Caustic Soda (Sodium Hydroxide): Formula, Uses, Grades and Bulk Buying

The short answer

Caustic soda is sodium hydroxide, formula NaOH, CAS 1310-73-2, molar mass 40.00 g/mol. It is a white, strongly alkaline ionic solid that dissolves readily in water to give a highly caustic solution, and it is the workhorse base of the chemical industry.

It is traded in two physical forms and a handful of strengths:

• Solution (lye) — usually 32 % or 47–50 % w/w, moved by road tanker, ISO tank, IBC or drum. This is what most large consumers buy.
• Solid — flakes, pearls (prills) or blocks at 98–99 % NaOH, in 25 kg bags or drums. Bought where freight distance makes shipping water uneconomic, or where no dissolving facility is wanted.

Three names for one substance cause most of the confusion: sodium hydroxide is the chemical name, caustic soda is the industrial name, and lye is the traditional name usually applied to the solution.

Names, formula and identifiers

A note on spelling and shorthand: NaOH, sodium hydroxide, caustic soda and caustic soda lye all appear on enquiries and all mean the same product. What actually has to be specified is the form and the concentration, covered below.

Why it is a strong base

Sodium hydroxide is an ionic compound. In water it dissociates essentially completely into a sodium ion and a hydroxide ion:

NaOH → Na⁺ + OH⁻

That complete dissociation is what makes it a strong base, as opposed to a weak one such as ammonia, which only partly ionises. A 0.1 % solution sits near pH 12.5; a 1 % solution near pH 13. It is the hydroxide ion that does the work in nearly every industrial application — neutralising acid, saponifying fat, dissolving amphoteric metal oxides, swelling cellulose.

Two behaviours follow from the same chemistry and matter in practice. Solid caustic soda is deliquescent: it pulls water out of the air until it dissolves in it, which is why bags must be kept sealed. And it reacts with atmospheric carbon dioxide to form sodium carbonate, so a solution left open slowly loses strength and gains carbonate — a specification issue for rayon and pharmaceutical grades.

How caustic soda is made

Essentially all commercial caustic soda comes from the chlor-alkali process: the electrolysis of sodium chloride brine, which splits it into caustic soda, chlorine and hydrogen. The three are produced in fixed proportion, which is why the caustic price is tied to chlorine demand.

• Membrane cell — the dominant modern route. An ion-exchange membrane separates the compartments, giving caustic at about 32 % with low chloride and no mercury. This is the standard technology for new plant.
• Diaphragm cell — older; produces weaker caustic with higher salt content, requiring evaporation and purification.
• Mercury cell — historic, gives very pure caustic, and is being phased out worldwide on environmental grounds.

Membrane caustic at 32 % is then evaporated to 47–50 % for shipment, or further to a melt that is flaked or prilled into solid product. If your specification limits mercury or chloride, the cell technology is the question to ask — not the grade name.

Grades and concentrations

Concentration and purity are independent. A 50 % technical lye and a 50 % food-grade lye are the same strength and carry very different certificates. Specifying only “caustic soda 50 %” tells a supplier nothing about which one you need.

What caustic soda is used for

• Alumina refining — the Bayer process digests bauxite in hot caustic to dissolve aluminium as sodium aluminate. One of the single largest consumers worldwide.
• Pulp and paper — the kraft process uses caustic with sodium sulphide to dissolve lignin and free the cellulose fibres; also used in de-inking and bleaching.
• Soaps and detergents — saponification of fats and oils to soap, and neutralisation of sulphonic acids to make anionic surfactants.
• Textiles — mercerising cotton under tension with strong caustic swells the fibre and permanently improves lustre, strength and dye uptake; also scouring and viscose manufacture.
• Water and effluent treatment — pH correction, alkalinity adjustment and precipitation of dissolved metals as hydroxides.
• Chemical manufacture — the feedstock for sodium hypochlorite, sodium silicate, sodium phenolate, sodium cyanide and a long list of sodium salts.
• Food — E524, used to cure olives, darken cocoa in Dutch processing, give pretzels their crust, and peel fruit and vegetables chemically.
• Biodiesel and oleochemicals — catalyst for transesterification, and refining of crude vegetable oils.
• Cleaning — CIP cleaning of dairy and brewery plant, drain and oven cleaners, paint stripping.

Handling, storage and materials of construction

• Always caustic into water. Dissolution is strongly exothermic and a concentrated make-up can reach boiling and spit. Add slowly, with cooling and agitation. Never the reverse.
• Freezing. A 50 % solution begins to crystallise around 12 °C, and the crystallisation point rises steeply above that concentration. Tanks and lines in cool climates need lagging and heat tracing; buyers without that normally take 32 %, which stays liquid far lower.
• Metals. Carbon steel is suitable for solutions up to about 50 °C. Above that, strong caustic causes caustic stress corrosion cracking of carbon steel, and nickel alloys or carefully selected stainless are required. Never use aluminium, zinc, tin, brass or galvanised steel — caustic attacks them and evolves hydrogen.
• Non-metallics. HDPE, polypropylene, PTFE, EPDM and rubber-lined steel all serve well. Glass is attacked slowly and is not suitable for long storage.
• Hydrogen. Contact with susceptible metals generates hydrogen in the tank headspace. Vent properly and keep ignition sources away.
• Carbon dioxide pick-up. An open solution absorbs CO₂ and converts to carbonate, losing strength. Keep tanks closed, with a vent drier where carbonate is specified tightly.
• Protection. Face shield over goggles, gauntlet gloves in neoprene or butyl, apron and boots. Caustic burns are painful and slow to appear on skin but cause immediate and serious eye damage. Flush any contact with running water for at least 15 minutes. An eyewash and safety shower must be within seconds of the work point.
• Segregation. Store away from acids, which neutralise violently, and away from aluminium and zinc.

Transport classification and documents

Solid sodium hydroxide ships as UN 1823, Class 8, Packing Group II. Sodium hydroxide solution ships as UN 1824, Class 8, with the packing group set by concentration. Both are corrosive, and both require UN-specification packaging and a dangerous goods declaration prepared before booking.

A caustic soda export consignment should carry a batch-specific certificate of analysis naming the standard the batch was released against; a 16-section GHS safety data sheet in the destination language where required; commercial invoice and packing list; certificate of origin; the dangerous goods declaration and container packing certificate; and for food or pharmacopoeial grade, the monograph reference and the food-contact, Halal, Kosher or heavy-metal declarations the destination asks for.

How to buy it properly

1. Form — solution or solid. If solid, flakes, pearls or blocks.
2. Concentration — 32 %, 47 %, 50 % for lye; 98 % or 99 % minimum for solid.
3. Standard — technical, rayon grade, FCC/E524, BP, USP, or your own specification sheet.
4. The limits that matter to your process — usually carbonate, chloride, chlorate, iron, sulphate and heavy metals. Rayon and pharmaceutical buyers should state iron and chlorate explicitly.
5. Packing and handling — ISO tank, IBC, HDPE drum or 25 kg bag, and whether your site can unload and store a solution that freezes at 12 °C.
6. Quantity, destination port and Incoterm — and the call-off pattern if it is a contract rather than a single lot.

Enquire with a specification

Caustic soda is supplied by Scapex Exports from qualified Indian manufacturers in lye and solid forms across technical, rayon, food and pharmacopoeial grades, with certificate of analysis, safety data sheet and full dangerous goods documentation.

Send your form, concentration, standard, impurity limits, quantity, packing preference and destination, and you will get a quotation against that specification rather than a generic one.

Frequently asked questions