Sodium Hydroxide for Industrial Use

Sodium hydroxide for industrial use, CAS 1310-73-2, molecular weight 40.00 g/mol, is supplied as anhydrous flakes, prills, micropearls, and aqueous solutions at concentrations from 25% to 50%. The commercial designation is governed primarily by form, assay, and chlor-alkali cell technology rather than by a unified model coding system; contractual product identifiers typically encode the manufacturing route and targeted impurity profile, such as membrane-cell 50% liquid or low-chloride rayon-grade solution. Solid anhydrous material melts at 318.4 °C, boils at 1388 °C, has a density of 2.13 g/cm³, and releases 44.51 kJ/mol on aqueous dissolution.

Manufacturing route controls the impurity fingerprint. Membrane-cell sodium hydroxide is formed as 32–35% catholyte and concentrated in multi-effect evaporators to 50%. Brine feed is softened and polished to calcium-plus-magnesium below 20 µg/L; ion-exchange membranes with sulfonate and carboxylate polymer layers limit chloride diffusion into the catholyte. The final product in merchant shipments is corrected to a NaOH assay of 50.00–50.50% and filtered through 1–5 µm cartridge filters before loading. Diaphragm-cell material contains higher sodium chloride and sodium chlorate as a consequence of catholyte composition; mercury-cell product is restricted in most jurisdictions and is not accepted in catalytic or food-contact specifications due to mercury residues.

What Limits Chloride and Chlorate in Membrane-Grade Caustic Soda?

Impurity thresholds determine suitability in catalytic and polymer applications. In membrane-cell 50% liquid, typical contractual maxima are sodium chloride at ≤50 mg/kg, sodium chlorate at ≤10 mg/kg, iron at ≤2 mg/kg, and mercury below instrumental detection limits because the membrane cell route excludes a mercury cathode. Diaphragm-cell material carries chloride at 1.0–1.5 wt% and chlorate at 500–1,500 mg/kg; these levels are acceptable in kraft pulp makeup and certain neutralization duties but are excluded from zeolite catalyst synthesis and chlorine-sensitive resin processing. Analysis is commonly reported under ASTM E291-18 and, for assay, ISO 979:1974.

Across Bayer-process alumina refineries, the addition point for 50% liquid caustic soda is typically the spent liquor return line ahead of the slurry heater or tube digester, where process temperatures range from 150 °C to 240 °C depending on bauxite mineralogy. Caustic concentration is controlled as 140–260 g/L Na₂O, with alumina-to-caustic ratio maintained between 0.60 and 0.70 for boehmite circuits; lime addition is used to displace carbonate from the liquor. Excess sodium carbonate above 2.5–3.0% of liquor solids reduces digestion efficiency and increases scaling on heat-exchanger tube surfaces, requiring periodic online acidizing with inhibited sulfuric acid.

Low-Chloride Limits Are Driven by Spinneret Corrosion and Cellulose Swelling

Rayon-grade sodium hydroxide solution is purchased against limits that protect spinneret capillaries and dissolution equipment. The 50.0% liquid product typically specifies chloride below 20 mg/kg, iron below 1 mg/kg, and chlorate below 5 mg/kg. Carbonate is limited to ≤0.30 wt% because carbonate precipitates in mercerizing baths and changes caustic diffusion into cellulose. At blend temperatures of 18–20 °C, viscosity of 50% sodium hydroxide is approximately 78–80 mPa·s; transfer pump sizing is corrected for this viscosity rather than water-like behavior. Rayon-grade product is filtered and stored in stainless steel or lined carbon steel tanks to prevent iron recontamination.

In kraft white liquor makeup, the product is dosed into the causticizer or directly into weak wash storage to replace sodium lost with lime mud and dregs. Operators monitor effective alkali as Na₂O, maintaining white liquor effective alkali at 80–120 g/L as Na₂O and sulfidity at 25–35% for softwood cooks. Diaphragm-grade 50% liquid is acceptable in open-cycle mills because chloride is purged in ash and dregs; however, closed-cycle mills with high recycle impose a chloride ceiling and may specify membrane-grade material to reduce corrosion in black liquor evaporator tubes. The product must be added slowly to the alkaline liquor circuit to avoid localized boiling, particularly when the weak wash temperature is above 80 °C.

Anion Exchange Resin Regeneration and Silica Control Limits

Demineralizer trains apply 4–10% sodium hydroxide solution at 40–50 °C to regenerate strong-base anion resins. The regeneration flow rate is held between 2 and 4 bed volumes per hour, with a slow displacement step to avoid channeling. Calcium and magnesium in the diluted regenerant are limited to ≤0.5 mg/L combined because hardness forms precipitates with silicate and blocks interstitial resin pores. Membrane-cell product reduces chloride leakage after regeneration compared with diaphragm-grade material; this is measured by rinse conductivity and silica breakthrough at the anion unit outlet. A final rinse to conductivity <5 µS/cm is required before service.

For packed-column acid gas scrubbing, 25% or 50% sodium hydroxide is diluted inline and metered with a pH control loop to hold alkaline scrubber blowdown between 8.0 and 9.5. The reaction with absorbed HCl produces sodium chloride, while absorbed SO₂ yields sodium sulfite and sodium bisulfite depending on oxidation potential. Compared with lime slurry, sodium hydroxide does not generate sulfate sludge or require slurry filter presses, but the operating cost per neutralization equivalent is higher. Column sizing is based on supplier packing factors and superficial gas velocities below 2.0 m/s to prevent entrainment.

When Sodium Hydroxide Replaces Sodium Carbonate or Hydrated Lime in Wastewater Neutralization

In wastewater neutralization, the replacement of sodium carbonate or hydrated lime with 25% sodium hydroxide is evaluated by titration curves and sludge generation. Sodium hydroxide reaches pH 9.0 without adding carbonate alkalinity or calcium hardness, but it creates a sharper pH response near the equivalence point; the control loop must use pulse-width modulation or variable-stroke metering pumps with a dead band no wider than ±0.2 pH. Hydrated lime has a solubility of 0.173 g/100 mL at 20 °C and produces calcium carbonate sludge, whereas sodium hydroxide is fully ionized. The absence of calcium is critical where downstream membranes reject hardness. Where the product is used in potable water treatment, the material must meet ANSI/AWWA B501-19 and NSF/ANSI/CAN 60.

Differences from potassium hydroxide are more evident in saponification and sulfonation. Potassium salts of fatty acids are more water-soft than sodium soaps; liquid castile and potassium carboxylate formulations require 45–50% KOH. Sodium hydroxide yields higher-melting sodium carboxylates and is used where bar hardness and detergency are governed by cation identity. In sulfonate neutralization, replacement of NaOH by KOH changes the viscosity and cloud point of the final surfactant. Sodium hydroxide is selected where sodium cation is desired or where potassium is excluded by discharge permits. On a delivered basis, 1,000 kg of 50% NaOH contains 500 kg active sodium hydroxide, while 1,000 kg of 45% KOH contains 450 kg active potassium hydroxide.

In cotton mercerizing, 25–30% sodium hydroxide at 15–20 °C is applied under controlled tension to swell the cellulose crystallites. Low-chloride membrane-cell caustic is preferred because chloride residues contribute to fabric tendering during subsequent drying. Residual alkali is recovered from wash water and concentrated in an evaporator, returning to the saturator at 28–30% strength. Carbonate above 0.3% in recovered caustic reduces the rate of cellulose swelling and can produce uneven dye uptake; continuous ranges monitor carbonate by titration and purge weak wash when carbonate exceeds the specified operating ceiling.

Interfacial polycarbonate polymerization uses 50% membrane-grade sodium hydroxide as an acid acceptor in the phosgenation of bisphenol A. Aqueous-phase pH is maintained between 10.0 and 11.0 to neutralize hydrogen chloride without excessive hydrolysis of the carbonate linkage. Chloride content in the caustic feed is controlled below 50 mg/kg because additional sodium chloride in the organic phase alters phase separation and can accelerate chlorinated byproduct formation. The material is added through an in-line pH-controlled feed manifold with a ±0.1 pH control band; this is a demanding application for membrane-cell product because diaphragm-grade chloride would require additional organic-phase washing.

Storage and transfer of 50% sodium hydroxide require heated and insulated tanks when ambient air is below 12 °C, because the crystallization point of 50% liquid is approximately 12 °C. Carbon steel is acceptable for ambient liquid service; EPDM or PTFE gaskets are used on flanged connections, while aluminum, zinc, tin, and galvanized steel are incompatible. Anhydrous flakes absorb water and carbon dioxide; packages are sealed and stored below 60% relative humidity to minimize caking. For dilution, caustic is added to water under agitation, never the reverse, because the heat of solution of 44.51 kJ/mol can raise local temperature above the atmospheric boiling point. Safety data sheets prepared under REACH (EC) No 1907/2006 Annex II must list the corrosion hazard and the required emergency response for both UN 1823 solid and UN 1824 solution.