Sodium hydroxide (NaOH), also known as caustic soda, is a caustic metallic base. It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner.
Of historic interest is the Leblanc process, which produced sodium carbonate, followed by roasting to create carbon dioxide and sodium oxide, which readily absorbs water to create sodium hydroxide. This method is still occasionally used. It helped establish sodium hydroxide as an important commodity chemical. The Leblanc process was superseded by the Solvay process in the late 19th century.
- Na2CO3 → Na2O + CO2
- Na2O + H2O → 2 NaOH
Sodium hydroxide may be formed by the metathesis reaction between calcium hydroxide (also known as lime) and sodium carbonate (also known as soda ash):
- Ca(OH)2 + Na2CO3 → CaCO3 + 2 NaOH
Sodium hydroxide is produced (along with chlorine and hydrogen) via the chloralkali process. This involves the electrolysis of an aqueous solution of sodium chloride. The sodium hydroxide builds up at the cathode, where water is reduced to hydrogen gas and hydroxide ion:
- 2 Na+ + 2 H2O + 2 e– → H2 + 2 NaOH
More accurately:
- 2 Na+Cl– + 2 H2O + 2 e– → H2 + 2 Cl– + 2 NaOH
The Cl– ions are oxidized to chlorine gas at the anode.
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