Potassium has a great affinity for oxygen. It will take that element out of almost any combination. Thrown on to the surface of water it rapidly bcaroines converted into oxide of potassium, and generates sufficicut heat to ignite the hydrogen that is set free, the latter carrying o®' a little vapour of potassium and burning with a beau tiful violet flame. At an elevated temperature potassium decomposes carbonic oxide, carbonic acid, or any gas that contains oxygeu. In chemical research potassium is frequently used for removing the last traces of oxygen from a hydrocarbon liquid, by rectifying the Latter over a small quantity of the metal. From what has been said it is evident that potassium must always be kept in purified rock oil, or naphtha, or some liquid that contains no oxygen.
The equivalent of potassium is 33•30.
Potassium and oxygen form three compounds :— Potassium and phosphorus may be fused together under rock oil, with production of a chocolate-brown phosphide of potassium. Water decomposes it into bypophosphiteof potash and into gaseous and solid pdiosphuretted hydrogen.
Potassints and sulphur form five different compounds, namely :— I. The first is formed on heating sulphate of potash in a current of hydrofien. Its colour is yellowish-brown, it is very deliquescent, and cauterises the skin. 2. The second is formed on passing excess of sulphuretted hydrogen through an alcoholic solution of potash, evapo rating in the air till it becomes turbid, and then exposing it in vacuo. It is fusible, and of an orange colour. 3. Tersulphido of potassium is obtained on passing bisulphide of carbon vapour over ignited carbonate of potash. 4. The tetraaulphide results when bisulphide of carbon vapour is passel over ignited sulphate of potash. 5. Pentasulphide of potassium in formed when a solution of any of elm preceding sulphides is boiled with excess of sulphur ; or, mixed with other potash salts, when caustic potash is boiled with excess of sulphur ; or when car bonnto of potash is fused with excess of sulphur. Lim- of sulphur is an old preparation, made by fusing together 69 parts of carbonate of potash with 40 of sulphur; it is mainly a mixture of sulphate of potash and tersulphide of potassium. With the exception of the first, all tho above sulphides of potassium yield sulphuretted hydrogen and free sulphur on being decomposed by a dilute acid, thus:— Suboxide of potassium This is formed when potassium is ex posed in a close vessel to a very small quantity of air. It is very unstable, and is probably only a mixture of metal and protoxide.
2. Protoxide of potassium (KO). Potash ; potassa.—This anhydrous oxide is formed when the metal is exposed to perfectly dry air; or when equivalent weights of water and potassium are brought together in a vessel from which air is excluded. It is very deliquescent, and when moistened with water becomes incandescent.
Hydrated oxide of potassium (KO,H0). Potash ; caustic potash.— This is best prepared by dissolving carbonate of potash in ten times its weight of water and then gradually adding half its weight of quick-lime, previously slaked and made into a cream with water ; carbonate of lime deposits, and caustic potash remains in solution. The resulting liquid frequently contains alumina. Solid hydrate of potash is obtained on evaporating the liquor, just mentioned, until tho neidue remains in a state of tranquil fusion ; it was formerly called lapis infermalis. It crystallises iu acute rhombohedra. The following is Dalton's table of the strength and boiling point of caustic potash solutions of different specific gravities:— 1'003.
- 3. Teroride of potassium (KO,) is formed when the metal is burned in excess of oxygen gas, or when caustic potash is fused In contact with air. It is a yellowish brown substance, having a crystalline texture, and readily parts with Its oxygen to combustibles, and even on dissolving 113 water.
Or it may be formed by fusing iodine with excess of iron, decom posing the iodide of iron solution by carbonate of potash, filtering, and evaporating to the crystallising point.
Iodide of potassium crystallises in white cubes, having a cooling somewhat bitter taste. It is very soluble in water, and, if pure, com pletely dissolves in six times its weight of alcohol. It readily dissolves iodine, forming a deep brown solution.
Potassium and bromine form bromide of potassium (KBr). It may be prepared by the same methods as the iodide, substituting bromine for iodine. Its appearance is also similar.
Potassium and chlorine form Clderide of potassium (KCl). Potassium burns in chlorine with even greater brilliancy than in oxygen ; the product is chloride of potassium. This salt may also be formed by dissolving carbonate of potash in hydrochloric acid, and evaporating to the crystallising point. Chloride of potassium is a bye-produet in many pharmaceutical and chemical operations, and, from the facility with which it may be decomposed, is useful as a source of other potash salts.