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my mentor told me that the KMnO4 solution specifically for TLC has added some bases like sodium hydroxide but we don't know why. I try to search on google, and found that the bases are usually by-products in the industrial method when preparing KMnO4 from Wiki.

Which are,

2 K2MnO4 + 2 H2O → 2 KMnO4 + 2 KOH + H2

3 K2MnO4 + 2 CO2 → 2 KMnO4 + 2 K2CO3 + MnO2

But is there any reason why they keep the bases?

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    $\begingroup$ KMnO4 is a bit too strong oxidiser to be totally stable in water and high pH lowers redox potentials. $\endgroup$
    – Mithoron
    Jun 24, 2023 at 15:17
  • $\begingroup$ The first reaction is the summary reaction of an electrolytic oxidation of $\ce{K2MnO4}$. $\endgroup$
    – Poutnik
    Jun 24, 2023 at 15:20

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Two reasons come to mind:

  • It's made that way. Potassium permanganate is made by oxidizing manganese dioxide (available as the mineral pyrolusite) with air in concentrated potassium hydroxide (to form initially potassium manganate (VI), $\ce{K2MnO4}$), and then diluting the solution (which disproportionates the potassium manganate(VI) to $\ce{KMnO4}$ plus part of the original $\ce{MnO2}$). Note that the manganate(VI) formation step does not work with sodium hydroxide, which explains why we usually have the potassium salt to begin with.

  • We don't want to tempt fate. Potassium permanganate solutions are only meta-stable. But it turns out that their decomposition tends to consume aqueous hydrogen ions. So starving the water of these can be expected to improve shelf life:

$\ce{4MnO4^- + 4H^+ -> 4 MnO2(s) + 2H2O + 3O2}$

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  • $\begingroup$ may I ask why the Mn(VI) formation cannot be carried out by NaOH? $\endgroup$ Jun 25, 2023 at 9:27
  • $\begingroup$ I have to guess on the explanation. Possibly the sodium compound is somehow stabilized at the Mn(V) stage, where the oxidation stops with sodium hydroxide. The less efficient oxidation of manganese would then cut down on permanganate yield upon disproportionation. $\endgroup$ Jun 25, 2023 at 10:29

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