Chemistry explained
What Is Caustic Soda?
One of the highest-volume industrial chemicals on earth, and one of the most underestimated hazards. What it is, what it does, and the handling rules that matter.
In short
Caustic soda is the common name for sodium hydroxide, NaOH — a strong alkali supplied as white flakes, beads or pearls, or as a solution. It dissolves in water to give a strongly basic liquid with a pH approaching 14, and it is used across pulp and paper, soap and detergent manufacture, alumina refining, textiles, water treatment and food processing.
What caustic soda actually is
Sodium hydroxide is an ionic solid made of sodium cations and hydroxide anions. Those hydroxide ions are what make it a strong base: in water it dissociates completely, flooding the solution with OH− and driving pH to around 13 at 0.5 per cent and near 14 at 5 per cent.
Two physical properties dominate how it must be handled. First, it is deliquescent — it absorbs so much moisture from the air that flakes left exposed will eventually turn into a puddle of concentrated solution. It also absorbs carbon dioxide, converting slowly to sodium carbonate and losing strength, which is why containers must be kept sealed and why an open drum steadily loses assay.
Second, dissolving it releases a great deal of heat. The heat of solution is large enough that adding concentrated caustic to a small volume of water can push the mixture past boiling point, flashing steam and throwing a corrosive liquid out of the vessel. This is the origin of the oldest rule in the trade: always add caustic to water, slowly, with agitation and cooling — never water to caustic.
Industrially it is made by the chlor-alkali process, electrolysing brine to produce caustic soda, chlorine and hydrogen together in fixed proportion. That coupling is why caustic pricing moves with chlorine demand: when PVC production is strong, chlorine demand pulls the electrolysers hard and caustic becomes abundant and cheap. When chlorine demand falls, caustic tightens. Anyone buying caustic in volume should watch the chlorine market, not the caustic market.
Flakes, pearls, solution: which form to buy
| Form | Typical assay | Best for | Watch out for |
|---|---|---|---|
| Flakes | 99% NaOH | Bulk import, general industrial use, 25 kg bags | Dust on handling; absorbs moisture fast once opened |
| Pearls / beads | 99% NaOH | Free-flowing dosing, automated make-up systems | Higher price than flakes for the same assay |
| Solid blocks | 98–99% | Legacy applications | Difficult and hazardous to break up |
| 50% solution | 50% w/w | High steady consumption, tanker delivery, no manual handling | Freezes near 12 °C; half the freight is water |
| 32% solution | 32% w/w | Cold climates, lower freezing point | More freight per tonne of NaOH delivered |
The trade-off is freight against handling. Solid caustic at 99 per cent carries no water, so you pay to ship only product — but someone has to open bags, handle a corrosive dusty solid and make up solution on site, with all the heat-of-solution risk that involves. Liquid at 50 per cent is pumped straight from a tanker into a tank with no manual handling, but half of every load is water and it freezes around 12 °C, so heated or traced lines are needed in cold climates.
As a rough rule: solution wins for a plant close to a supplier with steady high consumption; solid wins for long-distance import, intermittent use, or where tank farm capital is not justified. Most bulk import trade from Asia moves as 99 per cent flakes in 25 kg bags for exactly this reason.
What caustic soda is used for
Pulp and paper
The largest single consumer. Caustic is central to the kraft pulping process, which dissolves lignin to free the cellulose fibres, and it is used again in bleaching and in de-inking recycled paper.
Soap and detergent
Saponification — reacting caustic soda with fats and oils — is how hard soap has been made for centuries and still is. Caustic also produces the surfactant intermediates and the alkaline builders that go into modern detergent formulations.
Alumina refining
In the Bayer process, hot caustic solution dissolves alumina out of bauxite ore, leaving the iron and silicate impurities behind as red mud. Aluminium production is a very large consumer of caustic worldwide.
Textiles
Mercerisation treats cotton with concentrated caustic under tension, swelling the fibre permanently to improve lustre, strength and dye uptake. Caustic is also used for scouring, to strip natural waxes and sizing before dyeing.
Water and effluent treatment
Raising pH to precipitate dissolved heavy metals as insoluble hydroxides, neutralising acidic effluent, and correcting the pH depression that alum and ferric coagulants cause.
Food processing
At food grade, caustic is used for peeling fruit and vegetables, curing olives, making pretzels their characteristic colour, and in caustic clean-in-place systems throughout dairy and beverage plants.
Chemical manufacture
A feedstock or pH agent in a very long list of syntheses, and the standard scrubbing medium for acidic gases in stack gas treatment.
The safety rules that matter
Caustic soda is classified Skin Corrosion Category 1A — H314, causes severe skin burns and eye damage. It is worth being precise about why it is more dangerous than its reputation suggests.
Alkali burns are worse than acid burns of comparable strength. An acid coagulates surface protein and forms a barrier that limits how deep it goes. An alkali saponifies fat and dissolves protein, so it keeps penetrating for as long as it is in contact. A splash in the eye can cause irreversible corneal damage in under a minute, and there is often less immediate pain than with acid, so people delay irrigation.
- Eye protection is not negotiable. Chemical splash goggles plus a face shield wherever there is any possibility of splashing. Safety glasses are not adequate.
- Irrigate for at least fifteen minutes — the ERG recommends thirty for corrosives — and continue while transporting to medical care. Stopping early is the commonest mistake.
- Eyewash and safety shower must be present and unobstructed anywhere caustic is handled.
- Never use aluminium, zinc, tin or lead for vessels, scoops or pipework. Caustic attacks them and liberates flammable hydrogen gas. Use nickel, mild steel or appropriate plastics.
- Add caustic to water, never water to caustic, slowly, with agitation and cooling.
- Do not mix with acids without controlled neutralisation — the reaction is violently exothermic.
- Keep away from ammonium salts, which release ammonia gas on contact.
Full hazard data, exposure limits and transport classification are on our caustic soda safety data reference page.
What to check when buying
- Assay — 99 per cent minimum NaOH is standard for flakes and pearls. Confirm it on the certificate of analysis rather than the datasheet.
- Sodium carbonate content — the direct measure of how much CO2 the material has picked up, and therefore of packaging integrity and age.
- Sodium chloride and iron — matter in textiles, food and any application where staining or catalysis is a concern.
- Grade — food grade, rayon grade and technical grade differ substantially in specification and price. Buying rayon grade for a duty that needs only technical is a straightforward waste of money.
- Packaging — 25 kg PP-lined bags are standard; the inner liner is what keeps moisture and CO2 out, so specify and inspect it.
- Transport — UN 1823, Class 8, packing group II for the solid. Confirm your freight forwarder is set up for Class 8 before ordering.
Frequently asked questions
Is caustic soda the same as lye?
Effectively yes in everyday use. Lye traditionally refers to a strong alkali solution, and in modern usage almost always means sodium hydroxide. Strictly, potassium hydroxide is also a lye — it is the one used to make soft and liquid soaps, whereas sodium hydroxide makes hard bar soap.
What is the pH of caustic soda?
It depends entirely on concentration. A 0.05 per cent solution is about pH 12, 0.5 per cent about pH 13, and 5 per cent approaches pH 14. There is no single pH for the solid, since pH is a property of the solution. In practice, any working caustic solution should be treated as strongly corrosive regardless of where it sits in that range.
Why must you add caustic to water and not the other way round?
Because dissolving sodium hydroxide releases a large amount of heat. Adding water to solid or concentrated caustic concentrates that heat in a small volume of liquid, which can boil and flash, spraying a corrosive solution out of the vessel. Adding caustic slowly to a large volume of water spreads the same heat through enough liquid to keep the temperature controlled.
Can caustic soda be used in food?
Yes, at food grade. Sodium hydroxide is GRAS under 21 CFR 184.1033 as a pH control agent, and is used for fruit and vegetable peeling, olive curing, pretzel production and caustic clean-in-place in dairy and beverage plants. Food-grade material carries tighter limits on heavy metals and other impurities than technical grade, and technical grade must never be substituted.
What should not be stored near caustic soda?
Acids of any kind, because the neutralisation reaction is violently exothermic. Aluminium, zinc, tin and lead, which caustic attacks with the release of flammable hydrogen. Ammonium salts, which release ammonia gas. And food and feedstuffs, which must be segregated in both storage and transport.
How should caustic soda flakes be stored?
Cool, dry and well ventilated, in tightly closed original containers with the inner liner intact, in an area with no drain or sewer access. The container seal is doing real work: caustic absorbs both water and carbon dioxide from the air, so a poorly sealed drum steadily loses assay to sodium carbonate formation as well as turning sticky.
Products referenced on this page
Supplied in bulk from Chinese manufacturing plants with a batch certificate of analysis and the manufacturer’s safety data sheet against every consignment.