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    It takes up the iodine produced. Once its used up, the iodine can react with other iodine ions to form a complex with starch- solution turns dark. By knowing how much thiosulhate you have, you can also determine the rate of reaction.
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    it turns iodine into iodide ions, then when all the sodium thiosulfate has reacted with the iodine, as soon as the new iodine forms it can react with the starch giving a sharp colour change.
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    essentially it delays the starch reacting with the iodide so you can time the initial rate of reaction i think
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Updated: February 1, 2017
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