| HS Code | 754146 |
| Product Name | Tosoh Caustic Soda |
| Chemical Name | Sodium hydroxide |
| Chemical Formula | NaOH |
| Cas Number | 1310-73-2 |
| Molecular Weight | 40.00 g/mol |
| Appearance | Clear, colorless, odorless liquid (aqueous solution); white flakes or solid when anhydrous |
| Concentration | 48% w/w NaOH aqueous solution (typical industrial grade); solid grade ≥97% |
| Specific Gravity | Approximately 1.53 (48% solution at 20°C) |
| Solubility | Soluble in water, ethanol, methanol, and glycerol; insoluble in ether and acetone |
| Melting Point | 318°C (anhydrous) |
| Boiling Point | 1388°C (anhydrous) |
| Ph | Approximately 14 (1% aqueous solution at 25°C) |
| Vapor Pressure | Negligible at room temperature |
As an accredited Tosoh Caustic Soda factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Tosoh Caustic Soda pearls are packaged in durable 25 kg sealed polyethylene bags, ensuring safe handling, moisture protection, and easy storage. |
| Container Loading (20′ FCL) | Load Tosoh Caustic Soda in 20′ FCL, using sealed drums/bags, secure pallets, dry conditions, and proper PPE to prevent moisture and damage. |
| Shipping | Tosoh Caustic Soda ships as a corrosive alkaline solution or solid, typically in tank containers, ISO tanks, IBCs, or lined drums. Use acid-resistant equipment, secure ventilation, and protect from moisture. Clearly label UN 1824/1823, follow hazmat protocols, and avoid mixing with acids or reactive metals. |
| Storage | Store Tosoh Caustic Soda in a cool, dry, well-ventilated area away from moisture, acids, and incompatible metals. Keep containers tightly closed and clearly labeled. Use corrosion-resistant materials such as polyethylene or stainless steel. Provide secondary containment to prevent spills, and ensure access to eyewash stations and safety showers for emergency response. |
| Shelf Life | Shelf life is practically indefinite when stored sealed in original containers, protected from moisture and contamination. |
Low-temperature Bayer digestion circuits processing gibbsitic bauxite are adjusted to a total alkali charge of 140–180 g/L Na₂O, while high-temperature boehmitic digestion units operate between 220–250 g/L Na₂O and 220–265 °C in multi-pass tube digesters. Tosoh membrane-grade sodium hydroxide, received as 49 wt%, is metered into the spent liquor surge to compensate for sodium lost to reactive silica desilication products and red mud entrainment. In a low-temperature circuit, slurry residence time is held at 20–40 min for bauxite ground to 75% passing 75 µm, and the liquor leaving the digestor train is flashed in multistage letdown vessels to atmospheric pressure. Sodium aluminate concentration in the clarified green liquor is maintained at 0.55–0.65 molar Al₂O₃/Na₂O, which prevents premature gibbsite precipitation while limiting sodalite scaling on tube walls. Heat-exchanger fouling from cancrinite and sodalite scale reduces the overall heat-transfer coefficient over a campaign; periodic descaling is carried out with inhibited sulphuric acid at 85–90 °C. The alumina trihydrate precipitation section is seeded with fine gibbsite, and spent liquor is returned to digestion after evaporation to 180–220 g/L Na₂O, with sodium carbonate removed by lime causticisation to avoid caustic loss. The continuous closed caustic loop yields smelter-grade alumina, and the caustic inventory is sampled and titrated according to ISO 3195:1975. Chloride accumulates in Bayer liquor and promotes pitting corrosion in carbon steel evaporators, so the low chloride mass fraction of membrane-grade sodium hydroxide is preferred over diaphragm-grade material.
| Digestion variable | Gibbsitic low-temperature circuit | Boehmitic high-temperature circuit |
|---|---|---|
| Total NaOH as Na₂O | 140–180 g/L | 220–250 g/L |
| Digestion temperature | 145–155 °C | 220–265 °C |
| Residence time | 20–40 min | 45–90 min |
| Molar Al₂O₃/Na₂O in green liquor | 0.55–0.65 | 0.60–0.70 |
Caustic losses in the Bayer circuit are dominated by sodium aluminosilicate precipitation, not by alumina product entrainment. Reactive silica consumes sodium hydroxide during digestion to form desilication products such as sodalite and cancrinite, and those phases also contribute to heat-exchanger scale. Red mud washing recovers soluble sodium from thickened residue, but residual alkaline entrainment remains a controlling factor in caustic makeup demand. Process control in this application therefore relies on continuous analysis of free Na₂O, total Na₂O, and Al₂O₃ in plant liquor, rather than simple pH measurement, because the buffer capacity of sodium aluminate obscures free alkali trends at high caustic loadings.
Oxidative extraction in kraft pulp bleaching uses Tosoh caustic soda to raise pH to 11.0–11.8 in Eop towers after oxygen delignification. The alkali charge is split between the top of the medium-consistency pump and an injection ring at the tower bottom to avoid local pH spikes that would dissolve cellulose. A typical hardwood Eop stage receives 1.2–2.8 wt% NaOH on oven-dry pulp, 0.3–0.8 wt% hydrogen peroxide, and oxygen at 0.6–1.2 MPa. The tower operates at 90–105 °C with a retention time of 60–90 min. Residual alkali in the tower discharge is maintained at 6–10 g/L NaOH to prevent reprecipitation of dissolved xylan fragments; if residual alkali drops below 4 g/L, brightening reverses and metal ions carried from the pulp cause peroxide decomposition. Overcharging above 3.5 wt% NaOH accelerates alkaline hydrolysis and lowers intrinsic viscosity measured by ISO 5351:2010, so the extraction stage is controlled by on-line conductivity and residual-peroxide analyzers. The washed pulp then enters chlorine dioxide stages; the reduced kappa number and improved brightness stability from oxidative extraction reduce total chlorine dioxide demand. Effluent from this stage contains dissolved lignin, sodium carbonate, and organic acids; before discharge, the mill neutralizes it and controls adsorbable organic halogen compounds under EU Directive 2010/75/EU. The end product is bleached kraft pulp with 88–90% ISO brightness for printing and packaging grades.
Spanish-style green olive lye treatment uses membrane-grade sodium hydroxide diluted with decarbonated water to 1.8–2.8% w/v NaOH at 18–25 °C. Immersion time varies from 8 to 14 h depending on cultivar, fruit size, and storage temperature, with the lye front required to penetrate approximately two-thirds of the pulp thickness before the fruit is washed. Penetration is checked by cutting fruit and applying phenolphthalein; the exterior side of the cut surface should remain white while the interior remains pink. The lye hydrolyzes oleuropein and other bitter phenolics, softening the fruit and making the tissue permeable to fermentation. After lye treatment, the olives are washed in 2–3 changes of potable water over 8–12 h per change to remove free alkali, then placed in 5–7% NaCl brine for lactic acid fermentation. Fermentation typically proceeds for 60–90 days at 20–27 °C, during which Lactobacillus plantarum converts sugars to lactic acid and reduces the pH to below 4.6. The finished product is heat-pasteurized or refrigerated in acidified brine. Sodium hydroxide as a pH-adjustment and processing aid is permitted under FDA 21 CFR 184.1763, and the material must meet the Food Chemicals Codex monograph for caustic soda, including limits for lead, arsenic, and mercury. Residual caustic is not permitted in the finished olive, and the packed product is covered by Codex Stan 66-1981 for table olives.
In dairy evaporator and spray-dryer CIP circuits, a 1.0–2.5% w/v NaOH solution is circulated at 75–85 °C for 15–30 min, with a minimum return-line temperature of 65 °C to prevent redeposition of whey protein fouling. Membrane-grade caustic soda is specified because chloride mass fractions above 50–100 mg/kg promote stress-corrosion cracking in AISI 316L stainless steel surfaces, especially in hot evaporated-milk contact zones. The first alkaline wash hydrolyzes protein deposits, the second removes saponified fat and mineral complexes, and a final nitric acid step neutralizes and passivates the stainless steel.
| Cycle phase | Temperature | Concentration | Minimum contact time |
|---|---|---|---|
| First alkaline wash | 75–85 °C | 1.0–2.5% w/v NaOH | 15–20 min |
| Second alkaline wash | 70–80 °C | 0.8–1.5% w/v NaOH | 10–15 min |
| Nitric acid passivation | 55–65 °C | 0.5–1.0% v/v HNO₃ | 10–15 min |
The cleaning sequence is verified by conductivity monitoring of the final rinse and by ATP bioluminescence on critical control surfaces, with values above the plant-specific threshold triggering an automatic re-clean. Installation design follows ASME BPE-2022 for surface finish and 3-A Sanitary Standards for milk contact equipment. Final rinse water is controlled to ≤ 5 µS/cm conductivity and ≤ 20 mg/L sodium hydroxide residual. In multi-effect evaporators, caustic cleaning must not exceed 90 °C because of gasket degradation and thermal stress in plate-and-frame heat exchangers. The end product of this application is a validated clean surface, free of protein residues and ready for hot-water sanitization at 85–90 °C before production.
Continuous sodium hypochlorite generators are operated with Tosoh 48–50% caustic soda diluted to 18–20 wt% NaOH and chlorine gas at a molar ratio of 1.0–1.05 mol Cl₂ per mole NaOH. The gas sparger and reaction loop are fabricated from Hastelloy C-276 and titanium plate heat exchangers; cooling water at 10–15 °C removes the exothermic heat of reaction. The reaction mass is maintained at 18–25 °C, because above 30 °C sodium chlorate formation increases sharply and available chlorine loss becomes measurable within hours. Excess NaOH after reaction is held at 0.3–0.8 wt% to keep the final pH above 12.5, which suppresses hypochlorous acid formation and reduces decomposition. If the final pH falls below 11.5, chlorine off-gassing occurs and the product loses strength; if excess alkali exceeds 1.0 wt%, alkali in the finished bleach can precipitate magnesium and calcium from dilution water. Product is filtered through polypropylene cartridge filters and stored in HDPE or FRP tanks with caustic scrubbers on vents. The output, typically 12–15% available chlorine, is used in municipal water treatment and must comply with EN 901:2013 for sodium hypochlorite solutions used for drinking water.
To prevent free fatty acid carryover in continuous toilet soap production, the NaOH charge is calculated from the saponification value of each oil blend using ISO 3657:2013 and from the acid value of incoming fats. A 25–30 wt% caustic solution is mixed with molten tallow and coconut oil at 95–110 °C in a high-shear saponification reactor. The reaction is essentially complete within 15–45 min; the phase inversion from water-in-oil to oil-in-water occurs at approximately 70–80% saponification, producing a viscous neat soap mass with free alkali 0.05–0.20 wt% NaOH. Excess alkali above 0.30 wt% increases skin irritation and promotes soap hydrolysis during drying, while insufficient alkali leaves free fatty acid above 0.5% and accelerates rancidity. The neat soap moisture content is adjusted to 28–32 wt%, and glycerol is recovered via countercurrent wash columns for separate refining. The dried soap noodles are extruded into bar soap. Free caustic alkalinity in the final soap is measured by titration according to ISO 456:1973, and the saponification vessel contact surfaces are AISI 316L stainless steel to resist chloride-containing soap mass.
In cotton mercerization, yarn or open-width fabric is saturated with 18–24°Bé NaOH at 15–25 °C under controlled warp and weft tension. The caustic bath is filtered through an alkali recovery loop using ceramic membranes; recycled caustic is re-fortified to target density, and the working bath is continuously cooled to offset swelling exotherm. A dwell time of 30–120 s for open-width fabric allows cellulose I to cellulose II conversion, which increases fiber cross-sectional roundness, luster, and tensile strength while decreasing crystallinity. Immediately after saturation, the fabric enters a timed hot-water wash at 60–80 °C, followed by countercurrent cold rinses and neutralization with 0.5–1.0 g/L acetic acid. Residual alkali on fabric is controlled below 0.05% to avoid yellowing and uneven dye uptake in subsequent reactive or vat dyeing. Dimensional stability is tested according to ISO 6330:2012; tensile strength by ISO 13934-1:2013. Chemical inputs are screened against ZDHC MRSL v3.1 and REACH registration obligations for sodium hydroxide. The finished fabric is mercerized cotton for shirting, bed linen, and high-density woven goods.
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Tosoh Caustic Soda is an ion-exchange membrane electrolysis product composed primarily of sodium hydroxide with CAS 1310-73-2 in liquid and solid forms. The liquid grade is supplied at 48.0–49.0% sodium hydroxide by mass; the flake grade is supplied at a nominal 98.0% sodium hydroxide. The production route uses a perfluorosulfonate/perfluorocarboxylate bilayer membrane cell, which rejects chloride ions by Donnan exclusion while sodium ions migrate into the catholyte. Cell liquor is generated at 32–35% sodium hydroxide and then concentrated in multiple-effect nickel evaporators to the standard liquid concentration. The flake product is obtained by further concentration and drum flaking with surface solidification. The impurity profile of the membrane route is lower in sodium chloride, lower in mercury, and typically higher in sodium chlorate relative to mercury cell material, though sodium chlorate remains below 20 mg/kg in standard specifications. These characteristics are defined by the cell chemistry and evaporation train rather than by additive processing.
The standard grade designations are based on form and nominal concentration rather than numeric model codes. Liquid purchase orders use the descriptor 48% membrane-grade caustic soda solution; solid purchase orders use flake caustic soda 98%. Custom dilution below 48% is performed at selected distribution terminals, but standard transport and storage specifications are written for the concentrated liquid and solid forms. Release documentation identifies production lot, cell line, and analytical results rather than a product model number. This practice is common in chlor-alkali distribution and allows the same product to serve alumina, pulp, detergent, and chemical intermediate users without reformulation.
The 48% liquid exhibits a density of approximately 1.50 g/cm³ at 20°C and begins to crystallise near 10–12°C. Storage systems for unheated outdoor tanks therefore use either tank heating or continuous recirculation through insulated lines maintained at 20–30°C. The flake product deliquesces when relative humidity exceeds approximately 60%; hopper and silo systems require dry-air purging, vibratory discharge, and sealed rotary valves to prevent bridging. Dilution of the solid releases enough heat to raise local temperature above 80°C if water is added to the flake; the correct procedure is to add flake slowly to water under agitation. The liquid product is a strong base and should be stored separately from acids, ammonium salts, cyanides, and aluminium-containing equipment.
The standard purchase specification is controlled by lot-level titration, turbidimetric chloride analysis, and photometric or ICP-OES trace metal methods aligned with ASTM E291-18. Sodium carbonate concentration is a function of atmospheric exposure and evaporation residence time; liquid product typically carries 0.1–0.2% Na₂CO₃ on a 100% NaOH basis, and flake product may reach 0.5–1.0% through surface carbonation during solidification. Iron in the liquid is specified below 2 mg/kg; in flake the value is below 20 mg/kg due to contact with drying and flaking equipment. Aluminium, nickel, and copper are not routinely released in standard certificates but are controlled through brine polishing to avoid membrane fouling. Published data for trace metals below 1 mg/kg in this specific configuration is limited; users in catalyst or polymerisation processes should request lot-specific certificates of analysis.
| Parameter | Liquid 48% | Flake 98% | Standard method basis |
|---|---|---|---|
| Sodium hydroxide | 48.0–49.0 wt% | ≥98.0 wt% | acid titration, ASTM E291-18 |
| Sodium chloride | ≤0.01 wt% | ≤0.1 wt% | turbidimetric chloride |
| Sodium carbonate | ≤0.2 wt% | ≤1.0 wt% | acidimetric after barium precipitation |
| Iron as Fe₂O₃ | ≤0.001 wt% | ≤0.002 wt% | ICP-OES after matrix dilution |
| Sodium chlorate | ≤20 mg/kg | ≤20 mg/kg | ion chromatography |
The largest volume outlet for Tosoh Caustic Soda is alumina refining through the Bayer process. The 48% liquid is metered into bauxite digestion circuits at a caustic-to-alumina ratio set by reactive silica and gibbsite/boehmite balance. Typical consumption ranges from 0.25 to 0.45 tonnes of 100% NaOH per tonne of alumina, with the lower values associated with low-silica gibbsitic bauxite and the upper values with high reactive silica or high-temperature digestion. In kraft pulp make-down, the liquid is diluted to 18–25% active alkali before injection into continuous cooking and oxygen delignification systems. Direct addition to high-consistency transfer screws is avoided because local dilution heat can produce black liquor evaporation and fibre charring at the injection point.
The bilayer membrane rejects chloride ions while sodium ions migrate under a current density typically in the 3–6 kA/m² range. This yields cell liquor at 32–35% NaOH with sodium chloride concentrations that are lower than diaphragm cell product by roughly two orders of magnitude. Diaphragm-grade 50% caustic commonly contains 0.5–1.2% NaCl, whereas membrane-grade liquid is typically below 0.01%. In sodium hypochlorite manufacture, the lower chloride carryover changes the equilibrium distribution between hypochlorite, chlorate, and chloride during chlorination of caustic. Chlorate formation is suppressed when chlorine and caustic are reacted at pH 12–12.5 and temperature below 35°C; the use of membrane-grade caustic removes a portion of the chloride reservoir that otherwise shifts the product mixture toward chlorate during prolonged storage. Mercury cell material can have lower chloride but carries mercury at levels that require mercury-specific adsorbents under the Minamata Convention. Tosoh product is mercury-free, simplifying waste-water reporting and eliminating mercury removal equipment from storage areas.
| Constituent | Membrane cell (Tosoh) | Diaphragm cell | Mercury cell |
|---|---|---|---|
| Sodium chloride | ≤0.01 wt% | 0.5–1.2 wt% | ≤0.005 wt% |
| Sodium chlorate | ≤20 mg/kg | 50–300 mg/kg | ≤10 mg/kg |
| Sodium carbonate | ≤0.2 wt% | 0.1–0.3 wt% | ≤0.1 wt% |
| Iron as Fe₂O₃ | ≤0.001 wt% | 0.002–0.005 wt% | ≤0.001 wt% |
| Mercury | below routine detection limit | below routine detection limit | ≤0.1 mg/kg |
Unlined carbon steel is considered acceptable for continuous 48% liquid storage at ambient temperatures below 60°C, but welded seams and heat-affected zones become susceptible to stress-corrosion cracking when temperature and caustic concentration rise together. Nickel, nickel-plated steel, PTFE, and lined fiberglass are specified for pump internals and valve trim. Aluminium, zinc, tin, brass, and galvanised fittings are incompatible because of hydrogen evolution and rapid metal loss. The flake product should be kept separate from ammonium salts, cyanides, and combustible organics in dry form; confinement during unintended mixing can liberate toxic gases or initiate fire. When food-contact use is required, the application must be verified against 21 CFR 184.1763 or the applicable regional food chemical code, and the product should meet the analytical limits of the Food Chemicals Codex sodium hydroxide monograph. These limits are not automatically met by all industrial grades.
Sodium hydroxide from Tosoh is listed under REACH (EC) No 1907/2006 and is transported as UN 1824 solution and UN 1823 solid. GHS classification includes skin corrosion subcategory 1A with hazard statement H314. Emergency showers and eyewash stations are installed within 10 seconds travel distance of unloading and dosing points. The membrane production route does not use mercury cathodes, aligning with Minamata Convention obligations without the need for mercury monitoring in the product or plant waste streams.
In hot caustic washing of post-consumer PET flake, the wash bath is maintained at 1.5–2.5% sodium hydroxide and 85–90°C. Replacement of diaphragm-grade caustic with membrane-grade 48% liquid reduces the input of sodium chloride and chlorate into the recycled water loop. The benefit is not visible in once-through systems, but in plants operating 85–95% water recovery, chloride accumulation from repeated salt-containing alkali addition can raise bath conductivity above 5,000 µS/cm. At that level, stress-corrosion cracking of submerged stainless steel heating coils becomes more likely when chloride concentrates under scale deposits. Field observations from continuous lines indicate that the chloride inventory rises more slowly with membrane-grade material, but published head-to-head data for this specific configuration is limited. Free caustic control should use online conductivity or density sensors rather than pH alone; at 85°C, the pH response is compressed and cannot reliably distinguish 1.0% from 2.0% free alkali under high soil loading.
In soap saponification and detergent sulfonation, the 48% liquid is diluted to 20–30% before metering into high-shear mixers. Low chloride in membrane-grade caustic reduces the introduction of salt into the final soap curl and reduces the load on recycled glycerine recovery evaporators. Batch-to-batch variance in sodium chloride below the specification limit has not shown a measurable effect on saponification rate, but it influences the salt content of the neat soap phase and the viscosity of the resulting paste at 70–80°C. The flake product is used for dry detergent builders where liquid handling is not available; it must be ground immediately before dry blending because atmospheric moisture and carbon dioxide cause surface caking within hours at high humidity.
For sodium silicate production, the flake or liquid is dissolved to 30–40% NaOH before reaction with silica sand in a digester. Direct addition of flake to a hot silicate reactor can create local concentrations above 50% at the feed port and cause premature gelation before complete mixing. Iron and chloride introduced with caustic affect the colour and filterability of sodium silicate. Membrane-grade liquid with iron below 2 mg/kg and chloride below 0.01% is therefore preferred for detergent builders, catalyst binders, and high-clarity colloidal silica. Users with resin colour specifications below APHA 10 or catalyst pore-size tolerances below 5 nm should request lot-specific trace metal and chlorate data.
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