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Activated Carbon Iodine Number: What 900 vs 1100 mg/g Really Means

Every activated carbon enquiry starts with iodine number. It is a useful figure and a badly misused one. Here is what it actually measures, and why the higher number is not automatically the better carbon.

Ask ten suppliers for a quotation and ten will lead with iodine number. Ask what else matters and the conversation gets quiet. That gap is where bad purchases happen.

What iodine number actually measures

Iodine number is the milligrams of iodine adsorbed by one gram of carbon, measured under ASTM D4607 or an equivalent method. Iodine is a small molecule, so it fits into small pores. What the test really tells you is how much micropore volume the carbon has — pores below roughly 2 nanometres.

It is popular because it is cheap, fast and reproducible. It is a proxy for total surface area, and for coconut shell carbon it correlates reasonably well with performance in the applications that dominate the market.

Why 1100 is not simply better than 900

Micropores are excellent at capturing small molecules: chlorine, chloramine, many volatile organic compounds, and gases. If that is your application, a higher iodine number generally does mean better performance per kilogram.

But some jobs need mesopores — pores large enough for bigger molecules to enter at all. Colour bodies in sugar refining, humic acids in surface water, and large organics simply cannot fit into a micropore, no matter how many of them there are. A carbon with iodine 1100 and almost no mesopore structure can underperform a carbon with iodine 900 that has a wider pore distribution.

For those applications, ask for molasses number or methylene blue value instead. They probe the larger pores.

The three numbers nobody volunteers

Hardness / abrasion number

This one costs buyers real money and rarely appears on the first quotation. In gold recovery — carbon-in-pulp and carbon-in-leach — the carbon is tumbled with slurry for weeks. Soft carbon breaks down, and the fines carry your gold to the tailings. A 1100 iodine carbon that fractures is worse than a 950 carbon that survives the circuit.

The same applies to any packed bed: attrition means fines, fines mean pressure drop, pressure drop means backwashing.

Ash content

Ash is the mineral residue. In water treatment for drinking supply, high ash can leach into the treated water. Coconut shell carbon is naturally low in ash compared with coal-based carbon — that is one of its genuine advantages — but low is not zero, and the figure varies with how clean the shell was when it was carbonised.

Apparent density

You buy carbon by weight and use it by volume. Two carbons with identical iodine numbers but different densities will fill your vessel differently. This is a costing question as much as a performance one.

Matching grade to application

Application	What matters most	What to ask for
Drinking water, chlorine removal	Micropore volume, low ash	Iodine number, ash, mesh
Colour removal, large organics	Mesopore volume	Molasses number, not just iodine
Gold recovery (CIP / CIL)	Hardness, then activity	Abrasion number, iodine, density
Air and gas phase, solvent recovery	Micropore volume, CTC	CTC number, iodine, mesh
Food and beverage	Ash, purity, extractables	Ash, food-grade documentation

How to read an activated carbon quotation

• Is a test method named? "Iodine 1050" means little. "Iodine 1050 mg/g, ASTM D4607" means something.
• Is it a typical value or a batch certificate? A typical value is an average from a good production run. A certificate of analysis belongs to the specific batch you are paying for.
• Did they give you hardness without being asked? Suppliers who volunteer the inconvenient number are usually the ones whose numbers hold up.
• Does the grade match your job? If your application needs mesopores and the supplier only talks about iodine, they are selling a number rather than solving your problem.

What we do

We supply coconut shell activated carbon in granular and powdered form, with mesh sized to your requirement. We will tell you which grade suits your application and, where our grade is not the right one, we will say so. Samples are one kilogram, free, with the batch certificate of analysis — so you can verify against your own lab rather than take our word for it.