Sodium Hydroxide

Sodium hydroxide (chemical formula: NaOH) is a chemical compound composed of sodium, oxygen, and hydrogen. It is also known as caustic soda. It is a white, hard solid in the form of flakes or pellets, and dissolves rapidly in water while absorbing moisture from the air (hygroscopic).

NaOH is a strong alkali (base): when mixed with water, it completely dissociates and releases hydroxide ions (OH⁻), raising the pH to very high levels. Dissolution in water is exothermic, for which reason NaOH must be added to water, and not the reverse.

Physical and Chemical Properties

NaOH has a high melting point (above 300 °C) and is highly soluble in water. It reacts with acids to produce salt and water (neutralisation). It also reacts with aluminium and certain other metals, releasing hydrogen gas, which presents a fire and explosion hazard when stored in aluminium containers. When exposed to air, NaOH absorbs carbon dioxide and converts to sodium carbonate.

Production

NaOH is primarily produced by electrolysis of salt water solution (brine, NaCl). When electric current is passed through the solution, chlorine, hydrogen, and sodium hydroxide are obtained. This process is known as the chlor-alkali process. Technologies in current use include the membrane cell and diaphragm cell; the earlier mercury cell method has been phased out in many places for health and environmental reasons.

Uses

NaOH is one of the most widely used industrial chemicals. It is used in: soap and detergent production (saponification, converting fats into soap); paper mills (the kraft process, which removes lignin from wood); oil refining; aluminium production from bauxite (the Bayer process); water treatment to adjust pH; and strong cleaning agents for pipe flushing. It is also used in food processing (such as for bagels and olives) and in laboratory testing.

Hazards and Precautions

NaOH is corrosive: it can burn skin, eyes, and the digestive tract, and in particular destroys eye tissue causing permanent blindness. Its powder and aerosol are harmful to the respiratory system. Handling requires eye protection, impermeable gloves, and adequate ventilation. Emergency treatment consists of rinsing with large amounts of water for a prolonged period; neutralisation with acid is not recommended without medical supervision.