I am trying to oxidize toluene to a mixed salt $\ce{K/Na}$ benzoate/ benzoic acid using the potassium permanganate method, but I am using OTC sourced $\ce{KMnO4}$ which has a significant amount of sodium sulfite contamination.

Will that hinder the reaction? And is there a practical method for filtering said contaminants?

reaction vessel

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    $\begingroup$ I don't believe KMnO4 can have sodium sulfite in it. Sulfate, maybe. $\endgroup$ – Ivan Neretin Dec 7 '17 at 13:14
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    $\begingroup$ As @Ivan Neretin said. $\ce{KMnO4}$ is such a strong oxidiser that no sulphite could co-exist with it. It would be oxidised to sulphate. $\endgroup$ – Gert Dec 7 '17 at 13:19

How do you know your permanganate contains sodium sulphite?

It seems highly unlikely because $\ce{KMnO4}$ is a very strong oxidiser and sulphite is very easily oxidised to sulphate:

$$\ce{SO3^{2-}\to SO4^{2-} +2e-}$$

Also, it seems improbable to find a sodium salt in a potassium salt.

Even if the $\ce{KMnO4}$ contained some "surviving" $\ce{Na2SO3}$, during the oxidation of the toluene, it would quickly be oxidised to sulphate. At worst your benzoic acid yield would suffer slightly.

  • $\begingroup$ Sorry I wrote the wrong one. The KMnO4 is a filtering aid for home green sand water filtration systems and on the container it states that there is sodium sulfate at roughly 20-40% with KMnO4 at 60-100% $\endgroup$ – James Dec 14 '17 at 3:38

You can test any significant amount of sulfite contamination in $\ce{KMnO4}$ soln. using anion analysis. You can add dil. $\ce{HCl}$ to the soln. to evolve any sulfur dioxide gas or add any barium salt to produce $\ce{BaSO3}$(white precipitate).

But I don't think there would be any amount of sulfite left in the soln. as sulfite would be oxidized to sulfate as $\ce{KMnO4}$ is strong oxidizer.

$$\ce{3Na2SO3 + 2KMnO4 + H2O → 3Na2SO4 + 2MnO2 + 2KOH}$$

$\ce{MnO2}$ is a black insoluble salt that would turn the product solution black. It can be decanted/filtered off. Both sodium sulfate and potassium hydroxide are soluble and it is hard to distill off. But no need to worry, amount of sulfite contamination is so little that it is hardly taken into consideration. And what worse could happen? During oxidation of the toluene, sulfite would quickly oxidised to sulphate and your benzoic acid yield would suffer slightly by a little amount and that amount is insignificant.


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