Flakes or lye — the decision that shapes everything else
Caustic soda (sodium hydroxide, NaOH, CAS 1310-73-2) reaches industrial users in two practical forms. Flakes are a dry solid at 98–99% NaOH. Lye is a ready-made solution, normally at 31% or 50%. Both deliver the same chemistry. They create very different operations.
| Form | Concentration | Best suited to |
|---|---|---|
| Flakes | 98–99% NaOH | Textile processing, soap manufacture, users who dissolve to varying strengths |
| Pearls / pellets | 98–99% NaOH | Laboratory, pharmaceutical, precise small-scale dosing |
| Liquid lye | 50% NaOH | Continuous bulk users with heated storage and dosing pumps |
| Liquid lye | 31% NaOH | Water treatment and users needing a lower freezing point |
| Powder | 97–98% NaOH | Drain cleaner and chemical manufacture |
- Flakes cost less to freight because you are not paying to move water, and they store indefinitely if kept dry
- Flakes require a dissolving station, and dissolving NaOH in water is strongly exothermic — solutions can reach boiling if made too fast
- Lye arrives ready to dose, removes the dissolving hazard, and suits continuous processes
- 50% lye crystallises below about 12 °C, so it needs heated or insulated storage through a Pakistani winter
- 31% lye stays liquid at much lower temperatures, which is why water treatment plants often prefer it despite the extra freight
A practical rule: if your process needs one consistent strength around the clock, buy lye. If you need different strengths for different batches, or your consumption is irregular, buy flakes.
Key physical properties
| Property | Value |
|---|---|
| Chemical formula | NaOH |
| CAS number | 1310-73-2 |
| Molecular weight | 40.00 g/mol |
| Appearance | White solid, or colourless solution |
| pH (1% solution) | ~13 — strongly alkaline |
| Melting point | 318 °C |
| Boiling point | 1,388 °C |
| Solubility in water | 111 g/100 mL at 20 °C |
| Density (solid) | 2.13 g/cm³ |
| UN number | 1823 solid · 1824 solution |
Production route, and why it appears on a specification
Caustic soda is produced by chlor-alkali electrolysis of brine. Three cell technologies have been used, and the one behind your material affects its impurity profile.
| Cell technology | Notes | Status |
|---|---|---|
| Membrane cell | Ion-exchange membrane; most efficient, low mercury risk | Current industry standard |
| Diaphragm cell | Lower energy cost, higher salt carryover | Declining |
| Mercury cell | High purity but carries mercury contamination risk | Being phased out globally |
For food, pharmaceutical and any application with a heavy-metals limit, specify membrane-grade caustic and require the certificate to report mercury. This is one of the few places where naming the production route on a purchase order genuinely protects you.
What the specification should say
- Assay (% NaOH) with tolerance, and the form — flake, pearl, or solution strength
- Sodium chloride (NaCl) — the classic marker of diaphragm-cell material; matters in textile and any process sensitive to chloride corrosion
- Sodium carbonate (Na₂CO₃) — rises when material has been exposed to air, and indicates poor packaging or long storage
- Iron (Fe) — causes staining in textile processing and discoloration in soap
- Mercury — required for food, pharma and feed grades
- For flakes, also state the particle size or flake thickness if your dissolving equipment depends on it
Storage, handling and materials
Caustic soda is hygroscopic and reacts with carbon dioxide from the air. An open bag of flakes absorbs moisture, cakes solid, and slowly converts to sodium carbonate — losing strength and becoming difficult to handle.
- Store flakes in sealed bags in a dry, covered area off the floor; reseal opened bags immediately
- Suitable materials — carbon steel is acceptable for caustic solution at ambient temperature, along with HDPE, PTFE and rubber-lined steel
- Never use aluminium, zinc, tin, galvanised steel or copper alloys — caustic attacks them and generates hydrogen gas
- Dissolving flakes: add caustic to water slowly with agitation and cooling, never water to caustic, and never in a sealed vessel
- Caustic burns are deep and often painless at first, so eye protection and full face shield discipline matters more here than the immediate sensation suggests
Where it is used, and what that implies for grade
- Textile processing — mercerising cotton to improve dye uptake and lustre, plus scouring and bleaching; low iron and low chloride matter
- Soap and detergent manufacture — saponification of oils and fats, where iron causes colour defects in finished bars
- Water treatment — pH correction and alkalinity adjustment, usually as 31% lye for handling reasons
- Pulp and paper — kraft pulping and de-inking of recycled stock
- Vegetable oil and ghee refining — neutralising free fatty acids, requiring food-grade material
- Chemical manufacture — feedstock for sodium salts, and pH control across a wide range of syntheses
- Oil and gas — drilling fluid formulation and desulfurisation
What to put on the RFQ
- Form and strength — "98% flakes" or "50% lye", not "caustic soda"
- Membrane grade where mercury matters, stated explicitly
- Named limits for NaCl, Na₂CO₃ and iron where your process is sensitive
- Quantity per delivery and annual volume
- Pack — 25 kg bags, 50 kg bags, jumbo bags for flakes; IBC or tanker for lye — and whether you need palletised or loose
- Delivery city, site access, and whether lye storage is heated
- Documentation — batch COA, MSDS, and import records where relevant
What drives the price
- Chlorine demand, which sounds unrelated but is not — chlorine and caustic are produced together in fixed ratio, so weak chlorine demand raises caustic prices and vice versa
- Electricity cost, since chlor-alkali production is electricity-intensive
- Form — flakes carry a conversion cost over lye, but lye carries freight on its water content
- Pack format, with jumbo bags cheaper per tonne than 25 kg bags
- Freight and delivery city, plus heated storage requirements for 50% lye
- Exchange rate and duty, since Pakistan is supplied by a mix of domestic production and import
Sourcing considerations in Pakistan
- Domestic chlor-alkali capacity supplies a meaningful share of the market, with imports covering the balance — confirm origin where batch consistency matters
- Caustic is a Hazard Class 8 corrosive in both solid and solution form; transport and site handling need to reflect that
- Check that your dissolving or dosing equipment matches the form before ordering — a tanker of 50% lye arriving at a site set up for flakes is an expensive mismatch
- For food, pharmaceutical or feed use, keep the grade monograph and the mercury result in the purchase record