Compare forms on available chlorine, not on price per kilo
Chlorine (Cl₂, CAS 7782-50-5) reaches users in several forms. They are not interchangeable commercially, because what you are actually buying is available chlorine, and the content varies enormously.
| Form | Available chlorine | Best suited to |
|---|---|---|
| Chlorine gas | 100% | Large municipal and industrial plants with trained operators |
| Sodium hypochlorite | 10–16% | Most Pakistani users — municipal, industrial and institutional |
| Calcium hypochlorite | 65–70% | Small systems, remote sites, emergency treatment |
| Bleaching powder | 25–36% | Rural sanitation and premises disinfection |
| Chlorine dioxide | Generated on site | Large plants needing lower byproduct formation |
The comparison that matters is cost per kilogram of available chlorine delivered to your dosing point, including freight, storage losses and the safety infrastructure each form demands. Gas is by far the cheapest on that basis and by far the most demanding operationally. Calcium hypochlorite is the opposite — expensive per kilogram of chlorine, but it ships as a stable solid and stores for months.
Key physical properties of chlorine gas
| Property | Value |
|---|---|
| Chemical formula | Cl₂ |
| CAS number | 7782-50-5 |
| Molecular weight | 70.91 g/mol |
| Appearance | Greenish-yellow gas |
| Odour threshold | Detectable at about 0.5 ppm |
| Boiling point | −34.0 °C |
| Gas density at 0 °C | 3.17 kg/m³ — about 2.5 times heavier than air |
| Vapour pressure at 20 °C | 5.7 bar — stored as liquefied gas under pressure |
| Solubility in water at 20 °C | 7.3 g/L |
That density figure is the operational one. Chlorine gas sinks. A leak fills the lowest part of a building first, which is why chlorine rooms are designed with low-level extraction and why detection at floor level matters more than detection at head height.
What gas supply actually requires
Chlorine gas is supplied in 68 kg cylinders, 900 kg ton containers, or by tanker. Before quoting, a supplier will need to know whether your site can receive and handle them safely — and if you cannot answer, the honest recommendation is usually hypochlorite instead.
- A dedicated, ventilated chlorine room with low-level extraction discharging to a safe place
- Gas detection with alarm, sited at low level
- Emergency kit appropriate to the container type, and trained personnel who have practised using it
- Self-contained breathing apparatus and staff trained and fit-tested to wear it
- Secure cylinder restraint, and a means of moving ton containers safely
- A written emergency plan coordinated with local authorities
The economics only favour gas at scale, because that infrastructure is a fixed cost. For a plant dosing modest volumes, hypochlorite is almost always cheaper once safety infrastructure is included in the comparison.
Calcium hypochlorite: the option for remote sites
- At 65–70% available chlorine it carries far more chlorine per kilogram shipped than liquid hypochlorite, which matters when freight is long
- It is a stable solid with a shelf life measured in months rather than weeks
- Supplied as granules or tablets, so it suits erosion feeders and simple manual dosing
- It is a strong oxidiser — store away from organics, acids, ammonia and reducing agents, and keep it dry
- Never mix different hypochlorite products or add water to the container rather than the reverse
What to put on the RFQ
- Form, and available chlorine percentage — this is the figure quotations should be compared on
- Quantity expressed as kilograms of available chlorine per month, not just tonnes of product
- Pack — cylinder or ton container for gas, IBC or tanker for hypochlorite, drum or pail for calcium hypochlorite
- Whether cylinders are supplied on exchange and who owns them
- Delivery city, site access, and a statement of your storage and safety provision
- Documentation — batch COA showing available chlorine, MSDS, and drinking-water suitability certification if the end use is potable
What drives the price
- Chlor-alkali economics, since chlorine and caustic soda are produced together — weak caustic demand can leave chlorine cheap and vice versa
- Electricity cost, because chlor-alkali production is electricity-intensive
- Form, with gas cheapest per kilogram of available chlorine and calcium hypochlorite the most expensive
- Freight, which dominates for hypochlorite solution because most of the weight is water
- Cylinder or container rental and demurrage on gas supply
- Seasonal demand, which rises during monsoon and flood periods when water treatment loads increase
Sourcing considerations in Pakistan
- Domestic chlor-alkali production supplies much of the market, so chlorine is one of the more locally available industrial chemicals
- Chlorine gas is a Hazard Class 2.3 toxic gas — transport, storage and handling are tightly regulated and non-negotiable
- For potable water treatment, ask for confirmation that the product is suitable for drinking water use, and keep that documentation for regulatory review
- Match the form to the operator capability you actually have, not the one you plan to build — this is the most common and most consequential procurement error with chlorine