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
In redox titration, the titrant is either an oxidizing agent such as potassium permanganate, iodine, or cerium(IV), or a reducing agent such as sodium thiosulfate or ferrous ammonium sulfate. The reaction proceeds until all of the analyte has been oxidized or reduced, at which point the equivalence point is reached. The endpoint can be detected using a redox indicator such as starch for iodine or ferroin for cerium, or by potentiometric measurement.
Common Redox Titration Methods
Permanganate titration uses KMnO4, a deep purple strong oxidizing agent in acidic medium, with the endpoint detected by the persistence of a faint pink color; this method is used for analyzing Fe2+, oxalate, and hydrogen peroxide. Iodometric titration uses iodine (I2) 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. Dichromate titration uses K2Cr2O7 as a primary standard oxidizing agent for the determination of Fe2+ and organic compounds, offering the advantage of being stable and not requiring daily standardization. Cerimetric titration uses cerium(IV) sulfate as a strong oxidizing agent for determining Fe2+, As3+, and organic compounds in acidic solution.
Endpoint Detection
Visual indicators include starch (blue-black complex with I2), ferroin (red to pale blue with Ce4+), and diphenylamine (colorless to violet with Cr2O72-). For potentiometric detection, a platinum indicator electrode measures the change in potential during the titration, and the endpoint corresponds to the steepest point on the titration curve. Self-indication occurs with KMnO4, which serves as its own indicator as the intense purple color appears once excess permanganate is present.
Factors Affecting Accuracy
Many redox reactions require specific pH conditions — permanganate titrations must be performed in strong acid (1-2 M H2SO4) to prevent MnO2 formation. Some reactions such as permanganate-oxalate require heating to 60-80°C to proceed at a measurable rate. Oxygen in the air can oxidize certain analytes such as I- and must be excluded or corrected for.
Applications
Redox titration is used for determination of Fe2+ in ores, alloys, and pharmaceutical preparations; analysis of vitamin C (ascorbic acid) by iodometric titration; measurement of chemical oxygen demand (COD) in wastewater treatment; and assay of hydrogen peroxide, chlorine, and other disinfectants in commercial products.
