# Why can't iodide be oxidised to iodate in acidic medium?

Even if we use a strong oxidising agent like 'permanganate ion in acidic medium' it doesn't help oxidise iodide to iodate, but a relatively weaker oxidising agent 'permanganate ion in alkaline medium' helps to form iodate from iodide.

I have found one discussion similar to this but it's not convincing.

Even in alkaline medium the potential is negative although significantly lesser than in acidic medium. Then how does the reaction even proceed in alkaline but not in acidic?

• Draw yourself a chart of standard redox potentials for eventually involved redox systems as functions of pH, involving formally and implicitly [H+] or [OH-] to the redox potentials pH dependent values. See en.wikipedia.org/wiki/… Oct 18, 2020 at 7:43
• Note that there can be kinetic reasons, as $\ce{IO-}$ is more stable in alkaline solutins than $\ce{HIO}$ in acidic one. Oct 21, 2020 at 10:36

Permanganate oxidizes iodide to iodine $$\ce{I2}$$ at all pH values. In acidic conditions, the reaction is finished here. But in basic solution, the iodine $$\ce{I2}$$ is transformed into iodide and iodate according to $$\ce{3I2 + 6 OH^- -> 5 I- + IO3^- + 3 H2O}$$The mixture iodide + iodate reacts the other way as soon as the solution becomes acidic : $$\ce{5I^- + IO3^- + 6 H+ -> 3 I2 + 3 H2O}$$