Why does blue suddenly disappear in a classic iodine titration?
Sodium thiosulfate reduces iodine to colorless iodide:
I2 + 2 S2O3^2- -> 2 I- + S4O6^2-
Near the endpoint, starch makes the last traces of iodine intensely blue. When thiosulfate consumes that iodine, the blue vanishes.
Because the reaction has a fixed stoichiometric ratio, the color change becomes a quantitative measurement.
The same reducing chemistry also allows thiosulfate to neutralize residual chlorine in water
Global Chemical Database