Caustic Soda 10% Solution

Caustic Soda 10% Solution is an aqueous sodium hydroxide mixture supplied as product code CS-10-AQ. It contains 10.0 ± 0.5% w/w NaOH in water and is specified for pH neutralization, cleaning-in-place operations, saponification, and process alkalinity control. Sodium hydroxide has CAS Registry Number 1310-73-2, molecular weight 39.997 g/mol, and density of approximately 1.109 g/cm³ at 20 °C. The hydroxide molarity is approximately 2.77 mol/L, corresponding to a calculated pH of about 14.4 at 25 °C. For transport, the solution is classified as a corrosive liquid under UN 1824. Typical packages include 200 L HDPE drums, 1000 L intermediate bulk containers, and dedicated tank trucks. CS-10-AQ differs from 25% and 50% liquid caustic grades principally in delivered alkalinity per unit volume, freezing behaviour, and dilution exotherm.

A neutralization stoichiometry of 100 g NaOH per kilogram of product means that one metric tonne of CS-10-AQ neutralizes approximately 91 kg of hydrogen chloride or 123 kg of sulfuric acid. The solution is metered with positive-displacement diaphragm pumps into a reaction loop containing a static mixer or mechanically agitated flash tank. The pH probe should be placed at least 10 pipe diameters downstream of the caustic injection point, or after a residence time of 10–20 s, to avoid controller oscillation from local high-pH plumes. Because the product contains 90% water, a given stroke-length change produces a smaller hydroxide molar flux than the same change with 50% caustic, reducing overshoot at pH setpoints between 6.5 and 8.5. In a typical neutralization loop, a 0.75 kW mixer disperses the feed into an 8 m³ reaction tank; the lower viscosity of the dilute product avoids the metering-pump stall that can occur when unheated 50% solution is stored in lines below 15 °C.

What Happens When 10% Caustic Meets Amphoteric Metals in Wastewater pH Trim?

Wastewater containing aluminium, zinc, or lead is subject to metal redissolution if the pH overshoots the hydroxide solubility minimum. For aluminium-bearing rinse water, minimum solubility commonly lies between pH 6.5 and 7.5; above pH 9.0, aluminium hydroxide redissolves as soluble aluminate [Al(OH)₄]⁻. Zinc-bearing streams can redissolve above pH 10.5 as zincate. A 10% caustic solution is selected for these streams because the diluted alkali allows a broader proportional band on the pH controller without generating an uncontrolled high-pH plume. The heat of neutralization for a strong acid with NaOH is approximately −57 kJ per mole of water formed. In a 10 m³/h continuous-flow system, the bulk temperature rise from pH 2.5 to 7.0 is normally less than 2 °C, but local injection plumes may exceed 30 °C before complete mixing.

In metal precipitation service, the 10% product is paired with a coagulant and an anionic flocculant. Direct dosing into the flocculation tank is avoided because a local high-pH zone can break the floc polymer chain. Caustic is injected upstream of the rapid-mix stage where the velocity gradient G exceeds 300 s⁻¹. Jar tests are required to define the exact pH setpoint for each plant; published data for redissolution kinetics in specific aluminium-bearing resist rinses are limited.

Caustic Soda 10% in Clean-in-Place Loops and Carbonate Precipitation Control

In dairy, brewery, and pharmaceutical clean-in-place circuits, the 10% solution is diluted to operational concentrations of 1.0–2.0% w/w NaOH and heated to 60–85 °C. These conditions saponify fats and hydrolyse proteins while holding soil in suspension. The as-supplied 10% form is selected because its low-temperature fluidity permits unheated storage in a secondary containment bund, whereas 50% caustic may require heat tracing at temperatures below approximately 12 °C. Caustic cleaning is followed by potable-water rinse and dilute-acid rinse steps; residual caustic in low spots can cause pH shock to biological treatment if the rinse water is discharged without buffering.

In hard water, calcium and magnesium react with hydroxide to form precipitates. Magnesium hydroxide has a solubility product of approximately 5.6 × 10⁻¹² at 25 °C, and calcium carbonate may precipitate if carbon dioxide from air contacts the warm caustic solution. Published data for precipitation onset in 10% caustic under specific plant hardness conditions are limited, but field inspections of plate heat exchangers operating at 70 °C frequently report visible scale within weeks when no polyphosphate or polyacrylate scale inhibitor is used. Compatibility boundaries include ammonium salt formulations, which liberate ammonia gas upon contact, and aluminium equipment, which evolves hydrogen. The solution should not be mixed with strong acids, isocyanate prepolymers, or chlorinated solvents.

When 10% Sodium Hydroxide Replaces Soda Ash or 50% Membrane-Grade Caustic in pH Control

Compared with 50% membrane-grade caustic, the 10% solution delivers one-fifth the alkaline equivalent per metric tonne: 100 kg NaOH versus 500 kg NaOH. The lower concentration reduces the heat generated during addition to water, but increases freight and storage-tank volume per unit of alkalinity. The 10% grade is usually selected where mild alkalinity is required, where long suction lines create pump-priming concerns, or where the site does not operate heated storage for 50% solution. The 25% grade is intermediate in freezing point and viscosity but may not be stocked in all regions.

A chemical difference from soda ash is that caustic soda neutralizes acids without generating carbon dioxide. In a closed reactor or tank this avoids foam and gas-lock in discharge lines. However, the pH-versus-dose curve near neutral pH is steeper with caustic than with soda ash, so the metering pump requires a narrower validation band. When replacing soda ash in a pH-control loop, the stroke frequency of a diaphragm pump typically drops because the required molar dosage is lower; the controller proportional band should be widened and the injection point moved farther upstream to ensure complete mixing.

In batch saponification, the 10% solution is dosed into a jacketed reactor containing triglyceride oils at 80–90 °C. The lower caustic concentration reduces the local sodium-soap precipitation that can trap unreacted oil when 50% caustic is added directly. The reaction releases glycerol and sodium salts of fatty acids; the mixture is held at temperature until free alkali determined by titration with 0.1 mol/L HCl falls below 0.5% w/w.

In drinking-water treatment, a liquid sodium hydroxide product conforming to AWWA B501-19 may be used for pH adjustment and corrosion control. Dosage for low-alkalinity water is commonly 5–30 mg/L as NaOH, depending on source-water pH and alkalinity. The 10% solution is injected into a treated-water header upstream of a static mixer; the lower concentration permits finer control in small plants operating below 100 m³/h.

In food processing, sodium hydroxide is listed under 21 CFR 184.1763 as a direct food substance affirmed as GRAS when used in accordance with good manufacturing practice. Applications include pH adjustment in syrup refining, peeling of fruits and vegetables, and cleaning of food-contact lines. The 10% solution is often preferred for direct-contact pH adjustment because it avoids the larger heat of dilution of 50% material. Residual sodium is controlled by conductivity-based rinse verification; when rinse-water conductivity returns to within 10 µS/cm of source water, the line may be released.

In textile mercerization, 10% caustic is not used as the primary mercerizing bath, which normally requires 18–25% w/w NaOH at 15–30 °C. However, the 10% solution prepares dilute scouring baths and neutralizes acidic carbonizing residues. In pulp bleaching, 10% caustic is used to adjust alkali charge in extraction stages; if the slurry has high magnesium content, the solution is introduced through a dilution ring rather than a single injection quill to avoid local hydroxide precipitation. Published extraction-stage data for this specific 10% feed concentration are limited; pilot trials are required before converting from direct 50% or solid NaOH feed.