Final answer:
The volume of sodium thiosulfate required to titrate the sample is higher than that for the blank due to the presence of peroxides in the sample that produce iodine, which is then quantified by titration with sodium thiosulfate.
Step-by-step explanation:
When determining the peroxide value, the volume of sodium thiosulfate (Na₂S₂O₃) needed to titrate the sample is higher than the volume needed to titrate the blank because the sample contains peroxides that react with the KI to produce iodine (I₃⁻). This liberated iodine is then titrated with sodium thiosulfate. Since the blank does not contain peroxides, it will not produce as much free iodine, hence requiring less sodium thiosulfate for titration to reach the endpoint.
In the reaction, OCI⁻ is reduced to Cl⁻ with an excess of potassium iodide (KI), converting the chlorine into an equivalent amount of iodine (I₃⁻). The iodine is then titrated with sodium thiosulfate. The stoichiometry of the reaction shows that two moles of Na₂S₂O₃ are required for each mole of I₃⁻. Therefore, the more peroxide present in the sample, the more I₃⁻ is formed, and consequently, more moles of sodium thiosulfate are needed to reach the starch indicator endpoint, resulting in a higher volume of titrant used than with the blank.