By the YELI Technical Team · Updated August 2026 · 8 min read
Iodine and methylene blue value are the two screening numbers buyers compare first — if you need both on one COA for a gold circuit, see our gold recovery carbon hub and request the batch COA.
Open any activated carbon datasheet and the first number is almost always the iodine number: 900, 1000, 1200 mg/g. It is marketed as a shorthand for quality, but it is really a proxy for microporosity " + B + " nothing more. Used correctly it helps you screen suppliers; used alone it picks the wrong carbon. This guide explains what each test measures and how to read a COA without getting sold.
What Iodine Number Actually Measures
Iodine number (ASTM D4607) measures how much iodine, in milligrams, one gram of carbon adsorbs from solution. Iodine molecules are small, so the test mainly reflects the volume of micropores below about 2 nm " + B + " the same pores that remove small contaminants like THM precursors, chloramine, VOCs and many odour compounds. In practice, iodine values of 500" + EN + "1500 mg/g cover the commercial range: drinking-water GAC is typically 900" + EN + "1100, gold recovery carbon 1000" + EN + "1300, and some gas-phase grades push higher.
Iodine number is a screening tool, not a spec sheet. Two carbons with identical iodine numbers can perform completely differently in your process.
Why Iodine Alone Misleads
Iodine adsorption does not tell you about larger pores (meso- and macropores), which matter for removing colour bodies, large organics and many liquid-phase contaminants. A carbon can be "iodine-boosted" through aggressive activation at the cost of hardness and yield. That is why a cheap quote with an impressive iodine number can still fail in a contactor " + B + " the media crumbles, or the pores are the wrong size for your contaminant.
The Other Tests That Matter
Methylene blue number reflects mesopores (2" + EN + "50 nm) and correlates with decolorization duty " + B + " sugar, edible oils, and juice processing. Hardness (ASTM D3802) measures resistance to attrition; in backwashed water beds and gold circuits it directly controls fines, media top-up and metal losses. Moisture is a payable-weight issue: carbon is sold by weight, and 5% vs 15% moisture is money. Ash tells you about inorganic content and purity, which matters for food and pharmaceutical grades. Gas-phase buyers should also look at CTC adsorption (carbon tetrachloride activity) or working capacity for VOC duty, and butane working capacity (BWC) for gasoline vapour applications.
Lab Reports: Yours, Not Just Theirs
A supplier's COA is a starting point. For anything beyond a small trial, have the batch independently tested " + B + " SGS, Intertek, Bureau Veritas or an accredited local lab all run iodine, hardness, moisture and ash. Sampling matters too: the sample you receive should be drawn from your batch, not a golden sample kept on a shelf. If a supplier refuses or hesitates on independent testing, that is the answer to your question. Our supplier verification checklist covers the full screening process.
Specs Worth Asking For
For most liquid-phase applications, ask for a full COA line: iodine number, methylene blue (if decolorizing), hardness, moisture, ash, pH, and particle size distribution of the exact mesh. For gas phase, add CTC or working capacity. Then ask for the test methods used (ASTM, GB/T, ISO) so you can compare apples to apples " + B + " different standards can give different numbers for the same carbon.
Batch-Tested Carbon, Documented Per Lot
Every YELI Carbon batch ships with a COA and loading photos, and we support third-party inspection at origin. GAC, PAC, EAC and honeycomb grades with full spec lines.
Request Specs + COA Sample →The Bottom Line
Quality is a stack of measurements, not one headline number. Screen with iodine, decide with the full COA, and verify with independent testing before you commit. For a walkthrough of the buying process, start with our supplier verification checklist.