The graph below shows the redox titration curve of a 0.05 M Sn²⁺ solution titrated with 0.1 M Ce⁴⁺. The equivalence point indicates the volume of 0.1 M Ce⁴⁺ required to fully oxidize all of the Sn²⁺ in solution. If more than 100 mL of 0.1 M Ce⁴⁺ is required to titrate an equal volume of another 0.05 M Sn²⁺ solution, which of the following could be used to explain the inaccurate equivalence point?

Question

Quizplus · Accepted Answer

11ecba2d_1201_dfcc_a3bf_e719cf3731a9_MD0008_00 During a redox titration, a solution with a known concentration of an oxidizing agent (such as Ce⁴⁺) is slowly added to a solution with an unknown concentration of a species to be measured (analyte). As the oxidizing agent is added, the analyte is oxidized and the electrode potential changes. The voltage sharply rises when passing through the equivalence point, which occurs when all of the analyte in the solution has been oxidized. In the given titration of the Sn²⁺ analyte, 100 mL of 0.1 M Ce⁴⁺ are required to reach the equivalence point. At that point, all of the Sn²⁺ has been oxidized to Sn⁴⁺ according to the following reaction: 2Ce⁴⁺ + Sn²⁺ → Sn⁴⁺ + 2Ce³⁺ If only Sn²⁺ is being oxidized in the titration, then the consumption of 100 mL of 0.1 M Ce⁴⁺ correlates exclusively to the concentration of Sn²⁺. However, if a metal salt impurity that could also be oxidized by Ce⁴⁺ was in the solution, then the volume of 0.1 M Ce⁴⁺ consumed would be due to both the oxidation of Sn²⁺ and the oxidation of the metal impurity. The oxidation of a metal impurity would consume an additional amount of Ce⁴⁺ solution beyond the 100 mL required to fully oxidize the Sn²⁺. As such, the measured equivalence point would occur at a greater volume than the 100 mL of 0.1 M Ce⁴⁺ that is attributed solely to the oxidation of Sn²⁺. Because the oxidation of Sn²⁺ and the impurity cannot be distinguished in the titration, this introduces error and decreases the accuracy of the measured value of the equivalence point. (Choices A and B) The question states that the volume and Sn²⁺ concentration is the same in both of the titrated solutions. (Choice C) An impurity that could oxidize Ce³⁺ back to Ce⁴⁺ would regenerate some of the added titrant, and this would mean that less than 100 mL (not more) would be needed to reach the equivalence point. Educational objective: Impurities that undergo the same oxidation-reduction behavior as the analyte cannot be distinguished from the analyte and decrease the accuracy of the measured equivalence point in a redox titration.

The Graph Below Shows the Redox Titration Curve of a 0.05