The assay is the most important quality control test for iodine raw material. It tells you exactly how much pure iodine is in your sample and whether you are getting what you paid for.

The global standard method for iodine assay is iodometric titration with sodium thiosulfate. This method is recognized by major pharmacopoeias including the USP and EP. It is chosen for its accuracy, simplicity, and low cost.

The chemical reaction is simple but elegant. When iodine reacts with sodium thiosulfate, it is converted into colorless iodide. This color change is the heart of the entire test.

Here is how the test works. First, you accurately weigh about half a gram of the iodine sample into a stoppered flask. Then you add potassium iodide to help the iodine dissolve in water, because pure iodine does not dissolve well on its own. This step is critical. After the iodine dissolves, you dilute it with water and add a small amount of hydrochloric acid. You then titrate the solution with sodium thiosulfate. When the solution turns pale yellow, you add a few drops of starch indicator. The solution suddenly turns dark blue or purple-black. You continue titrating slowly until the blue color disappears completely and the solution becomes colorless. The volume of thiosulfate used in this process directly tells you the purity of the iodine. Each milliliter of 0.1 normal sodium thiosulfate equals 12.69 milligrams of pure iodine.

For pharmaceutical grade iodine, the USP standard requires purity between 99.8 and 100.5 percent. The range slightly above 100 percent is allowed due to analytical tolerance and reference material variation, not because of cheating.

Performing this test correctly requires high attention. Iodine is volatile and sublimes at room temperature, so the flask must stay stoppered and you must work quickly. Direct light can oxidize iodide and cause errors, so perform the titration away from light. Also, never add starch indicator at the beginning of the test because it will cause errors in detecting the end point.

A complete iodine analysis includes other tests alongside the assay. Non-volatile residue must not exceed 0.05 percent. Chloride and bromide impurities are strictly limited. Heavy metals are allowed up to 20 parts per million, and arsenic up to 3 parts per million. Moisture content must stay below 0.1 percent.

When you request a Certificate of Analysis from your supplier, always look for the assay value reported as percentage I₂, the method used (USP or EP), and the date of analysis. Never accept a generic specification sheet. Always request analysis results for your specific lot.

In simple terms, iodometric titration with sodium thiosulfate and starch indicator is the gold standard for iodine purity testing. For pharmaceutical applications, 99.8 percent purity or higher is required. This simple but precise test ensures you receive exactly what you paid for.