Sulphuric acid
Cheap, effective, and the most dangerous product a small water treatment plant is likely to hold. Where it is genuinely needed, nothing substitutes for it economically. Where it is not, do not have it on site.
Sulphuric acid, H₂SO₄, is supplied at 96–98% concentrated or as 30–50% dilute solution. In water treatment it depresses pH ahead of aluminium coagulation, neutralises alkaline effluent before discharge, and splits soapstock in edible oil refinery streams.
One milligram per litre of 98% acid neutralises about 1.0 mg/L of alkalinity as CaCO₃. Where the required pH drop is modest and the site has limited chemical competence, hydrochloric acid at 30–33% is easier and safer to handle for a modest cost penalty.
Specification
| Parameter | Technical, 98% | Dilute, 50% |
|---|---|---|
| H₂SO₄ content | 96.0–98.5% | 49.0–51.0% |
| Iron as Fe | ≤ 50 ppm | ≤ 30 ppm |
| Chloride | ≤ 20 ppm | ≤ 15 ppm |
| Density at 20 °C | 1.83–1.84 kg/L | 1.39–1.40 kg/L |
| Appearance | Colourless to pale straw, oily | Colourless |
| Freezing point | ≈ 3 °C at 98% | ≈ −35 °C |
Dose calculation
| Alkalinity to neutralise, as CaCO₃ | 100% H₂SO₄ | 98% acid | mL of 98% per m³ |
|---|---|---|---|
| 50 mg/L | 49 mg/L | 50 mg/L | 27 mL |
| 100 mg/L | 98 mg/L | 100 mg/L | 55 mL |
| 200 mg/L | 196 mg/L | 200 mg/L | 109 mL |
| 300 mg/L | 294 mg/L | 300 mg/L | 164 mL |
| 500 mg/L | 490 mg/L | 500 mg/L | 273 mL |
Safety and materials
- Always add acid to water, never water to acid. Dilution is violently exothermic — adding water to concentrated acid can boil and eject it explosively. This is the rule that must be tested on every operator before they are allowed near the tank.
- Dilute in stages with cooling, target below 20% w/w per stage, and allow the batch to cool between additions.
- Segregate from caustic, hypochlorite and cyanide-bearing streams. Separate bunds, separate delivery points, unambiguous line labelling.
- Materials, concentrated: carbon steel is acceptable above 93% because of the passive film, and is the normal storage material. PTFE, PVDF, glass-lined steel and Alloy 20 for pumps and valves.
- Materials, dilute: the passive film is lost. Use PVC, CPVC, PVDF, PTFE, FRP with vinyl ester, or rubber-lined steel. Never carbon steel on dilute acid — corrosion is rapid and generates hydrogen.
- Protection: full face shield, acid-resistant gloves and apron, closed footwear. Safety shower and eyewash within ten seconds, tested weekly.
Frequently asked questions
Sulphuric or hydrochloric acid for pH correction?
Sulphuric is cheaper per unit of acidity and does not add chloride. Hydrochloric at 30–33% is far safer to handle, less prone to violent dilution behaviour, and does not add sulphate. Where the plant has limited chemical competence, or where added sulphate would feed sulphate-reducing bacteria downstream, hydrochloric is worth the cost difference. Where cost dominates and handling is competent, sulphuric wins.
How much sulphuric acid do I need?
Roughly 1 mg of 98% acid per mg of alkalinity as CaCO₃ to be neutralised, which is about 55 mL per cubic metre for 100 mg/L of alkalinity. That is the arithmetic; the real answer comes from titrating a sample of the actual stream, because buffering capacity varies widely and the arithmetic assumes complete neutralisation.
Do I need acid at all?
Often not. Acid dosing is needed where raw pH sits above the coagulant’s effective window and cannot be brought down any other way. Switching from alum, which needs pH 5.8–7.5, to PAC at 5.5–9.0 or ferric chloride at 4.0–11.0 frequently removes the acid dosing system entirely. Removing a concentrated acid installation from a plant is usually worth more than the chemical saving it represents.
What grade should I specify for an African site?
93% rather than 98% if there is any possibility of ambient temperatures near freezing, because 98% acid solidifies at about 3 °C. On coastal and lowland sites, 98% is standard. For small plants, a 30–50% dilute delivery removes the on-site dilution hazard entirely and is often the right call despite the freight on the water.