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From a description of my experiment:

Potassium iodide crystals are place in the middle of filter paper soaked with potassium sulphate and phenolphthalein. Two electrodes are placed on the filter on two opposite sides. The electrodes are connected to a battery. The objective of this experiment is to see the movement of ions.

Why soak the filter paper in potassium sulphate and not just soak it in potassium iodide?

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For this experiment, you'll be seeing an interesting color change when Iodine is oxidized at the anode to form the brownish $\ce{I2}$ on the electrode and yellow $\ce{I3-}$ in solution.

The reduction of $\ce{H+}$ to form $\ce{H2~(g)}$ and $\ce{OH-}$ will turn the indicator pink at the cathode.

The cool thing about your experiment is that you will not see any redox reaction at all (at either electrode) until iodide ions diffuse to the anode! That is, unless you use a high voltage battery, $\gtrapprox2.0~\mathrm{V}$, you will not have sufficient energy to oxidize $\ce{SO4^2-}$ to $\ce{S2O8^2-}$.

This makes the experiment an exercise in patience, and then sudden color display! Measuring the time it takes can yield data that can help you calculate the diffusion coefficient of $\ce{I-}$ if you wanted to take it another step!

In summary: Soaking the filter paper with KI would result in the electrodes producing color changes immediately. I suspect the sulfate was used in this experiment to illustrate the diffusion of dissolved ions, because the redox reaction would not take place until the iodide ions reached the anode.

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  • $\begingroup$ What is S2O4? Presumably you mean oxidation of sulfate to persulfate or something, but could you correct the formulae? $\endgroup$ – Curt F. Jul 28 '15 at 0:41
  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. I agree with Curt, that S2O4 does not make much sense, maybe you mean $\ce{2SO4^2- <=> S2O8^2- + 2e-}$? $\endgroup$ – Martin - マーチン Jul 28 '15 at 3:28
  • $\begingroup$ I have updated your post with chemistry markup. If you want to know more, please have a look here and here. Please do not use markup in the title field, see here for details. States of aggregation should not be subscripted, it is not wrong, but the recommendations (Sec. 2.1.) are different. $\endgroup$ – Martin - マーチン Jul 28 '15 at 3:54
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    $\begingroup$ Whilst this is a good answer I don't quite see how it answers the question. Perhaps you could be more specific. $\endgroup$ – bon Jul 28 '15 at 9:50
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    $\begingroup$ @Dan Burden You should edit this into your answer. $\endgroup$ – bon Jul 28 '15 at 13:11

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