Sulphuric Acid Dosing Equation
Sulphuric acid is the most common acid for pH reduction in water treatment. Whether you dose 98% concentrated acid neat or a diluted solution, the calculation is a mass balance — required dose × flow ÷ stock strength. Always add acid to water, never water to acid, and use acid-rated wetted materials (PVC-U, PVDF, PTFE).
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
- Caustic Soda (NaOH) Dosing Calculator — pH lift · alkali dosing
- General Chemical Dosing Calculator — any chemical · ppm → L/hr
- Caustic Soda Dosing Calculator — pH Raise · Neutralisation
- All Chemical Engineering Calculators →
Frequently Asked Questions
Sulphuric acid (H₂SO₄) is the most common acid for pH reduction. Typical uses: lowering pH before reverse-osmosis to control the Langelier Saturation Index and prevent scaling, alkalinity reduction in cooling-tower and boiler make-up water, regeneration of strong-acid cation exchangers, and neutralising alkaline effluent before discharge. The required dose depends on the raw-water alkalinity and the target pH, so confirm by titration or jar test.
First find the acid demand (mg/L as H₂SO₄) needed to drop the alkalinity/pH to target — from a titration curve or jar test. Then: dose rate (L/hr) = (flow m³/hr × demand mg/L) ÷ C_stock, where C_stock = % w/w × SG × 10. For 98% acid, C_stock ≈ 1803 g/L. Example: 100 m³/hr needing 20 mg/L → (100 × 20) ÷ 1803 = 1.11 L/hr of 98% acid.
Commercial concentrated sulphuric acid is 98% w/w with a specific gravity of about 1.84, giving roughly 1803 g/L of H₂SO₄. Battery/dilute grades (≈30–37%) have SG around 1.2–1.3. Diluting acid is strongly exothermic — always add acid slowly to water, never the reverse, and allow for the heat of dilution in tank design.
For concentrated (98%) acid use PVDF, PTFE or suitable stainless (limited); PVC-U and CPVC are fine for dilute acid but check the concentration/temperature chart. Elastomers: Viton (FKM) is generally preferred over EPDM for strong acid. Use a diaphragm metering pump with acid-rated wetted ends, a calibration column, and bunded storage.
Sulphuric acid is highly corrosive and the dilution reaction releases significant heat. Always add acid to water, never water to acid. Provide acid-resistant PPE (face shield, gloves, apron), eyewash/safety shower, ventilation, secondary containment (bunding) sized to the largest vessel, and clear ‘ADD ACID TO WATER’ labelling. Follow the product SDS and local regulations.
Sulphuric Acid Dosing for pH Correction
Sulphuric acid (H₂SO₄) is the workhorse acid for reducing pH and alkalinity in water and process streams. In reverse-osmosis pre-treatment it is dosed to bring the feed pH down so that the Langelier Saturation Index stays negative and calcium-carbonate scale cannot form on the membranes. In cooling towers and boilers it controls the alkalinity of the circulating water, and in effluent plants it neutralises alkaline discharge before it leaves site.
The dosing calculation
The dose rate is a straightforward mass balance: dose rate (L/hr) = (flow × required dose) ÷ C_stock, where the required dose (mg/L, as H₂SO₄) comes from the water's alkalinity and the target pH, and C_stock (g/L) = % w/w × SG × 10. For 98% concentrated acid at SG 1.84 that is about 1803 g/L of active H₂SO₄, so a small volume of neat acid treats a large flow — which is exactly why accurate metering and good mixing matter.
Finding the required dose
Acid demand is not a fixed number — it depends on the buffering capacity (alkalinity) of the raw water. The reliable way to size it is a titration curve or jar test: add measured acid to a sample and record pH until you reach target. As a planning figure, roughly 1 mg/L of alkalinity (as CaCO₃) needs about 0.5 mg/L of H₂SO₄ to neutralise, but always confirm against your own water analysis.
Pumps, materials and safety
Use a diaphragm metering pump with acid-rated wetted ends (PVDF/PTFE for concentrated acid), a calibration column for verification, and bunded storage sized to the largest vessel. The dilution of sulphuric acid is strongly exothermic, so always add acid to water, never water to acid, and design for the heat released. Provide acid-resistant PPE, an eyewash/safety shower, and follow the product SDS and local regulations.
Related: for raising pH instead, see the caustic soda (NaOH) dosing calculator; for any other chemical use the general chemical dosing calculator.