Potassium carbonate, K₂CO₃

The Potassium carbonate, K₂CO₃ is a constituent of the ashes of wood and other vegetable products, and is obtained by extracting with water, the purified product being known as "pearl-ash." The origin of the world " potash " has occasioned some controversy. It has been attributed by some to the name of the chemist Pott (1692-1777), but it appears to have been in use before his birth. Three other explanations have been advanced: the use of pots, and later boilers, in the concentration of the aqueous extract of the crude ashes; the employment of pots in the incineration of the wood; and the fact that ashes collected under the cooking-pots on the hearth fires of the Middle Ages. The first of these three explanations seems to be the most plausible.

Modern manufacturing methods depend on the conversion of potassium chloride into carbonate by electrolysis; by the Le Blanc process; or by the action of magnesium carbonate, carbon dioxide, and water:

3MgCO₃ + 2KCl + CO₂ + H₂O = 2MgKH(CO₃)₂ + MgCl₂.

The precipitated double carbonate is decomposed by hot water:

2MgKH(CO₃)₂ = 2MgCO₃ + K₂CO₃ + CO₂ + H₂O.

Potassium carbonate is also manufactured from the spent wash of the spirit-distiller, and from the residual liquor of the wool-scourer.

Potassium carbonate is a white solid. Its melting-point is given by various experimenters as 878.6° C., 880° C., 885° C., 873.1° C., 887.5° C., 891° C., 894° C., 897.3° C., 897.7° C., 900° C. For the density the mean value is given as 2.29; a more recent determination gives 2.3312 at 17° C. Its specific heat is 0.206 between 17° and 47° C., and 0.2162 between 23° and 99° C. At 970° C. the vapour-pressure is 1.68 mm.; and at 1130° C. it is 5.0 mm. The heat of formation from the elements is recorded as 275.37 Cal., 278.8 Cal., and 281.1 Cal. Potassium carbonate exhibits diamagnetism.

Several hydrates have been described, but their constitutions are not definitely settled. At 25° c. the solubility is 113.5 grams per 100 grams of water, and at 130° c. it is 196 grams. The aqueous solution has a strong alkaline reaction, due to hydrolytic dissociation. It forms various primary carbonates by interaction with atmospheric carbon dioxide, and unites with hydrogen peroxide yielding compounds of the formulae K₂CO₃,3H₂O₂ and K₂CO₃,2H₂O₂,½H₂O.