10.9 : Redox Titration: Other Oxidizing and Reducing Agents

Besides iodine, other oxidizing or reducing agents can serve as titrants in redox titrations. Common oxidizing titrants include KMnO₄, cerium(IV), and K₂Cr₂O₇. The choice of oxidizing titrants depends on factors like stability, cost, analyte strength, and reaction rate between the analyte and titrant. KMnO₄ is a strong oxidizing titrant that reduces from Mn(VII) to Mn(II) in a highly acidic solution, simultaneously oxidizing the analyte to a higher oxidation state. In this case, KMnO₄ acts as a self-indicator, turning colorless upon reaction completion.

Reducing analytes can also be titrated using acidic cerium(IV) solutions, with redox indicators employed for endpoint detection. The analyte is oxidized during titration while Ce(IV) reduces to Ce(III). Potassium dichromate (K₂Cr₂O₇), a moderately strong oxidizing titrant, reduces to Cr³⁺ in acidic solutions. The dichromate endpoint is observed using a redox indicator.

In contrast, most reducing titrants are oxidized by atmospheric oxygen, making them unsuitable for direct titration of oxidizing analytes. For example, thiosulfate is used indirectly as a typical reducing titrant for iodometric titrations. However, an acidified iron (II) solution can be used to titrate some strong oxidizing analytes, such as cerium(IV), chromium(VI), and vanadium(V).