[shelanachium]

Perbromate [BrO4]- though pretty stable once made (only perchlorate among oxyhalogen ions is less reactive) proved very elusive for decades but was finally obtained by F2 oxidation of bromate solutions. All the reactions producing it appear to involve atomic oxygen as an intermediate.

[FeO4]2- is stable only in strongly alkaline solutions and is one of the strongest oxidants that can exist in aqueous solution. Acidify and you get O2 and Fe3+. I suspect that atomic O may form on acidification of ferrate solutions - has anyone tried adding ferrate (VI) to an acidified bromate solution to see if perbromate results? Perbromate like perchlorate forms salts of limited water-solubility with large monovalent cations such as K+ and heavier alkali metals, and organic cations. This makes it easy to separate from bromate. Or if only small amounts are obtained, it could be co-precipitated with a perchlorate.

[woelen]

I do not expect a good yield in this reaction, because of the transient properties of the medium in which it would occur. In the F2 oxidation, an alkaline solution of KBrO3 is used and this stable situation is maintained and F2 is bubbled through the solution. An alternative is adding XeF2 to such a solution.

In your suggestion, however, no perbromate can be formed in the alkaline ferrate(VI) solution, and certainly not in the acidic Fe(3+) solution. So, only during the very short time in which there is atomic oxygen, there could be a reaction. I expect that only O2 will be produced on acidification.

I could try this experiment. I have bromate salts (K and Na salts), and I also have iron(III) compounds. Making the ferrate(VI) is quite cumbersome though. If I get any perbromate, how could I easily detect this in the big mess, which will be produced?

Your idea is interesting though...

[shelanachium]

If [BrO4]- forms it would co-precipitate with (say) KClO4, the formation of any bromide on reduction of the precipitated KClO4 would indicate [BrO4]- had been present. The precpitate would need careful washing (ethanol/water?) to remove any included bromate.

I see the problem if the bromate solution must be strongly acid. The [FeO4]2- solution must be alkaline for ferrate to persist; might it be possible to use a bromate solution buffered to just the acid side of neutrality so that ferrate still has high oxidising power but the pH is more favourable for [BrO4]- formation?

[cybercavalier]

I can only give thoughts...

https://chemistryfanatics.wordpress.com/2013/06/29/experiment-4-preparation-of-potassium-ferrate/
Fe + 3 KNO₃ + 2 KOH  K₂FeO₄ + 3 KNO₂ + H₂O

As Potassium Ferrate can be prepared burning from iron fillings and potassium nitrate, can bromate and nitrate be burn to produce perbromate? That is to say:
KBrO₃ +  KNO₃ KBrO₄+ KNO₂
Anhydrous condition can be used...

To remove KNO₂, ammonium bromide could be used to produce more potassium bromide that could be recycled. But Ammonium nitrite and nitrate (just in case excess of KNO₃) are explosive in nature...

Ammonium nitrite, nitrate, potassium bromide are much water soluble than perbromates. So precipitation separation could work...