Caustic Soda Liquid 50%

Liquid caustic soda 50% is an aqueous sodium hydroxide solution containing 50.0 ± 0.5 wt% NaOH, CAS 1310-73-2, molar mass 40.00 g/mol. The product is assigned commercial designations such as 50% membrane-cell grade and 50% diaphragm-cell grade; low-chlorate variants are supplied for rayon and organic peroxide processes. Typical bulk properties at 20 °C include density 1.525–1.530 g/cm³, dynamic viscosity approximately 78 mPa·s, and vapour pressure below 1.3 kPa. Equilibrium crystallization begins near 12 °C, and the atmospheric boiling point is approximately 140–143 °C. Total alkalinity is determined by titration under ASTM E291-18. Sodium chloride and sodium carbonate levels are controlled by cell technology and evaporation conditions. Transportation classification is UN1824, Class 8, Packing Group II.

Certificate of analysis documents for bulk shipments typically report NaOH, sodium carbonate, sodium chloride, iron, and chlorate or sulfate depending on product model. Total alkalinity under ASTM E291-18 is a co-determination of NaOH and Na₂CO₃; hydroxide alkalinity is obtained by subtracting the carbonate contribution. Sodium chloride is determined by potentiometric titration with silver nitrate after acidification. Iron is determined by ICP-OES after sample dilution and acidification. Sodium chlorate is measured by iodometric titration where low-chlorate grade is specified; typical low-chlorate limits are ≤0.10 wt% as NaClO₃. Density is checked at 20 °C by oscillating U-tube with ±0.0005 g/cm³ repeatability. Viscosity is measured with a rotational viscometer at 20 °C and 20 rpm, with calibration traceable to certified viscosity standards.

What Limits Storage Temperature and Pump Selection for 50% Liquid Caustic Soda?

Crystallization at 12 °C is the critical processing boundary. At 10 °C, the solution can form a slush or solid phase that blocks transfer lines and reduces pump suction; field reports from outdoor unloading stations describe solidification first at low-flow drain lines and instrument bridles. Production-scale storage therefore uses vertical welded carbon steel tanks with side-mounted external heat exchangers or electric heat tracing to maintain bulk temperature at 20–30 °C, with low-temperature alarm at 15 °C and high-temperature limit at 40 °C to reduce carbon steel corrosion. The dynamic viscosity of 78 mPa·s at 20 °C is appreciably higher than the approximately 3 mPa·s observed for 32% NaOH at the same temperature; suction piping is sized for 0.6–1.2 m/s velocity and short flooded suction. ASME B73.1 centrifugal pumps with ductile iron casings and PTFE/EPDM mechanical seals are used for bulk transfer, while magnetically driven centrifugal pumps with PTFE/PFA wetted components handle low-flow dosing at 0.5–5 m³/h. Progressive cavity pumps with EPDM stators are selected where carbonate particles are present above 0.20 wt%. Field failure modes include mechanical seal leakage from sodium carbonate crystals formed when atmospheric CO₂ contacts unstirred caustic surfaces in open tanks; closed-top storage with nitrogen padding or desiccant-filtered vent air maintains carbonate below 0.20 wt%. Diaphragm-grade material can show a lower apparent freezing point because higher sodium chloride content depresses the equilibrium solidification temperature; published data for this specific configuration is limited.

Membrane-Grade and Diaphragm-Grade Specifications Create Distinct Purity Boundaries

The production process governs impurity loading. Membrane-cell grade typically contains ≤0.03 wt% sodium chloride and ≤0.20 wt% sodium carbonate. Diaphragm-cell grade carries higher chloride, typically 0.8–1.2 wt%, and may contain chlorate and sulfate residues at levels that influence organic peroxide synthesis, viscose spinning, and pharmaceutical neutralisation. The following specification matrix represents typical industrial certificate-of-analysis values for bulk 50% liquid product. Chloride speciation is measured by silver nitrate potentiometric titration; carbonate is isolated by barium chloride precipitation before acidimetric titration.

In diaphragm-cell operations, sodium chloride content can shift by ±0.2 wt% between cell liquor batches; downstream chlorate-sensitive processes therefore specify membrane or low-chlorate grade to avoid oxidation side reactions. Low-chlorate rayon-grade 50% liquid is controlled to ≤0.10 wt% NaClO₃ and is used in viscose dope spinning where chlorate can degrade spinnerette finish.

Neutralisation of acidic waste streams uses 50% liquid caustic soda in pH-correction skids with metering pumps rated for 50–150 L/h and final pH set points of 7.0–9.5 for discharge compliance. In edible-oil refining, 0.05–0.15 wt% NaOH is added during neutralisation to remove free fatty acids; soapstock separation is performed in disc-stack centrifuges. In batch saponification, the stoichiometric relationship is 1 mol NaOH per 1 mol fatty acid, with 50% liquid introduced under high-shear mixing to prevent gel formation. In crude glycerin neutralisation, 50% NaOH is used to adjust pH to 5.0–7.0 before methanol recovery in biodiesel plants; static mixers and recycle loops are adequate for this service. Simple pH correction with static mixers and recirculation loops requires no further elaboration.

A batch neutralisation mass balance example: 1,000 kg of 98% sulfuric acid requires 1,598 kg of 50% liquid NaOH at stoichiometric ratio, based on 2 mol NaOH per 1 mol H₂SO₄ and solution density 1.525 g/cm³. Continuous skids use pH trim rather than fixed ratio for excursions.

Alkaline cleaning in metal finishing uses 5–15 wt% NaOH baths at 65–90 °C in stainless steel tanks with immersion heater bundles. 50% liquid is metered into the cleaner tank by conductivity-controlled chemical feed; oil and grease removal is monitored by free-alkalinity titration. Aluminum profile etching uses 40–60 g/L NaOH at 60–70 °C; aluminum-containing tanks require lined or stainless steel construction because caustic attack releases hydrogen. Chlorine absorption scrubbers and hypochlorite generators using 50% liquid maintain sodium hypochlorite at 10–15 wt% free chlorine, with packed-column cooling to keep the scrubber below 30 °C to minimize chlorate formation.

When 50% Liquid Caustic Soda Replaces Solid Flakes or Pearls in Continuous Dosing

Replacement of solid anhydrous caustic soda with 50% liquid changes storage, dissolution, and heat-management requirements. Solid flakes and pearls typically contain 98–99 wt% NaOH; 50% liquid contains 50.0 wt% NaOH, meaning that 1.96 tonnes of 50% liquid delivers 0.98 tonne NaOH equivalent. The liquid eliminates dissolution dust but must be maintained above 12 °C. Table 2 compares properties relevant to continuous dosing.

Continuous dosing systems for 50% liquid use magnetic flow meters with PTFE/PFA liners or Coriolis mass flow meters with Hastelloy C-276 wetted parts. Solid systems use loss-in-weight feeders and agitated dissolving vessels; ambient relative humidity above 50% causes caking and feeder blockage. Published data for solid feeder accuracy at high humidity is limited. 50% liquid reduces dust exposure but increases freeze risk in unheated outdoor lines. Dilution to working concentrations below 5 wt% is performed by adding caustic to water under agitation, not the reverse. The dilution skid should maintain outlet temperature below 80 °C to protect polypropylene piping and should include a temperature indicator with alarm.

Alumina Refining, Pulp Digestion, and Neutralisation Service Requirements

In Bayer-process alumina refining, 50% liquid caustic soda is diluted into process liquor to maintain caustic concentrations of 200–260 g/L Na₂O at digestion temperatures of 145–175 °C. Wetted piping and digestors in this service use nickel alloy 200/201 or controlled low-carbon austenitic stainless steel because carbon steel at temperatures above 80 °C and high caustic concentration is susceptible to caustic stress corrosion cracking. In kraft pulp mills, 50% liquid is added to white liquor storage or the causticizer to maintain total titratable alkali at 85–120 g/L as NaOH. In food-plant clean-in-place service, 21 CFR 184.1763 and Food Chemicals Codex grade are required for food-contact cleaning; typical cleaning solutions are 2–4 wt% NaOH at 60–80 °C with contact times of 10–20 minutes. In water treatment, caustic soda increases the Langelier Saturation Index of low-alkalinity softened water, with target pH 7.5–8.5 to reduce distribution-system corrosion; feed is controlled by proportional pH loops with 0.5–2.0 L/min diaphragm pumps.

Carbon steel is used for ambient 50% NaOH storage but post-weld heat treatment and stress relieving are recommended at temperatures above 50 °C because caustic stress corrosion cracking can initiate at weld residual stress. Nickel alloy 200/201 is specified for evaporators and heating surfaces above 80 °C. EPDM and PTFE are suitable gasket materials; fluorocarbon elastomers are not recommended in strong caustic because of dehydrofluorination. Polypropylene and PVC are acceptable for ambient vent lines but not for continuous service above 80 °C and 60 °C, respectively. Ball valves with PTFE seats and Hastelloy C-276 trim are used in continuous duty.

Wetted materials exclude aluminum, zinc, tin, and brass because 50% NaOH reacts with these metals to release hydrogen; concentrated acid neutralisation skids require dilution water and cooling jackets to keep the reaction mass below 80 °C. Storage tanks are welded carbon steel with high-level interlocks and secondary containment. Pumps use EPDM/PTFE wetted components for intermittent service; Alloy 20 or nickel alloy trim may be required for high-temperature continuous duty. Occupational exposure limits for sodium hydroxide mist are 2 mg/m³ as an 8-hour TWA under OSHA PEL and 10 mg/m³ as the NIOSH IDLH; local exhaust ventilation is specified for open tanks. Spill verification uses pH measurements with alkali-resistant electrodes. The product is classified as corrosive under UN1824, Class 8, Packing Group II.