During an iodometric titration iodide is oxidised to iodine by an oxidiser, and then titrated using standardised sodium thiosulfate. I have had no problems using iodate as the oxidiser, however, using sodium hypochlorite (bleach) causes trouble for some reason. Instead of the iodine dissolving in the solution upon addition of acid/KI, the iodine forms black crystals which fail to dissolve even upon addition of sufficient sodium thiosulphate (even when excess is added). The solution is still black, but the crystals become readily apparent during titration.

I suspected this to be due to insufficient KI, and have used both solid and aqueous KI, however I have tested this and this is not true.

Does anyone know what is causing the crystals to form?


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