Redox Titration: Iodimetry and Iodometry

Iodometry and iodimetry are analytical methods used to determine the concentration of oxidizing or reducing agents using iodine. In iodometric titrations, the oxidizing analyte solution is usually acidified and treated with an excess of iodide ions, which generates an equivalent amount of iodine in equilibrium with triiodide. The released iodine is subsequently titrated directly against a standardized reducing agent. As the dilute iodine color becomes pale yellow, a few drops of freshly prepared starch solution are added, creating an intense dark blue starch–iodine complex. The titration is continued until the disappearance of the blue color, allowing the determination of the analyte's concentration through the stoichiometric relationship between iodine, the oxidizing analyte, and the reducing titrant. The titration should be performed quickly and vigorously stirring for accurate results.

In iodimetric titrations, a reducing analyte is titrated directly against a standard iodine solution in the presence of starch. The starch acts as an indicator, forming a dark blue starch–iodine complex when the iodine is in excess. As the titration progresses, the reducing analyte reacts with the iodine, causing the iodine concentration to decrease. When all the reducing analyte has reacted and is completely oxidized, any additional iodine added will not be reduced, resulting in the formation of the dark blue starch–iodine complex. This color change signifies the endpoint of the titration.

To determine the concentration of the analyte, the stoichiometry of the reaction (represented by the balanced chemical equation) and the volume of the standard iodine solution used during the titration are considered. By knowing the amount of iodine that reacted with the analyte and the stoichiometric relationship between the iodine and the analyte, the concentration of the reducing analyte in the original sample can be accurately determined.