Starch is added near the endpoint of an iodine titration when the iodine concentration is low, as the starch-iodide combination is not highly soluble in water. This keeps I2-related errors from remaining adsorbed on the complex and going undetected.
Iodometry is one of the most significant redox titration methods. Many organic and inorganic compounds react immediately, quickly, and quantitatively with iodine.
Iodometry is a volumetric analysis that can be used to determine the amount of both reducing and oxidizing substances, through direct iodine titration or by titration of iodine with sodium thiosulfate (Na2S2O3).
Why is a starch added at the end of the reaction of iodometric titration and not in the initial stage?
When the iodine solution is lower, starch is added at the end. The trapped molecules of iodine do not react with Na S O, the endpoint is diffuse. If the starch was added earlier in the titration, an iodine-starch complex would develop, resulting in a reddish-brown endpoint.
Starch and iodine produce a deep blue complex, adding just a few drops of reducing agent will transform the solution from nearly black to clear. This is considerably easier to observe than the fading yellow color of the iodine alone.
The color change becomes noticeable only at the equivalence point, which is why we don't add the starch earlier. Iodine color fades more slowly on its own, giving a better idea of whether we are close to (pale yellow) or far from the equivalence point (red).
It means we can titrate more rapidly without the starch if we know we're not close there yet, and then adding the starch near the end makes it simpler to observe if any unreacted iodine remains.
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