Before you sign a purchase agreement for iodine, verify these seven quality parameters with your supplier. Missing even one can cost you time, money, and customer trust.

1. Assay (purity) – the obvious but not the only metric
The most basic parameter. Industrial applications need 99.0–99.5%. Pharmaceutical requires 99.8–100.2% (on dried basis). But beware: a high assay number can hide other problems. Always look beyond this single value.

2. Chloride and bromide content – the hidden competitors
Iodine is often contaminated with other halogens, especially chlorine and bromine. These impurities alter reactivity and can poison catalysts. Maximum acceptable limits: chlorides below 0.05%, bromides below 0.02% for most sensitive applications. For display films, even lower.

3. Heavy metals – a toxicity and performance risk
Lead, arsenic, mercury, and iron are common metal contaminants. Lead and arsenic must stay below 10 ppm (pharmaceutical) or 20 ppm (industrial). Iron above 50 ppm can discolor final products and catalyze unwanted side reactions. Always check the COA for individual metal values, not just “heavy metals as total.”

4. Non-volatile residue – the invisible sludge
When you heat iodine to drive off the vapor, anything left behind is non-volatile residue (NVR). This includes metal salts, ash, and organic decomposition products. High NVR (above 0.1%) suggests poor refining. It will accumulate in your equipment and contaminate your final product.

5. Moisture content – the stability killer
Wet iodine is unstable iodine. Moisture accelerates sublimation, promotes corrosion, and can trigger unwanted reactions. Acceptable moisture is below 0.2% for most grades. For pharmaceutical use, many buyers specify below 0.1%. Iodine should arrive dry enough to flow freely without clumping.

6. Particle size distribution – small details matter
Iodine crystals range from fine powder to 10 mm lumps. Your application may prefer one over the other:

· Fine powder (under 500 microns): Dissolves fastest, ideal for solutions
· Prills (1–3 mm spheres): Flow best, minimal dust
· Crushed crystals (3–10 mm): Traditional form, slower dissolution
Always specify your requirement. Receiving the wrong particle size can disrupt your batching process.

7. Visual appearance – your first quality check
Pure iodine is uniformly blue-black with a metallic luster. Warning signs include:

· Gray or brown discoloration → possible contamination
· White or yellow crystals on container walls → iodine has sublimed and recrystallized, indicating temperature abuse or poor sealing
· Damp or clumpy appearance → moisture ingress
Trust your eyes. If the material looks wrong, test before using.

The Bottom Line: Do not buy iodine on price alone. A cheap shipment with high chlorides or heavy metals will cost you far more in failed batches and equipment damage. Demand a complete COA, and know how to read it.