Sodium Hydroxide (NaOH) Dosing Equation
Sodium hydroxide is the standard alkali for raising pH, neutralising acids, and CIP-cleaning RO and UF membranes. It is supplied as 50% (SG 1.53), 32% (SG 1.35) or 25% (SG 1.27) liquor. 50% is the cheapest per kg NaOH but freezes at 12 °C — heated tanks needed below that.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
Related dosing calculators
Other chemical-specific dosing calculators in the same series — same formula, different defaults:
- Chemical Dosing Calculator (generic) — the universal seed page
- Lime Dosing Calculator — pH Correction · Alkalinity Boost
- Phosphate Dosing Calculator — Boiler · Cooling · Anti-Corrosion
- Chlorine Dosing Calculator — Drinking Water · Cooling Towers
- All Chemical Engineering Calculators →
Frequently Asked Questions
For low-alkalinity water (<20 mg/L), about 5–15 mg/L NaOH per pH unit. For a 100 m³/hr stream raising 2 pH units: ~20 mg/L NaOH × 100 m³/hr ÷ 765 g/L (50% NaOH) = 2.6 L/hr. Always validate by inline pH probe — alkalinity and CO₂ buffering vary.
50% NaOH freezes at 12°C and crystallises in cool weather. Storage tanks are typically heat-traced and insulated to keep it >18°C. 32% and 25% caustic remain liquid down to −20°C, but cost more per kg active.
RO high-pH CIP uses 0.1–0.2% NaOH (1,000–2,000 mg/L) at pH 11–12 and 35–40°C, recirculated for 60–90 min. For a 200 L cleaning solution: dose = 200 L × 1500 mg/L ÷ 765,000 mg/L (50%) = 0.39 L of 50% caustic per cleaning.
Never — the heat of neutralisation can boil off the line and the resulting Na salt can scale fittings. Keep dose lines separated by at least 5 m of pipe with full mixing in between, or dose into independent legs of a Y-mixer.
50% NaOH is severely corrosive — Class 8 dangerous good. Splash on skin causes deep burns within seconds. PPE: full chemical suit, face shield, neoprene gloves, splash boots. Eyewash and safety shower within 15 m of dose tanks is mandatory.