Acid and iodide ions are added to bleach forming iodine. This is brown, but a blue filter is selected for colorimetry. This can then be titrated against sodium thiosulphate. The thiosulphate ion reacts with the liberated iodine to form a straw yellow collour. Starch can then be added at this stage turning the solution blue, and then more thiosulphate can be added untill it turns colourless.
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