Sodium Hydroxide (Caustic Soda, Lye, Flake/Pearl/Liquid NaOH)

I. Physical Properties

1. Appearance: Pure substance is a white solid, available in flake or granular form; industrial liquid caustic soda is a colourless viscous liquid.
2. Strong hygroscopicity: Rapidly absorbs moisture from air upon exposure, leading to caking.
3. Heat release on dissolution: Dissolving in water releases a large amount of heat; adding water to solid caustic soda causes boiling and splattering. Soluble in methanol and glycerol, insoluble in acetone.
4. Susceptibility to degradation: Absorbs CO₂ from air, forming sodium carbonate, which reduces alkalinity and renders it ineffective.
5. High freezing point: High-concentration liquid caustic soda tends to crystallise at low temperatures, potentially blocking pipelines.

II. Chemical Properties

1. Strongly alkaline: Aqueous solutions can reach pH above 13; a monobasic strong base capable of neutralising all inorganic and organic acids.
2. Reaction with amphoteric metals: Reacts with aluminium, zinc, and lead at room temperature, releasing flammable and explosive hydrogen gas.
3. Saponification: Undergoes saponification with animal and vegetable fats and oils, breaking down grease—a core principle in degreasing and soap manufacturing.
4. Corrosive to silicates: Prolonged contact with glass or ceramics corrodes silicon dioxide; therefore, it cannot be stored long-term in glass-stoppered reagent bottles.
5. Salt metathesis: Precipitates heavy metal ions such as copper, nickel, zinc, and iron—commonly used in wastewater treatment.
6. Non-flammable: Does not burn or support combustion.

III. Hazard Characteristics

1. Strongly corrosive (Class 8 corrosive substance): Contact with skin, eyes, or respiratory tract causes severe and irreversible burns; pain sensation is delayed, and initial injury may go unnoticed.
2. Exothermic hazard: Neutralisation with acids and dissolution in water both generate intense heat, which may cause liquid splashing.
3. Incompatibility: Must not be stored or mixed with hydrochloric acid, sulfuric acid, sodium sulfide, hydrogen peroxide, sodium persulfate, or aluminium powder, as toxic or flammable gases may be produced.
4. Environmental hazard: Discharge of highly alkaline solutions into water bodies significantly raises pH, killing aquatic organisms; classified as hazardous wastewater.

IV. Detailed Industry Applications

[1. Wastewater Treatment]

1. pH adjustment: Neutralises acidic wastewater (from electroplating, pickling, and chemical processes).
2. Heavy metal precipitation: Used with sodium sulfide and flocculants to precipitate copper, zinc, nickel, and other heavy metals.
3. Demulsification: Breaks oil-in-water emulsions in oily wastewater and cutting fluid effluents.
4. Sludge dewatering: Adjusts sludge alkalinity to improve flocculation performance.

[2. Chemical Industry]

1. Basic chemical feedstock: Production of soaps, stearic acid, and glycerol.
2. Pulp and paper: Pulping and cooking to decompose wood lignin; chemical recovery in black liquor processing.
3. Synthetic detergents: Synthesis of surfactants such as sodium dodecylbenzenesulphonate.
4. Organic synthesis: Used as a reagent for saponification, hydrolysis, and neutralisation reactions.

[3. Textile and Dyeing]

1. Mercerisation of cotton fabrics to enhance lustre and dye uptake.
2. Desizing and scouring: Removal of sizing agents, oils, and impurities from textiles.
3. pH adjustment of dye baths.

[4. Metallurgy / Electroplating / Surface Treatment]

1. Alumina extraction (Bayer process for dissolving alumina from bauxite).
2. Metal degreasing: Alkaline degreasing of steel and aluminium workpieces.
3. pH regulator for electroplating bath solutions; stripping of defective coatings.

[5. Food Processing (Food-Grade Sodium Hydroxide)]

Used for cleaning food equipment, peeling fruits and vegetables (e.g., peaches, potatoes), refining edible oils, and decolourising and neutralising sugar. Only food-grade material is permitted; industrial-grade caustic soda must never come into contact with food.

[6. Construction and Cleaning]

1. Drain unblocking: Decomposes grease and hair blockages.
2. Industrial heavy-duty degreasers and floor scale-removal formulations.