A redox titration is a type of titration based on a redox reaction between the analyte and titrant. Redox titration may involve the use of a redox indicator and/or a potentiometer. Common examples of a redox titration is treating a solution of iodine with a reducing agent and using starch as an indicator. Iodine forms an intensely blue complex with starch. Iodine (I2) can be reduced to iodide (I−) by e.g. thiosulphate (S2O32−), and when all iodine is spent the blue colour disappears. This is called an iodometric titration.
Most often of all, the reduction of iodine to iodide is the last step in a series of reactions where the initial reactions are used to convert an unknown amount of the solute (the substance you want to analyze) to an equivalent amount of iodine, which may then be titrated. Sometimes other halogens (or halogenoalkanes) than iodine are used in the intermediate reactions because they are available in better measurable standard solutions and/or react more readily with the solute. The extra steps in iodometric titration may be worth while because the equivalence point, where the blue turns a bit colourless, is more distinct than some other analytical or may be by volumetric methods.