iodometric

Iodometric analysis, or iodometry, is a redox titration method in which iodine species are generated and consumed to quantify an oxidizing agent or reducing substance. The technique commonly relies on converting iodide (I-) to iodine (I2) with an oxidant, then titrating the liberated I2 with a standard solution of thiosulfate (S2O3^2–). A starch indicator is often used to visualize the endpoint.

The principle involves two main steps. First, the sample’s oxidant oxidizes iodide to iodine: Ox + 2 I- → Red + I2. In the second step, the produced iodine is titrated with thiosulfate: I2 + 2 S2O3^2– → 2 I- + S4O6^2–. The titration is typically carried out in acidic solution to maintain iodine in the proper form and to minimize side reactions. The endpoint is detected by a starch–iodine complex, which is blue; it disappears when all iodine has been reduced, indicating the end point.

Procedure and standardization commonly involve adding excess potassium iodide to the acidified sample to liberate I2, then titrating with a standardized sodium thiosulfate solution until a pale yellow color remains. A starch indicator is added to reveal the endpoint. Thiosulfate solutions are standardized against a primary iodometric standard such as potassium iodate (KIO3) or iodine.

Applications of iodometric titration include determination of oxidants such as chlorine, hypochlorite, iodate, and periodate, as well as certain reducing substances like sulfite. The method is valued for accuracy and ease of use, though it requires careful standardization and control of reagents to avoid side reactions and solution instability.