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)

In this approach you measure an oxidizing substance by generating iodine and then titrating that iodine with a standard thiosulfate solution. The oxidizing component in the sample converts iodide to iodine; the amount of iodine formed is quantified by the thiosulfate titration, because iodine reacts with thiosulfate in a known 1:2 stoichiometry (I2 + 2 S2O3^2- → 2 I- + S4O6^2-). The endpoint is typically detected with a starch indicator, which turns blue in the presence of iodine and becomes colorless as all iodine is reduced.

This is why the process is called iodometry: iodine serves as the intermediate whose quantity reflects the amount of oxidant in the sample. Direct titration would measure the oxidant without forming iodine, and iodimetry involves different uses of iodine or a different titration approach.