|The metal was first isolated by Davy by electrolysis of the hydroxide, and shortly afterwards Gay-Lussac and Thenard obtained it by the action of iron on the carbonate. Charcoal can be substituted for iron, one process consisting in the decomposition at low red heat of potassium hydrogen tartrate, with formation of a mixture of charcoal and potassium carbonate, the reduction to metal being subsequently effected at a higher temperature: |
K2CO3 + 2C = 2K + 8CO.
If the temperature is too low, there is danger of the liberated potassium combining with carbon monoxide to form a highly explosive compound, potassium carbonyl, C6(OK)6. Another modification of the method consists in heating a mixture of tar and potassium carbonate or hydroxide at dull redness, and subsequently raising the temperature of small portions of the carbonaceous mass to bright redness, the molten metal being tapped off. The fused hydroxide can also be allowed to flow over heated charcoal, and the metal distilled off.
When equivalent weights of sodium and potassium hydroxide are mixed, and fused in absence of air, sodium monoxide is formed, hydrogen evolved, and potassium distils at a temperature of about 670° C. The metal can be condensed, and the process is claimed to be applicable to its manufacture.
Latterly the production of potassium by the electrolytic process has become of industrial importance, partly on account of the purity of the product, and partly because the attendant risk is much less than with the chemical methods. The ordinary Castner process for the isolation of sodium is inapplicable to potassium, Lorenz having shown that part of the potassium is dissolved in the molten state by the fused hydroxide, and part is vaporized and after condensation in minute drops is diffused throughout the liquid mass. The effect is that the liberated metal tends to combine with the anion at the anode. The difficulty is avoided by surrounding the iron-wire cathode with a cylinder of magnesite to prevent diffusion of the metal to the anode, by employing as low a temperature as possible, and by excluding air. An anode of sheet iron is immersed in the fused hydroxide outside the magnesite chamber. As substitutes for the hydroxide, potassium nitrate, and also potassium chloride with an admixture of fluoride to lower the melting-point, are employed. With the nitrate the cathode is an aluminium vessel.