Sodium hypochlorite
The one product on this site whose specification is meaningfully different by the time it reaches you. Everything about buying hypochlorite for an African plant follows from that.
Sodium hypochlorite, NaOCl, is supplied at 10–15% available chlorine as a pale yellow alkaline liquid. It is used for disinfection, sulphide and odour oxidation, ferrous iron oxidation, biofouling control and equipment sanitation.
It decomposes continuously. At 25 °C a 12% solution loses roughly 2–5% of its available chlorine per month; at 35 °C that becomes 10–15% per month; above 40 °C, or in sunlight, decay is faster again. Buy it in small, frequent lots against a stated available-chlorine figure at the point of delivery rather than at the point of manufacture.
Specification and decay
| Parameter | Value |
|---|---|
| Available chlorine at manufacture | 12.0–15.0% w/v |
| Available chlorine, delivery specification | Typically 10.0–12.0% w/v |
| Free alkali as NaOH | 0.3–1.0% |
| Sodium chlorate | ≤ 0.5%, rises with age and heat |
| Iron as Fe | ≤ 0.5 ppm — iron catalyses decomposition |
| Copper as Cu | ≤ 0.1 ppm — copper catalyses decomposition strongly |
| pH | 11.0–13.0 |
| Density at 20 °C | 1.16–1.24 kg/L |
| Storage temperature | Approximate loss per month, 12% product | Strength after 3 months |
|---|---|---|
| 15 °C | 1–2% | ≈ 11.5% |
| 25 °C | 2–5% | ≈ 10.7% |
| 30 °C | 5–10% | ≈ 9.5% |
| 35 °C | 10–15% | ≈ 8.0% |
| 40 °C in sunlight | 20%+ | ≈ 6.0% or less |
Dose ranges
| Duty | Dose as available chlorine | Notes |
|---|---|---|
| Final effluent disinfection | 5–20 mg/L | 30 min contact; residual 0.5–1.0 mg/L |
| Potable disinfection | 1–3 mg/L | Residual 0.2–0.5 mg/L at the far point |
| Sulphide oxidation | 2.2 mg Cl₂ per mg S²⁻ to sulphur; 8.9 to sulphate | Partial oxidation is cheaper and usually enough for odour |
| Ferrous iron oxidation | 0.64 mg Cl₂ per mg Fe²⁺ | Followed by coagulation and filtration |
| Ammonia breakpoint | 7.6 mg Cl₂ per mg NH₃-N | Expensive; rarely the right route on a large stream |
| Biofilm and slime control, cooling | 0.5–2 mg/L continuous or 5 mg/L shock | Watch chloride build-up in closed circuits |
| Tank and line sanitation | 50–200 mg/L | Rinse thoroughly before returning to service |
Handling
- Never mix with acid. Any acid, including the acidic coagulants on this site, liberates chlorine gas. Segregate bunds and label lines unambiguously. This is the most common serious chemical incident in small water treatment plants.
- Never mix with ammonia or ammonia-bearing effluent in storage. Chloramine formation is intended in some treatment schemes but is a hazard in a tank.
- Vent everything. Decomposition generates oxygen; sealed containers pressurise and split.
- Materials: HDPE, PVC, CPVC, PVDF, PTFE, titanium, EPDM and Viton. Not mild steel, not stainless of any grade in prolonged contact, not copper alloys, not natural rubber.
- Store cool, dark and vented. Shade is worth more than any other single storage measure — a shaded tank at 28 °C holds strength roughly three times as long as an unshaded one at 40 °C.
- Protection: splash goggles, face shield, gloves, apron. Eyewash within reach.
Frequently asked questions
How long does sodium hypochlorite last in storage?
It depends almost entirely on temperature. At 25 °C a 12% product loses 2–5% of its available chlorine per month; at 35 °C, 10–15%. Plan storage for four to eight weeks of consumption, not six months, and shade the tank. Buying a large cheap lot that then sits in the sun is a false economy — you will dose more product for the same result and eventually miss a disinfection target.
Should I specify 10% or 15% available chlorine?
For hot climates, 10–12%. Higher-strength product decomposes proportionally faster, so a 15% solution loses more absolute chlorine per month than a 10% one. The extra freight on the weaker product is usually less than the chlorine you would lose from the stronger one over an African supply line and storage period.
How do I check the strength I actually received?
Iodometric titration is the reference method and takes a few minutes with a burette, potassium iodide and sodium thiosulphate. A hypochlorite test strip or a portable photometer is adequate for routine checks. Test on arrival and record it — the delivery specification should be an available-chlorine figure at delivery, not at manufacture, and it cannot be enforced without a measurement.
Can hypochlorite be dosed with ferric chloride?
Not at the same point and never in the same line. They react. Where both are needed — hypochlorite to oxidise sulphide or ferrous iron, ferric chloride to coagulate — separate the injection points with sufficient reaction time between them and confirm the sequence on the bench first.