Is there a difference between potassium manganate and permanganate? [duplicate]








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[Nomenclature of MnO4^-: is it an oxyanion?](/questions/32985/nomenclature-of-mno4-is-it-an-oxyanion)

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Is potassium manganate the same as potassium permanganate?


Also, I have seen potassium manganate written as potassium manganate (VII) or potassium manganate (VI). 


What do the roman numerals stand for?




No, they are somewhat different. $\ce{K2MnO4}$ is potassium manganate and $\ce{KMnO4}$ is potassium permanganate. Notice the two potassium for each $\ce{MnO4}$.


The Roman numerals mark the oxydation state. So, manganate will have a (VI) and permanganate will have a (VII).


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There are two different nomenclature concepts at work here. One includes *potassium manganate* and *potassium permanganate* and the other uses *potassium manganate(VII)* and *potassium manganate(VI).* These two do not mix, and each of the two has an unambiguous name for each compound. Thus, to answer your title question, potassium permanganate and potassium manganate are **not** the same.


The more easily explained difference is that between manganate(VI) and manganate(VII) (the cation is irrelevant; as always). The Roman numerals represent oxidation states as they always do. In both cases, we are dealing with $\ce{MnO4^n-}$ and the oxidation state numeral tells us the exact charge. If you do your maths correctly, you will arrive at the following:


$$\begin{align}\text{manganate(VI)} && \ce{\overset{{+VI}}{Mn}\overset{-II}{O}\_4^n-} & \Longrightarrow &&\ce{MnO4^2-}&& \Longrightarrow &\ce{K2MnO4}\\
\text{manganate(VII)} && \ce{\overset{{+VII}}{Mn}\overset{-II}{O}\_4^n-} & \Longrightarrow &&\ce{MnO4-}&& \Longrightarrow &\ce{KMnO4}\end{align}$$


The two substances differ by the number of potassium counterions present in the crystal; potassium manganate(VI) has two per manganate(VI) anion while potassium manganate(VII) has one.


For reasons not entirely known to me, it was decided once that manganate(VI) — $\ce{MnO4^2-}$ — should be the ‘standard manganate’. It fits in well if the analogy to sulphate ($\ce{SO4^2-}$) and chromate ($\ce{CrO4^2-}$) which all have the $\mathrm{+VI}$ oxidation state and same charge; but that’s the last difference. *-ate* names are *not* commonly used for the highest oxidation state only (e.g. chlorate $\ce{ClO3-}$) and they do *not* refer to a certain oxidation state (e.g. phosphate with the oxidation state $\mathrm{+V}$). Whatever the reasons were, manganate was decided to be $\ce{MnO4^2-}$, i.e. the manganate(VI) you just heard about above.


The other ‘manganate’-like anion, manganate(VII) from above, is in a *higher* oxidation state. At least this part of the nomenclature is consistent: the higher oxidation state is marked by the prefix *per.* Thus, *permanganate* must have a higher oxidation state than *manganate.*


**Summary:** *Manganate* **always** maps to *manganate(VI)* and vice-versa; *permanganate* **always** maps to *manganate(VII)* and vice-versa.


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