adding to the boiling solution a quantity of slaked lime, equal in weight to half the carbonate of potash used. The resulting compounds are carbonate of lime, which falls as a precipitate, and hydrate of potash, which remains in solution; the changes being expressed by the equation:

The clear supernatant fluid is removed by decantation, or by means of a siphon, into a clean silver or iron basin, and is rapidly evaporated till it flows tranquilly like oil; it is then either cast into cylinders in metallic moulds, or is poured upon a cold slab, and solidifies on cooling. As so obtained it is very impure, but by solution in alcohol and evaporation a very pure article is produced.

Hydrated potash, on solidifying after fusion, occurs as a hard, grayish-white, opaque body, with a crystalline fracture, which may be readily again fused into a colourless oily fluid, but which on y volatilises at a very high temperature. It is soluble in about half its weight either of water or of alcohol, and rapidly absorbs both carbonic acid and moisture from the atmosphere. It acts as a powerful caustic, and quickly destroys both animal and vegetable tissues, and hence its solutions can only be filtered through asbestos or pounded glass or sand. Its affinities are so powerful that few vessels are capable of resisting its influence. Its solution must be preserved in glass bottles into the composition of which no oxide of lead enters, as it has the property of dissolving this oxide out of the glass. Vessels containing silica (porcelain, earthenware, &c.) are decomposed, and platinum itself is oxidised when heated in contact with it.

The salts which potash forms with acids are for the most part readily soluble in water, and colourless, unless (as, for example, in permanganate of potash) the acid is coloured. Most of them are crystallisable, and they all communicate a violet tint, characteristic of potash, to the flame of spirit of wine and to that of the blowpipe. Many of them occur in animals and vegetables, and the ashes of plants contain them in large quantity.

Carbonate of Potash, $K\_2CO\_3$, is obtained by burning plants in dry pits, dissolving the ashes in water, evaporating till the sulphates, chlorides, &c. separate in crystals, and then boiling the mother liquid to dryness in iron pots. The quantity of pure carbonate of potash contained in it is liable to great variation, and for pharmaceutical purposes it must be dissolved in water and crystallised, the crystals containing about 20 per cent. of water. Carbonate of potash is extremely deliquescent, and is soluble in less than its own weight of water, but is insoluble in alcohol. It has an acrid, alkaline taste, and its reaction upon test-paper is strongly alkaline. It is a compound of great importance, both as a chemical reagent and as entering largely into the preparation of most of the other compounds of potash, and into the manufacture of soap and glass. The commercial carbonate is often called Pearl Ashes. Bicarbonate of Potash, $KHCO\_3$, is obtained in white rhombic prisms, by passing a current of carbonic acid gas through a strong solution of carbonate of potash. These crystals are permanent in the air, but are decomposed by heat; water and carbonic acid being evolved, and the simple carbonate left. This salt is much less soluble than the carbonate, requiring four parts of cold water for its solution, which is nearly neutral to test-paper, and has a much milder taste than the preceding salt. It is employed as an antacid in medicine. The Sulphate, $K\_2SO\_4$, and Bisulphate, $KHSO\_4$, may be prepared by treating potash with sulphuric acid. Nitrate of Potash has been already described under the head NITRE. Chlorate of Potash, $KClO\_3$, occurs in white rhomboidal tablets of a pearly lustre. It has a cooling taste like that of nitre. It fuses at a gentle heat without decomposition, but on increasing the heat it gradually gives off all its oxygen, and is converted into chloride of potassium, according to the equation :

It is not very soluble, as it requires for solution 16 parts of cold and 1.7 parts of boiling water. It even surpasses nitrate of potash as an oxidising agent; and if combustible substances, such as carbon, sulphur, or phosphorus, be heated or forcibly rubbed with it, a detonation or explosion occurs. This salt is employed in the manufacture of Matches (q.v.), in certain operations in calico-printing, and for filling the friction-tubes employed for firing cannon : the best mixture for these tubes consisting of 2 parts of this salt, 2 of sulphide of antimony, and 1 of powdered glass. A mixture known as White Gunpowder, consisting of chlorate of potash, dried ferrocyanide of potassium, and sugar, has been employed for blasting purposes, but its preparation is accompanied by so much danger that it is seldom used. This salt does not occur as a natural product, but may be obtained along with chloride of potassium by passing a current of chlorine gas through a hot solution of caustic potash. The two salts are easily separated by crystallisation, as the chlorate is comparatively insoluble, and the chloride extremely soluble. Hypochlorite of Potash can only be obtained in solution. Under the title of Eau de Javelle, it is sold as a bleaching agent. It is obtained by passing chlorine gas through a cold dilute solution of carbonate of potash, when chloride of potassium and hypochlorite of potash are formed, from which the chloride may be removed by crystallisation. The Phosphates of Potash, formed by the different varieties of phosphoric acid, are sufficiently noticed in the articles PHOSPHORUS and MANURE. The Silicates of Potash are important compounds in connection with the manufacture of glass; they also enter into the composition of Fuchs's Soluble Glass (see GLASS), or Water-glass, and have been employed as a coating by which the decay of magnesian and other limestones may be prevented. The Chromate and Bichromate of Potash are noticed in the articles CHROMIUM and CALICO-PRINTING. The haloid salts of potassium may be passed over very briefly. The Chloride of Potassium, $KCl$, is obtained in large quantity in the preparation of chlorate of potash, or may be procured by burning potassium in chlorine gas, when the result of the brilliant combustion which takes place is this salt. In its general characters it closely resembles common salt, $NaCl$, except that the former communicates a violet and the latter a yellow tint to the flame of alcohol. It is a constituent of sea-water, of salt marshes, and of many animal and vegetable fluids and tissues. The Bromide and Iodide of Potassium are noticed in the articles BROMINE and IODINE. Fluoride of Potassium, $KF$, possesses the property of corroding glass. There are several sulphides, the most important being the Liver of Sulphur, prepared by fusing together carbonate of potash and sulphur. Besides its use in skin diseases, it is much employed by florists to prevent mildew on roses. The Yellow and the Red Prussiate (or the Ferrocyanide and Ferrieyanide) of Potash are noticed in the article FERRIDCYANOGEN. The Cyanide of Potassium, $KCy$, may be procured by heating potassium in cyanogen gas, when brilliant combustion occurs, and the resulting product is this salt. It may, however, be more cheaply and easily prepared by fusing together 8 parts of ferrocyanide and 3 of carbonate of potassium. This salt forms colourless deliquescent crystals very soluble in water. It exhales an odour of hydrocyanic acid, and is nearly as poisonous as that acid. Its great deoxidising power at a high temperature renders it a valuable agent in many of the finer operations of metallurgy.

The following are the ordinary tests for the potassium compounds : (1) Solution of tartaric acid added in excess to a moderately strong solution of a potassium salt gives after some time a white crystalline precipitate of cream of tartar (see TARTARIC ACID). The result is hastened by stirring or shaking. (2) Solution of bichloride of platinum gives a crystalline yellow precipitate, which is a double salt of bichloride of platinum and chloride of potassium. If not previously acid, the mixture to be tested should be acidulated with hydrochloric acid. (3) The violet tint occurring in the presence of potassium in the outer flame of the blowpipe, or in the flame of spirit, has been already noticed. (4) The spectrum of a flame containing potassium exhibits a characteristic bright line at the extreme limit of the red, and another one at the opposite violet limit of the Spectrum (q.v.).

In medicine the following compounds are used : Caustic Potash, or Hydrate of Potash, KOH, which occurs in hard white pencils. From its power of dissolving the animal tissues, it is sometimes used as a caustic, although its great deliquescence renders it somewhat difficult to localise its action to the desired spot. In bites of venomous serpents, mad dogs, &c. it may be applied with advantage, and it is useful in destroying warts and fungoid growths of various kinds. Solution of Potash, commonly known as Liquor potassæ, is obtained by the process already given for the preparation of hydrate of potash. Liquor potassæ, in combination with a tonic infusion, is of service in cases of dyspepsia which are accompanied with excessive acidity of the stomach, such, for example, as often occur in habitual spirit-drinkers. It is also frequently given with the view of rendering the urine alkaline, or of diminishing its acidity in cases in which that secretion is too acid. The usual dose is ten drops, gradually increased to as much as a fluid drachm. Acetate of Potash, $KC\_2H\_3O\_2$, is obtained by the action of acetic acid on carbonate of potash, and occurs in white foliaceous satiny masses. In its passage through the system it is converted into carbonate, and thus renders the urine alkaline. In small doses, as from a scruple to a drachm, it acts as a diuretic, and is of service in some forms of dropsy. Combined with other potass-salts, it is much given in acute rheumatism. The two carbonates and the sulphurated potash have been already referred to.

The Chlorate of Potash has come much into use as a popular remedy for sore throats. For this purpose it is usually employed in the form of compressed pellets, which are allowed to dissolve slowly in the mouth.