Redox titration (oxidation-reduction titration) is a quantitative analytical method in which the reaction between the titrant and the analyte involves a transfer of electrons. It is widely used to determine the concentration of oxidizing or reducing agents in a sample.

Principle of Redox Titration

1. The titrant is either an oxidizing agent (e.g., potassium permanganate, iodine, cerium(IV)) or a reducing agent (e.g., sodium thiosulfate, ferrous ammonium sulfate).
2. The reaction proceeds until all of the analyte has been oxidized or reduced, at which point the equivalence point is reached.
3. The endpoint can be detected using a redox indicator (e.g., starch for iodine, ferroin for cerium) or by potentiometric measurement.

Common Redox Titration Methods

1. Permanganate Titration: KMnO4 (deep purple) is used as a strong oxidizing agent in acidic medium. The endpoint is detected by the persistence of a faint pink color. This method is used for analyzing Fe2+, oxalate, and hydrogen peroxide.
2. Iodometric Titration: Iodine (I2) is used as an oxidizing agent, or iodide (I-) is oxidized to iodine, which is then titrated with sodium thiosulfate (Na2S2O3) using starch indicator near the endpoint.
3. Dichromate Titration: K2Cr2O7 is a primary standard oxidizing agent used for the determination of Fe2+ and organic compounds. It offers the advantage of being stable and不需要 daily standardization.
4. Cerimetric Titration: Cerium(IV) sulfate is a strong oxidizing agent used for determining Fe2+, As3+, and organic compounds in acidic solution.

Endpoint Detection

1. Visual Indicators: Starch (blue-black complex with I2), ferroin (red to pale blue with Ce4+), and diphenylamine (colorless to violet with Cr2O72-).
2. Potentiometric Detection: A platinum indicator electrode measures the change in potential during the titration; the endpoint corresponds to the steepest point on the titration curve.
3. Self-Indication: KMnO4 is its own indicator, as the intense purple color appears once excess permanganate is present.

Factors Affecting Accuracy

1. pH Control: Many redox reactions require specific pH conditions. Permanganate titrations must be performed in strong acid (1-2 M H2SO4) to prevent MnO2 formation.
2. Temperature: Some reactions (e.g., permanganate-oxalate) require heating to 60-80°C to proceed at a measurable rate.
3. Competing Reactions: Oxygen in the air can oxidize certain analytes (e.g., I-) and must be excluded or corrected.

Applications

1. Determination of Fe2+ in ores, alloys, and pharmaceutical preparations.
2. Analysis of vitamin C (ascorbic acid) by iodometric titration.
3. Measurement of chemical oxygen demand (COD) in wastewater treatment.
4. Assay of hydrogen peroxide, chlorine, and other disinfectants in commercial products.