Iodometry
Iodometry, also known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point.
Note that iodometry involves indirect titration of iodine liberated by reaction with the analyte , whereas iodimetry involves direct titration using iodine as the titrant.
Iodometry is commonly used to analyse the concentration of oxidizing agents in water samples, such as oxygen saturation in ecological studies or active chlorine in swimming pool water analysis. To a known volume of sample, an excess but known amount of iodide is added, which the oxidizing agents oxidizes iodide to iodine. Iodine dissolves in the iodide-containing solution to give triiodide ions, which have a dark brown color.
The triiodide ion solution is then titrated against standard thiosulfate solution to give iodide again using starch indicator:
Together with reduction potential of thiosulfate:
The overall reaction is thus:
For simplicity, the equations will usually be written in terms of aqueous molecular iodine rather than the triiodide ion, as the iodide ion did not participate in the reaction in terms of mole ratio analysis.
The disappearance of deep blue color due to the decomposition of the iodine-starch clathrate marks the end point
The reducing agent used does not necessarily need to be thiosulfate; stannous chloride, sulfites, sulfides, arsenic(III), and antimony(III) are commonly used alternatives. At higher pH (> 8)) At low pH would also react with the thiosulfate:: :S2O32− + 2 H+ → SO2 + S + H2O
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