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I'm currently doing an IB chemistry individual investigation on the effect of adding bleach to water on the water's dissolved oxygen content. I've used Winkler's method for this and I'm just wondering whether it is possible to do Winkler's method without the presence of iodide ions $(\ce{I-})$, i.e. without adding chemicals like potassium iodide?

I used $\ce{NaOH},$ $\ce{MnSO4},$ $\ce{H2SO4}$ in this experiment (and $\ce{Na2S2O3}$ as the titrant) and I'm not too sure whether I used potassium iodide. However, I've obtained a result and found out a relationship where the dissolved oxygen content increases as I add more bleach into the water.

So my question is: is it possible to do a Winkler's method experiment without the presence of iodide ions? If so, could I know how my experiment worked?

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  • $\begingroup$ You shouldn't have used the winkler method for bleach analysis. Why not just stick to the iodine-thiosulfate method given in the textbook? Pg 421 in the Pearson HL book. $\endgroup$ – 124c41 Oct 22 '19 at 2:51
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Bad news is that Winkler's method will fail in the presence of bleach or any other oxidizing agent besides oxygen. Neither sodium thiosulfate nor potassium iodide are compatible with bleach, whether it is a oxygen bleach or a chlorine bleach.

I used $\ce{NaOH},$ $\ce{MnSO4},$ $\ce{H2SO4}$ in this experiment (and $\ce{Na2S2O3}$ as the titrant) and I'm not too sure whether I used potassium iodide. However, I've obtained a result and found out a relationship where the dissolved oxygen content increases as I add more bleach into the water.

As a good scientist of future, you always write whatever you are doing at the time of experiment in your notes.

Your results are skewed towards higher numbers because sodium thiosulfate is being consumed by released iodine not only from the oxygen reaction but also from the bleach.

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