Sodium hydroxide (NaOH, also called caustic soda or lye, CAS 1310-73-2) is made industrially by the chlor-alkali process — the electrolysis of brine (saltwater). Passing a direct current through purified sodium chloride solution splits it into three commercial products at once: sodium hydroxide collects at the cathode, while chlorine and hydrogen gas are co-produced. The single overall reaction is 2 NaCl + 2 H2O → 2 NaOH + Cl2 + H2. Almost all of the roughly 80 million tonnes of caustic soda produced worldwide each year comes from this route; an older lime-soda (causticizing) process is now largely historical.
How Is Sodium Hydroxide Made? The Chlor-Alkali Process
Sodium hydroxide is produced commercially by electrolyzing brine, a saturated solution of sodium chloride. The chemistry is the same in every plant; what differs is the cell design that keeps the chlorine and the caustic apart. Strip a chlor-alkali plant down to its core and you get four steps.
- Brine preparation and purification. Rock salt or solar salt is dissolved in water, then treated to remove calcium, magnesium and sulphate. Membrane cells are unforgiving here — impurities above single-digit parts-per-billion foul the membrane, so brine polishing is the single biggest quality lever in the plant.
- Electrolysis. Direct current drives the reaction inside a divided cell. At the anode, chloride ions are oxidized to chlorine gas: 2 Cl− → Cl2 + 2 e−. At the cathode, water is reduced to hydrogen gas and hydroxide ions: 2 H2O + 2 e− → H2 + 2 OH−.
- Caustic formation. The hydroxide ions pair with sodium ions in the cathode compartment to give sodium hydroxide. Membrane cells deliver this as a clean ~32–35% NaOH solution.
- Concentration and finishing. The cell liquor is evaporated to the standard 50% commercial liquid, or further dried and prilled into the solid beads, flakes and pearls sold for handling and shipping economy.
The two co-products are not waste. Chlorine feeds PVC, water disinfection and dozens of organic syntheses; hydrogen is burned for process heat or used to make hydrochloric acid. That shared economics is why caustic soda and chlorine prices move together — a plant cannot make one without the other, so a glut of chlorine demand can pull caustic supply along with it whether the caustic market wants it or not.
The Three Chlor-Alkali Cell Technologies
All three cell types run the same overall reaction but separate the products differently, and that choice sets purity, energy use and environmental footprint. The membrane cell now dominates new capacity; the mercury cell is being retired.
| Process | How it works | NaOH purity / concentration | Status / notes |
|---|---|---|---|
| Membrane cell | Cation-exchange membrane lets only Na+ cross from the anode side to the cathode side, keeping chloride out of the caustic. | Highest purity, very low salt (<~50 ppm NaCl); cell liquor ~32–35%, evaporated to 50%. | Modern standard and most common; lowest electrical energy of the three. Sensitive to brine impurities. |
| Diaphragm cell | A porous diaphragm (historically asbestos, now polymer composites) slows mixing but lets some brine through, so caustic carries salt. | Lower purity; cell liquor ~10–12% NaOH contaminated with NaCl, requiring heavy evaporation. | Still in service, mainly in North America; higher energy for concentration, but tolerant of less-pure brine. |
| Mercury cell | Sodium forms an amalgam with a flowing mercury cathode, which then reacts with water in a separate denuder to give very pure caustic. | Very high purity, chloride-free, high concentration directly. | Largely phased out over mercury-emission concerns (e.g. EU exit by 2017 under the Minamata Convention/BREF). Almost no new capacity. |
The Historical Lime-Soda (Causticizing) Route
Before electrolysis took over, caustic soda was made by causticizing — reacting sodium carbonate (soda ash) with slaked lime: Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3. The calcium carbonate precipitates out and the caustic stays in solution. It works, but it is energy-hungry, caps out at modest concentration, and ties NaOH supply to soda-ash supply. The chlor-alkali process displaced it because electrolysis yields higher-purity caustic plus two saleable co-products from one cheap feedstock: salt. The lime-soda route survives today mainly inside kraft pulp mills, which regenerate their own cooking liquor rather than buy caustic.
What Sodium Hydroxide Is Used For
Caustic soda is one of the highest-tonnage industrial bases on the market because it is a cheap, concentrated source of hydroxide. The largest pulls on demand:
- Pulp & paper — kraft pulping and bleaching; one of the single biggest end uses worldwide.
- Soaps & detergents — saponification of fats and oils into soap; the original use that gave lye its name.
- Alumina (Bayer process) — digesting bauxite to extract alumina for aluminium production.
- Water treatment — pH adjustment and remineralization control in municipal and industrial water.
- Food processing — food-grade NaOH (E524) for olive curing, pretzel and bagel bathing, peeling and cocoa processing.
- Biodiesel — as a transesterification catalyst converting vegetable oils to methyl esters.
- Textiles — mercerizing cotton to improve strength, lustre and dye uptake; also viscose/rayon manufacture.
- Chemical processing & cleaning — neutralizing acids, scrubbing acid gases, and CIP (clean-in-place) degreasing.
Grade follows the application. Membrane-cell rayon and food grades carry tight chloride and heavy-metal limits; a paper mill or a drain-cleaner blender can run a standard technical 50% solution. Specify the grade against the use — over-specifying purity for a tolerant application just adds cost.
Safe Handling: NaOH Is Highly Corrosive
Be blunt about this: sodium hydroxide is highly corrosive and causes severe chemical burns to skin and eyes, with permanent eye damage possible from even brief contact. It is dangerous in solid form, as dust, and as solution. Dissolving it in water is strongly exothermic, and so is neutralizing it — both release enough heat to boil and spatter caustic if you rush. Treat every form of it as a burn hazard, not a mild irritant.
- PPE is mandatory. Chemical splash goggles (a face shield for bulk transfers), nitrile or neoprene gloves, and chemical-resistant apron or suit. No bare skin near open caustic.
- Add caustic to water, never water to caustic, and add slowly with stirring to control the heat of dilution.
- Have an eyewash and safety shower within reach wherever caustic is handled.
- Store in tightly closed, labelled, corrosion-resistant containers away from acids and from aluminium, zinc and tin — caustic attacks those metals and liberates flammable hydrogen.
Always work from the product Safety Data Sheet for the specific grade and concentration you have on site; the guidance below is general and does not replace it.
How to Neutralize Sodium Hydroxide
Neutralization is an acid-base reaction that converts caustic to a near-neutral salt and water — for example NaOH + HCl → NaCl + H2O. For small spills, a weak acid is safer and more controllable than a strong one: dilute acetic acid (white vinegar) or citric acid solution will bring the pH down without the violent, heat-spiking reaction a concentrated strong acid produces.
- Put on goggles, gloves and an apron, and work in a ventilated area.
- Add the diluted acid slowly to the caustic, stirring, so the exothermic reaction stays controlled and does not spatter.
- Check pH with strips or a meter as you go; the target is roughly neutral (pH ~7). Stop when you get there — overshooting into strong acid just creates a second hazard.
- For industrial volumes, neutralization belongs in a controlled process under your EHS procedures, not a mop bucket.
How to Dispose of Sodium Hydroxide
Concentrated caustic is a regulated corrosive waste — do not pour it onto the ground or into waterways, where its high pH harms aquatic life. The usual route is to neutralize to near-neutral pH (above), confirm the result, and dispose per local regulations; small, fully neutralized, well-diluted quantities may be allowed down the drain with copious water, but verify against your jurisdiction first. For bulk or uncertain waste, use a licensed hazardous-waste contractor. Always identify quantity and concentration before choosing a method, and keep the SDS disposal section on hand.
Caustic Soda Supplier — Bulk Sourcing from RawSource
RawSource supplies caustic soda in bulk to manufacturers, blenders and processors. We source caustic soda beads alongside flakes and 50% liquid solution, so you can match the form to your dosing and handling setup rather than retrofit your process to whatever is on hand. Where a process calls for a different alkali, we also supply related products such as potassium hydroxide and sodium carbonate (soda ash).
To get a quote, send three things: grade and form (beads, flake, or 50% solution), volume (drums, totes/IBCs, or bulk tanker), and delivery point and cadence. We confirm specification and availability against your application and source to it. For background on writing a clean RFQ and qualifying suppliers, see our comprehensive guide to chemical procurement.
Frequently Asked Questions
How is sodium hydroxide made?
Industrially, sodium hydroxide is made by the chlor-alkali process: electrolyzing brine (a concentrated sodium chloride solution) with a direct current. The overall reaction is 2 NaCl + 2 H2O → 2 NaOH + Cl2 + H2, co-producing chlorine and hydrogen. Most plants use membrane cells, which give the highest-purity caustic at the lowest energy.
Is caustic soda the same as lye and sodium hydroxide?
Yes. “Caustic soda,” “lye” and “sodium hydroxide” are three names for the same compound, NaOH (CAS 1310-73-2). “Lye” is the traditional name, common in soap-making; “caustic soda” is the standard industrial trade name; “sodium hydroxide” is the chemical name. Potassium hydroxide (KOH) is sometimes called “potash lye” but is a different chemical.
What is the chemical formula of sodium hydroxide?
The chemical formula is NaOH — one sodium ion (Na+) bonded to one hydroxide ion (OH−). Its molar mass is about 40 g/mol, and its CAS number is 1310-73-2. In solid form it appears as white beads, flakes, pearls or pellets; it is also widely supplied as a 50% aqueous solution.
Is sodium hydroxide an acid or a base?
Sodium hydroxide is a strong base, not an acid. It dissociates completely in water to release hydroxide ions, giving a high pH (a 1% solution is around pH 13–14). Because it is such a strong base, it reacts readily with acids in neutralization reactions and is highly corrosive to skin, eyes and many materials.
How is sodium hydroxide neutralized?
Neutralize it by slowly adding a dilute acid while stirring and checking pH toward neutral (~7). For small spills, a weak acid such as citric acid or vinegar (acetic acid) is safer and easier to control than a strong acid. The reaction is exothermic, so add the acid gradually and wear goggles, gloves and protective clothing.
Is sodium hydroxide dangerous?
Yes. Sodium hydroxide is highly corrosive and causes severe burns to skin and eyes, with risk of permanent eye damage from brief contact. Dissolving and neutralizing it release heat and can spatter. It must be handled with chemical splash goggles, gloves and an apron, kept away from acids and reactive metals, and used per its Safety Data Sheet.
What forms does caustic soda come in?
Caustic soda is supplied as solids — beads, flakes, pearls or pellets — and as a liquid, most commonly 50% aqueous solution. Solids ship and store more economically and are dissolved on site; the 50% solution is convenient for direct dosing into a process. The right form depends on your handling, storage and metering setup.
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