Iodine number explained: what it measures and what it hides

The activated carbon iodine number is usually the first figure a buyer looks at on a grade sheet, and often the only one. It is easy to read, which is exactly why it is easy to misuse. This note explains what the figure measures, what it can suggest and where it stops being useful. It is written for someone choosing or comparing carbons, and for students meeting the term for the first time.

What is actually measured

The activated carbon iodine number is the amount of iodine, in milligrams, that one gram of carbon takes up from a solution under set conditions. The unit on a sheet is mg/g. The method most listings name for it is ASTM D4607. We have not bought the standard, so check the edition before you write it into a contract.

Notice what is not in that sentence. The test does not measure surface area directly. It does not measure how well the carbon removes colour, a pollutant or a metal. It measures how much of one small molecule the carbon holds under one set of conditions.

What it suggests

Iodine is a small molecule, so it reaches the finest pores. A higher number is therefore often read as a sign of more fine-pore space. That is a reasonable first reading, and it is why the figure is common on sheets for liquid-phase work. It is only a first reading. The standard text itself is reported to warn that iodine number and surface area cannot be treated as a fixed pair, so a sheet that converts one into the other deserves a question.

Numbers you will meet

Ranges differ by grade and by supplier. One supplier table we read lists minimum values from the 800s up to the 1300s mg/g across its grades. A buyer request on a trading listing asked for 1150 to 1300 together with a high hardness. These are single examples from one table and one listing, not market norms. Use them to see the scale, not to set your own limit.

Three ways the figure misleads

First, a limit is not a result. “Iodine number minimum 900” says the producer will not ship below 900. It does not say the lot you will receive is 900 or 1100. Ask whether the sheet shows a limit, a typical value or a measured result for your lot.

Second, the best number for the application is not the highest number. Some uses need larger pores because the molecule to be removed is large. A carbon with a very high iodine number but few large pores can underperform there. Ask the supplier which pore range the application needs and what test shows it.

Third, the number depends on the sample. Moisture, grain size and sampling change the result. Ask whether the figure is stated on a dry basis and how the sample was drawn.

Reading it next to the other figures

Iodine number says little alone. Read it with:

• Hardness, because soft grains break down in service whatever their pores.
• Ash, because mineral content can affect use and washing.
• Mesh size, because grain size changes how fast the carbon reaches balance with the liquid.
• Moisture, because water in the bag is part of the weight you pay for.

Our reading card puts these terms side by side with the method each one is usually tested by. Open it from the grade sheet reading card page and keep it next to the sheet you are checking.

What this means for shell charcoal buyers

The activated carbon iodine number belongs to the finished carbon, not to the charcoal that comes before activation. Kora supplies coconut shell charcoal and charcoal powder as feedstock and does not make or sell activated carbon, so Kora has no iodine figure to give you. What the charcoal can contribute is a sound starting point. Ask for the properties the charcoal does have: fixed carbon, ash, moisture and, if you need them, volatile matter and hardness. The glossary explains each of these terms in the same plain way.

A short checklist for the next grade sheet

1. Write down the iodine figure and mark it as limit, typical value or lot result.
2. Ask for the method number and edition.
3. Ask whether the figure is on a dry basis.
4. Ask which pore range the application needs and how the supplier knows.
5. Read the hardness, ash and mesh figures before you trust the iodine figure.

Ask us

If a grade sheet has a term you cannot place, or you are choosing charcoal to activate and want to know what Kora can state and cannot state, send us the question. Use the contact page and say which sheet or which charcoal you are looking at. We answer what we can and say so plainly when a question is outside what Kora supplies.