Alkali

Alkali, a substance that produces hydroxide ions, OH^—, when dissolved in water. The term originates from the Arabic al-qili, “ashes of the saltwort plant”, where it meant the hydroxides and carbonates of potassium and sodium, leached from plant ashes. The term is now generally applied to any base that is soluble in water: the hydroxides and carbonates of ammonium (NH₄) and the other alkali metals; and the hydroxides of calcium, strontium, and barium. The carbonates and ammonium hydroxide give only moderate concentrations of hydroxide ions and are termed mild alkalis. The hydroxides of sodium and potassium, however, produce hydroxide ions in high enough concentration to destroy flesh; for this reason they are called caustic alkalis. Solutions of alkalis turn red litmus blue, neutralize acids, are electrical conductors, and feel slippery. This slippery feeling is due to the saponification of fats in the skin as they react with the alkali to form soap.

Sodium hydroxide (caustic soda), NaOH, is an important commercial product, used in making soap, rayon, and cellophane; in processing paper pulp; in petroleum refining; and in the manufacture of many other chemical products. Sodium hydroxide is manufactured principally by electrolysis of a common salt solution, with chlorine and hydrogen as important by-products.

Sodium carbonate, Na₂CO₃, one of the mild alkalis, is manufactured from natural deposits or made from common salt solutions by the Solvay process. It is used in the manufacture of glass and as a cleaning agent and water softener.