Sodium Hydroxide (Caustic Soda): Uses, Grades and Safe Handling

Sodium hydroxide, known as caustic soda or lye, is a strong inorganic base with the formula NaOH and CAS number 1310-73-2. It is among the largest-volume industrial chemicals, produced mainly through the chlor-alkali process alongside chlorine. For traders it sells as solid flakes, pearls or prills, and as aqueous solutions called liquor, most often at 30 to 50 percent concentration.

Common uses

• Soap and detergent manufacture, where it saponifies fats and oils.
• Pulp and paper, to separate lignin from wood fibre.
• Alumina refining, where the Bayer process dissolves bauxite in hot caustic solution.
• Water treatment, for pH adjustment and neutralising acidic streams.
• Chemical synthesis, including sodium hypochlorite, sodium phosphates and many sodium salts.
• Food processing, such as peeling fruit and curing olives, where food-grade material applies.
• Drain cleaners, oven cleaners and industrial cleaning-in-place systems.
• Textile processing, including mercerising cotton.

Hazard classification

Sodium hydroxide is classified as corrosive to skin (category 1A, H314: causes severe skin burns and eye damage) and commonly as corrosive to metals (H290). The signal word is Danger and the pictogram is the corrosion symbol (GHS05). Diluted solutions can fall into lower categories because the harmonised entry carries specific concentration limits, which is exactly why a solution of 2 percent caustic is labelled differently from 50 percent liquor. Using the concentration-based rules correctly is part of classifying a mixture under GHS.

The practical dangers are chemical burns that may not hurt immediately, permanent eye damage from even small splashes, and a strong exothermic reaction when solid caustic is dissolved in water. Dissolving always means adding the solid slowly to cold water with stirring, never water onto solid caustic, because localised boiling can spit corrosive liquid.

Handling and PPE

Use chemical splash goggles and a face shield, gloves rated for caustic (nitrile or neoprene for short contact, thicker rubber for immersion), a chemical-resistant apron and closed footwear. Provide an eyewash station and safety shower close to the work area. Your Section 8 should state these controls clearly, and Section 4 first aid should specify immediate flushing with water for at least 15 minutes and medical attention for any eye contact.

Storage and incompatibilities

Store in closed containers made of compatible materials such as polyethylene or carbon steel, and keep it away from acids, which cause violent reactions, and from aluminium, zinc and tin, which it attacks and which can release flammable hydrogen gas. Solid caustic absorbs moisture and carbon dioxide from the air, so keep containers tightly closed. See our storage compatibility guide for how to separate hazard groups in a warehouse.

Grades and specifications

Buyers specify caustic soda by assay (for example 98 percent minimum for flakes, or 48 to 50 percent for liquor), sodium carbonate content, chloride, iron and sodium chlorate. Membrane-cell grade has lower chloride than diaphragm-cell grade, which matters for some processes. State the process route and grade on your specification and certificate of analysis so the buyer knows what they are getting.

What your SDS and label need

Use the exact CAS number and EC number, the right concentration in Section 3 for liquors, corrosion pictogram and signal word on the label, and transport information in Section 14: caustic soda is a regulated dangerous good in transport, with entries for solid and for solution. Check the current UN numbers and packing group for your shipment. You can view the classification we hold on our sodium hydroxide page.