Caustic soda (sodium hydroxide, NaOH) is one of the most widely traded industrial chemicals. It is the standard source of strong alkalinity for detergents and cleaning, water treatment, textiles, pulp and paper, alumina refining, biodiesel, food processing and chemical synthesis. Buying it well means matching three things to your process: the physical form, the grade and the specification limits. This guide explains each one and gives typical values you can use as a starting point for your own specification.
What caustic soda is and how it is made
Almost all caustic soda is produced by electrolysing brine in chlor-alkali plants, which make chlorine, hydrogen and sodium hydroxide together. Three cell technologies exist. Membrane cells, now the dominant technology, produce a solution of roughly 30–33% NaOH with very low salt content, which is then concentrated to 50%. Diaphragm cells produce a weaker liquor that contains salt, and after concentration their 50% product typically still carries around 1% sodium chloride. Mercury cells, once common, have been phased out in Europe and in many other regions.
Solid caustic soda is made by evaporating 50% liquor to an anhydrous melt. The melt is either cooled on a rotating drum and scraped off as flakes, or sprayed through a prilling tower to form pearls. Flakes and pearls are therefore chemically the same material. Their differences are physical and come from how the melt is solidified.
The three physical forms
Flakes. Irregular, thin white flakes, typically around a millimetre thick. Flakes are the most widely produced solid form and are well suited to batch dissolution, where the solid is added to a make-up tank and stirred. They produce more dust than pearls and can bridge in hoppers. See the caustic soda flakes product page for the listed specification and pack options.
Pearls. Small, free-flowing spheres, typically under about one millimetre in diameter. Their uniform size gives faster, more predictable dissolution and accurate dosing from screw feeders and weigh hoppers, with less dust. They are the usual choice for automated dosing, powder blending and any application where precise weighing matters. See caustic soda pearls.
Liquid (lye). Sodium hydroxide solution, most often traded at 48–50% for long-distance bulk shipment and at around 30–32% for local distribution and direct dosing. Liquid removes the dissolving step and the heat it releases, and it can be pumped straight into the process. The trade-offs are that you pay to transport water and you need bulk storage, receipt facilities and, in cold climates, trace heating or insulation: a 50% solution begins to crystallise at around 12 °C.
Comparison: flakes vs pearls vs liquid (indicative)
- NaOH content: flakes typically 98–99% minimum; pearls typically 98–99% minimum; liquid usually 30–32% or 48–50% by weight.
- Dissolution: flakes moderate and can clump; pearls fast and even; liquid needs dilution only.
- Dosing: flakes suit batch addition; pearls suit automatic feeders and weighers; liquid suits metering pumps.
- Dust and caking: flakes produce more fines and can bridge; pearls are low-dust and free-flowing while kept dry; liquid has no dust but can crystallise in the cold.
- Typical packs: flakes and pearls in 25 kg bags, one-tonne big bags and steel drums; liquid in IBCs, road tankers and ISO tank containers.
- Best fit: flakes for batch make-up and general duty; pearls for automated dosing, blending and precise weighing; liquid for high, continuous consumption with bulk storage on site.
Grades of caustic soda
Technical or industrial grade. The standard grade for cleaning, water treatment, pH control and most chemical processing. Specifications focus on NaOH content, carbonate, chloride and iron.
Rayon grade. A high-purity grade with tight limits on iron, heavy metals, silica and other trace elements, used in viscose fibre production and other processes that are sensitive to metals.
Food grade. Sodium hydroxide is authorised as a food additive (E524), mainly as an acidity regulator, and is also used as a processing aid, for example in peeling fruit and vegetables, refining edible oils and curing olives. Food-grade material must meet the E524 purity criteria in Regulation (EU) No 231/2012 and its GB equivalent, or the Food Chemicals Codex monograph in other markets. Permitted uses and levels are set by Regulation (EC) No 1333/2008 and the UK retained equivalent, so check the current annex for your application.
Pharmacopoeia and reagent grades. Material tested to a pharmacopoeia monograph such as the Ph. Eur. or USP, or to reagent specifications, for pharmaceutical and laboratory use. Expect tighter limits, more documentation and a higher cost.
Choose the lowest grade that reliably meets your process and regulatory needs. Paying for pharmacopoeia material to neutralise effluent adds cost without benefit, but using technical grade in a food or pharmaceutical process is a compliance failure.
Typical specification parameters
The values below are indicative for solid technical-grade caustic soda. Always agree limits in writing and check them against the certificate of analysis for each batch.
- NaOH (assay): typically 98.0–99.5% minimum, depending on grade.
- Sodium carbonate (Na2CO3): typically 0.4–1.0% maximum. Carbonate rises when the product absorbs carbon dioxide from the air.
- Sodium chloride (NaCl): often 0.01–0.05% maximum for membrane-cell product, and higher for diaphragm-derived material.
- Iron (Fe): typically 10–50 ppm maximum for technical grade, and lower for rayon grade.
- Other limits you may see: sodium chlorate, sodium sulfate, silica, appearance and, for food grade, heavy metals such as lead and mercury.
For liquid caustic soda, the specification is stated on the solution: NaOH content by weight, with carbonate, chloride, chlorate and iron limits, and sometimes density at 20 °C (indicatively about 1.52 g/cm³ at 50% and 1.35 g/cm³ at 32%).
Choosing by application
- Detergents, soap and cleaning: technical-grade flakes or pearls, with pearls where dosing is automated. See our detergents and institutional cleaning hub.
- Water and effluent treatment: technical grade; liquid for large continuous dosing and pearls for smaller or intermittent use. See our water and effluent treatment hub.
- Textile scouring and mercerising: technical or low-iron grades, because iron can mark white goods.
- Viscose and cellulose processing: rayon grade.
- Biodiesel: caustic soda is a common transesterification catalyst, with potassium hydroxide as the alternative. Low water and carbonate content matter, because both reduce catalyst activity and promote soap formation.
- Food processing: food grade only, with documents that show conformity with the E524 purity criteria.
Packaging and logistics
Solid caustic soda is usually packed in 25 kg bags of woven polypropylene or multi-wall paper with a polyethylene liner, in flexible intermediate bulk containers (big bags) of about one tonne, or in steel drums. Because caustic soda is a Class 8 corrosive dangerous good, the packaging must be UN-approved for its packing group and marked accordingly. A 20-foot container commonly carries in the region of 25–27 tonnes of bagged product, depending on palletisation and the road-weight limits at destination. Liquid caustic soda moves in road tankers, rail cars, ISO tank containers and 1,000-litre IBCs.
Shipping documents typically include the commercial invoice, packing list, bill of lading, certificate of analysis, safety data sheet and dangerous goods declaration, plus a certificate of origin where preferential duty or destination rules require one. Our HS code, CAS and UN1823 reference covers codes and transport classification in more detail.
Storage and safe handling
- Store sealed packs in a dry, covered area. Caustic soda is strongly hygroscopic and absorbs carbon dioxide from the air, so open bags cake and slowly convert to carbonate.
- Keep it away from acids, and from aluminium, zinc, tin and their alloys, including galvanised steel, which react to release flammable hydrogen. Keep it away from ammonium salts too, which release ammonia.
- Carbon steel and stainless steel suit most caustic duties at ambient temperature, and polyethylene and polypropylene are common for tanks and pipework. Check compatibility at your operating temperature and concentration.
- Always add caustic soda slowly to cold water while stirring, never water to caustic. Dissolving releases a great deal of heat and can cause boiling and spattering.
- Wear chemical goggles with a face shield, alkali-resistant gloves such as nitrile, neoprene or PVC, and protective clothing. Provide an eye-wash station and safety shower where the product is handled.
- After eye contact, rinse immediately with plenty of water for an extended period and get medical help. The safety data sheet for the supplied product is the controlling document.
Under CLP, caustic soda is classified as causing severe skin burns and eye damage (H314) and as possibly corrosive to metals (H290).
Documents to request with a quotation
- The product specification or technical data sheet for the offered grade
- A certificate of analysis for the batch to be shipped
- A safety data sheet in the language and format required at destination
- For food or pharmaceutical use, a conformity statement against the relevant purity criteria or monograph
- For imports into the EU or GB, confirmation of which company holds the REACH or UK REACH registration covering your import and use
- Any quality or management-system certificates you rely on: request the certificate scope with your quotation and check that it covers the producing site and product
Comparing quotations
Compare offers like for like: the price per tonne of 100% NaOH for solutions, the same Incoterms (for example FOB, CFR or DAP), the same packaging and the same specification limits. A cheaper offer with higher carbonate or iron, or with bags that are not UN-approved, can cost more in rejected batches or delayed customs clearance than it saves. For a closer look at the two solid forms, read caustic soda flakes vs pearls.
Frequently asked questions
Are caustic soda flakes and pearls the same chemical?
Yes. Both are sodium hydroxide at typically 98–99% NaOH. They differ only in physical form, which affects dissolving speed, dosing and dust.
What is the difference between 32% and 50% liquid caustic soda?
Only the water content. A 50% solution costs less to transport per tonne of NaOH but begins to crystallise at around 12 °C, while a 32% solution stays liquid at much lower temperatures and is easier to dose directly.
Is caustic soda the same as lye?
Lye is a common name for caustic soda solution, and is sometimes also used for potassium hydroxide solution. In trade, lye or soda lye usually means sodium hydroxide solution.
How long can solid caustic soda be stored?
In sealed, undamaged packaging and dry conditions it is stable for a long time, and many producers quote a shelf life of two years or more. Once a bag is opened, the product absorbs moisture and carbon dioxide, so use it promptly and reseal part-used packs.
Can technical-grade caustic soda be used in food production?
No. Food use requires a food-grade product that meets the E524 purity criteria, or the equivalent standard in your market, with documents to prove it.
Does caustic soda need special transport arrangements?
Yes. It is a Class 8 corrosive dangerous good (UN1823 as a solid, UN1824 as a solution) and must be packed, labelled and documented accordingly for road, sea and air.