Iodometry

Iodometry, or iodometric titration, is a redox analytical technique that uses the iodine–iodide system to quantify oxidizing or reducing substances. In a typical assay, an oxidant in the sample oxidizes iodide (I−) to elemental iodine (I2); the amount of I2 generated is proportional to the oxidant present.

The liberated iodine is then titrated with a standard solution of sodium thiosulfate (S2O3^2−). The reaction I2 + 2 S2O3^2− → 2 I− + S4O6^2− proceeds until all I2 is consumed. A starch indicator is often used near the endpoint, where the blue starch–iodine complex disappears. From the volume of thiosulfate consumed, the quantity of oxidant can be calculated; equivalently, the method can reveal the reducing capacity of a sample by the amount of I2 formed.

Applications are wide in environmental, pharmaceutical, and food analysis. The method is commonly used to determine residual oxidants such as free chlorine, chlorine dioxide, and hypochlorite in water, as well as other oxidants that can convert I− to I2. It is also employed to quantify reducing agents in iodometric formulations and to study redox properties of various compounds. The approach is valued for its simplicity, inexpensive reagents, and good accuracy when reagents are properly standardized.

Procedural notes include accounting for substances in the sample that can consume iodine or thiosulfate, standardizing thiosulfate solutions, and ensuring proper storage of iodine stocks. Endpoint detection can be visual via starch indicator, or enhanced by potentiometric or colorimetric methods.