Same carbon, two labs, 60 mg/g apart: where the iodine number difference comes from
Buyers reject loads over a number that depends on the test method. Four variables explain almost every dispute, and three of them are in the sample bag.
The dispute in one sentence
A shipment tested 1,020 mg/g at the loading port and 962 mg/g at the buyer's laboratory. Neither lab made an error, and the 58 mg/g gap is entirely explained by method and sample handling. It still cost somebody a 6% price reduction.
Iodine number is a proxy measurement: it counts how much iodine the carbon adsorbs from a standard solution, and converts that to a surface area estimate. Anything that changes access to the surface changes the number.
Four variables and their typical effect
| Variable | Range in practice | Effect on result |
|---|---|---|
| Sample drying | As-received vs dried at 150 °C | 15–35 mg/g, wet reads low |
| Particle size | As-shipped vs ground to <75 µm | 20–40 mg/g, coarse reads low |
| Contact time | 30 s vs 60 s shaking | 10–25 mg/g, short reads low |
| Iodine concentration standardisation | Fresh vs aged starch indicator | 5–15 mg/g either direction |
What to write into the contract
Specify the method, not just the number: ASTM D4607, sample ground to pass 75 µm, dried at 150 °C for 3 hours, 30 minutes contact with agitation. Then specify the tolerance: ±30 mg/g between labs, with a third laboratory as umpire at the seller's cost if the gap exceeds it.
Specify sampling too. A 25 kg lot must be riffled to a 100 g laboratory sample, not scooped. Scooping from the top of a bag selects for fines, and fines always read higher because they expose more surface per gram.
Practical protocol for a disputed load
- Stop discharging. Once the load is blended into a silo, the sample no longer represents anything.
- Take 8 increments per container from different depths, combine, riffle to 2 kg, split into three sealed samples.
- Send one to each lab and hold the third sealed, with the seal number recorded on the shipping document.
- If the gap exceeds 30 mg/g, send the held sample to a third lab and accept its result as binding.
The number that matters more
For most water-treatment buyers, iodine number is a purchasing habit rather than a performance requirement. Two carbons reading 950 and 1,020 mg/g can behave identically in a column if their pore size distribution and hardness differ in the right direction.
If a buyer is rejecting on iodine alone, ask what the application actually needs — butane number, molasses number, hardness, or a column test. Half the time the specification can be rewritten around a measurement that predicts performance, and both sides stop arguing about a proxy.
Other notes
Send the raw material, target tonnage and finished spec. You get a configuration, a utility schedule and a budget range — not a brochure.