All the metals of this family are monovalent, and replace each other, atom for atom, in chemical compounds.

POTASSIUM.

Occurrence.

Symbol, K. Atomic weight=39.15.

In combination this element is widely distributed in nature. It forms an essential constituent of many of the common silicates and rocks which form the earth's crust. From these rocks, by processes of disintegration, the potassium compounds find their way into the soil, from whence they are absorbed by plants, which can only flourish in a soil that contains compounds of potassium. Most of the potassium found in plants is present in combination with organic acids.

From the vegetable kingdom, potash passes directly into the bodies of animals. The material known as suint, which is the oily perspiration of the sheep, that accumulates in, and is extracted from the wool, consists of the potassium salt of an organic acid (sudoric acid). In the form of chloride and sulphate, potassium is present in sea-water and many mineral springs. As nitrate it is found as a crystallised efflorescence upon the soil, notably in Peru and Chili, where it is associated with sodium nitrate. The largest supplies of potassium compounds are met with in the great saline deposits of Stassfurt, where the element is found as chloride (KCI) in sylvine, as a double chloride of potassium and magnesium (KCl,MgCl2,6H2O) in carnallite, and as a mixed sulphate in kainite (K2SO,,MgSO4, MgCl2,6H,O).

Modes of Formation.-(1.) The method by which Davy first effected the isolation of potassium was by the electrolysis of potassium hydroxide: the method may be illustrated by the experiment represented in Fig. 131. A small quantity of potassium. hydroxide is gently heated in a platinum capsule, which is connected to the positive terminal of a powerful battery. A stout platinum wire, flattened out at one end, is made the negative pole. When this is introduced into the fused potash, a brisk evolution of gas takes place, and minute beads of metallic potassium make their appearance in the liquid and upon the negative electrode, some of which ignite upon the surface. The decomposition takes place according to the equation—

(2.) Potassium may also be obtained by allowing melted potassium hydroxide to pass over iron turnings heated to whiteness, when the magnetic oxide of iron is formed

4KHO+3Fe Fe3O4+2H2+2Kg.

This is known as Gay-Lussac and Thénard's method.

(3.) The method devised by Brunner, and modified by Wöhler,

Deville, and others, consisted in heating to whiteness an intimate mixture of potassium carbonate and carbon. This mixture was obtained by first igniting in a covered iron pot crude tartar (hydrogen potassium tartrate, or cream of tartar), which was thereby decomposed as indicated by the equation

2HKC,H,O=K,CO2+3C+5H2O+4CO.

The charred mass was then introduced into an iron retort (P, Fig. 132), and strongly heated in the furnace, when the potassium carbonate was reduced by the carbon, as follows

K2CO2+2C=3CO+K1.

As soon as the potassium began to be formed, which was readily

ascertained by the green appearance of the vapour, seen on looking in at the open mouth of the retort, the condenser was attached. This consisted of a flat, shallow iron tray, 6 (Fig. 133), upon which was fitted the cover a, two portions being clamped together. The

a

FIG. 133.

object of this special form of condenser is to cool the potassium as rapidly as possible, for it is found that unless the metal is quickly cooled, it combines with the carbon monoxide, forming a highly explosive compound (believed to have the composition Ke(CO)6). By the use of this form of condensing apparatus the formation of this compound is reduced to a minimum, but not entirely prevented.

(4.) A more recent method by which potassium (and sodium)

is prepared on a manufacturing scale, was devised by Castner

(1886). In this process potassium hydroxide is strongly heated with a carbide of iron, having approximately the composition CFe (This compound is obtained by heating a mixture of pitch and finely divided iron.)

The potassium hydroxide, with the powdered carbide of iron, is introduced into large egg-shaped retorts, one of which is represented in Fig. 134. These retorts are placed upon hydraulic lifts, so that they can be lowered away from their covers, to the ground

level, in order to be discharged at the end of the distillation. The retorts are heated by gaseous fuel, and the metal, as it distils, is passed into long narrow cast-iron condensers, from which it drops into iron pots, and is protected from oxidation by mineral oil. The reaction which takes place may be represented by the equation

6KHO+2C=2K,CO,+3H,+2K.

By avoiding any excess of carbon, no carbon monoxide is produced, and hence there is no formation of the explosive compound of this gas with potassium.

(5.) At the present time potassium is manufactured almost exclusively by electrolytic methods, as described under sodium.

Properties.-Potassium is a lustrous, white metal, which at ordinary temperatures is sufficiently soft to be moulded between the fingers; at o° it is brittle, and shows a crystalline fracture. The metal is readily crystallised by melting a quantity of it in a vacuous tube, and when it has partially solidified, pouring the still liquid portion to the other end of the tube. Potassium melts at 62.5°, and when boiled gives an emerald-green vapour. The metal is rapidly acted on by ordinary air, its freshly cut surface becoming instantly covered with a film of oxide, which, by absorption of atmospheric moisture and carbon dioxide, passes first into the hydroxide and finally into the carbonate. Potassium is therefore usually preserved beneath naphtha, or some other liquid devoid of oxygen.

When potassium is volatilised in a vacuous tube, the thin film of metal which condenses upon the cool portion of the tube is seen to possess a rich violet-blue colour, when viewed by transmitted light. The density of potassium vapour is about 20(Dewar and Scott), showing that in the vaporous condition the molecules are monatomic.

Potassium dissolves in liquefied ammonia, forming a deep indigo solution (page 276). When potassium is thrown upon water, that liquid is decomposed with sufficient energy to cause the ignition of the liberated hydrogen (page 172). When heated in carbon dioxide, potassium takes fire, forming potassium carbonate and carbon (page 304). Heated in carbon monoxide, it forms the explosive compound already mentioned. Potassium takes fire spontaneously in contact with the halogens, forming the haloid compounds of the metal. When heated in hydrogen, it absorbs the gas, forming a brittle lustrous substance, which inflames spontaneously in the air. This compound has the composition KH.. When heated in nitric oxide, potassium burns, forming a mixture of potassium nitrate and nitrite (Holt and Sims).

Oxides of Potassium.—When potassium is heated in ordinary air, it takes fire and burns, giving rise to a mixture of the oxides of the metal. Perfectly dry air or oxygen is without action upon potassium.

Potassium Peroxide, KO4, may be obtained by melting potassium in an atmosphere of nitrogen, and gradually displacing the nitrogen by moderately dry oxygen. It is also produced by heating potassium in nitrous oxide.