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Physical Properties

Caustic soda is also known as sodium hydroxide, caustic, and lye. Anhydrous (100%, solid) sodium hydroxide has a chemical formula of NaOH and a molecular weight of 40.00.

Basic Properties of Caustic Soda Solution 50%
Vapor pressure 1.5 mmHg @ 68°F (0.20 kPa @ 20°C)
Boiling point Approximately 293°F (145°C)
Freezing point Approximately 58°F (14°C)
pH 14
Specific gravity 1.52 g/ml @ 68°F (20°C)*

*Based upon pure (salt-free) caustic soda solutions data from International Critical Tables of Numerical Data, Physics, Chemistry and Technology, Volume III, First Edition, page 79. Refer to Density Tables listed in "Additional physical property information" that is provided later in this section for other concentrations and temperatures.

Caustic soda, as a 50% solution, is an odorless and colorless liquid. In all forms, caustic soda is highly corrosive and reactive. Caustic soda solution reacts readily with metals such as aluminum, magnesium, zinc, tin, chromium, bronze, brass, copper, and tantalum. Galvanized (zinc coated) materials should be avoided. Contact with acids, halogenated organics, organic nitro compounds, and glycol should be avoided. It reacts with most animal tissue, including leather, human skin, and eyes. It also reacts readily with various reducing sugars (i.e., fructose, galactose, maltose, dry whey solids) to produce carbon monoxide.

Upon cooling, the viscosity of the solution increases rapidly as the temperature falls below 65°F (18°C). For additional information, see Viscosity Table for Pure (salt-free) Caustic Soda Solution (16KB PDF)

Stability and Storage Life

Caustic soda solution is a stable product but its storage life is dependent upon the storage conditions. If the caustic is exposed to air, a change in the product quality will be seen over time, since the caustic soda solution will pick up carbon dioxide to form sodium carbonate (Na2CO3) solids. In addition, iron pick up is common in carbon steel storage vessels or in lined carbon steel storage vessels where the liner has been damaged. Therefore, minimizing its exposure to air and its direct contact with iron containing metals will extend the storage life of caustic soda solution.

Additional Physical Property Information:

Freezing Point Curve for Caustic Soda Solutions (16KB PDF)
Densities of Pure (salt-free) Caustic Soda Solution at Various Concentrations and Temperatures (16KB PDF)
Densities of Dow 50% Caustic Soda Solution, Commerical and Membrane Grades, at 30°C (86°F) (16KB PDF)
Densities of Pure (salt-free) Caustic Soda Solution (volumetric units) at 20°C (68° F) (21KB PDF)
Enthalpy-Concentration Table for Pure (salt-free) Caustic Soda Solution (17KB PDF)
pH Versus Concentration Table for Caustic Soda Solution (16KB PDF)
Dilution Temperature Curves for 50% Caustic Soda Solution at Three Initial Temperatures (22KB PDF)