Why do you have to add a weak acid like acetic acid to liberate the iodine when titrating sodiumhypochlorite?


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John McCann Profile
John McCann answered

Titrating sodium hypochlorite against what? Acetic acid? There isn't iodine in either of these species.

What is the indicator?

thanked the writer.
Marlon Delchot
Marlon Delchot commented
Thanks John. The titration is carried out with thiosulphate (Na2S2O3)solution after adding potassium jodide (KI). The iodine which is then formed is titrated using starch as indicator. In the meantime I found the answer myself. There might be small traces of calciumchlorate (CaClO3) present. This will decompose if a strong acid like hydrochloric acid is added. The chlorate will also liberate iodine from KI, which is then titrated too. Ofcourse this will lead to higher (false) results.
John McCann
John McCann commented
That makes a bit more sense!

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