A process wherein the sample liberates iodine, which is then titrated with a standard solution of sodium thiosulfate is called:

A process wherein the sample liberates iodine, which is then titrated with a standard solution of sodium thiosulfate is called:

• A. Direct titration
• B. Residual titration
• C. Iodometry
• D. Iodimetry

Answer: (C)

Think about redox titration methods that use iodine as an intermediate. In this approach, the substance in the sample oxidizes iodide or otherwise releases iodine into the solution. That iodine isn’t the analyte itself being measured directly; instead, you determine how much iodine is present by titrating it with a standard sodium thiosulfate solution. The reaction I2 + 2 S2O3^2- → 2 I- + S4O6^2- shows how thiosulfate reduces iodine to iodide, so the amount of thiosulfate used tells you how much iodine—and thus how much oxidizing power—was in the sample. The endpoint is often detected with starch, which forms a blue complex with iodine and disappears as iodine is consumed.

This is called iodometry (iodometric titration) because iodine is produced in situ and then titrated with thiosulfate. It wouldn’t fit as a direct titration of the analyte, since iodine is the intermediate being measured. It also differs from iodimetry, which would involve titrating iodine itself with a reducing agent rather than using the sample to liberate iodine first.