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Author: Subject: Preparation of Bromine
Tsjerk
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[*] posted on 5-8-2021 at 08:35


Yes, so the salt and sulfuric acid together make Caro's acid. Without acid the salt is quite tame.
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Keras
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[*] posted on 5-8-2021 at 10:08


Quote: Originally posted by Tsjerk  
Yes, so the salt and sulfuric acid together make Caro's acid. Without acid the salt is quite tame.


But the redox potential of the anion is unchanged.
Wikipedia says the redox potential of HSO₅⁻ + 2 H⁺ + 2 e⁻ → HSO₄⁻ + H₂O is +1.81 V

2 Br⁻ → Br₂ + 2 e⁻ E = -1.025 V

So: HSO₅⁻ + 2 H⁺ + 2 Br⁻ → HSO₄⁻ + H₂O + Br₂ has ∆E ~ 0.8 V

Ok, you need to use an acid to get the reaction going, but HCl should suffice.

[Edited on 5-8-2021 by Keras]
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RustyShackleford
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[*] posted on 5-8-2021 at 14:13


Quote: Originally posted by Keras  
Quote: Originally posted by RustyShackleford  
Oxone works very well. I obtained an 88% yield using: 50g NaBr, 80g oxone and 60g 50% sulfruric acid.


What's the purpose of the sulphuric acid here? Isn't oxone already oxidising enough to get the bromine out?

in retrospect i believe it may be unnecessary, as i have heard from someone else that it works without any acid at all (atleast in the case of pure persulfate). I added it because i calculated/designed the reaction using the normal mechanism w peroxide, which relies on HBr forming.
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[*] posted on 5-8-2021 at 14:15


Quote: Originally posted by Keras  
Quote: Originally posted by Tsjerk  
Yes, so the salt and sulfuric acid together make Caro's acid. Without acid the salt is quite tame.


But the redox potential of the anion is unchanged.
Wikipedia says the redox potential of HSO₅⁻ + 2 H⁺ + 2 e⁻ → HSO₄⁻ + H₂O is +1.81 V

2 Br⁻ → Br₂ + 2 e⁻ E = -1.025 V

So: HSO₅⁻ + 2 H⁺ + 2 Br⁻ → HSO₄⁻ + H₂O + Br₂ has ∆E ~ 0.8 V

Ok, you need to use an acid to get the reaction going, but HCl should suffice.

[Edited on 5-8-2021 by Keras]

HCl would be terrible in this case, as it would form chlorine gas and interhalogens. bisulfate would likely work though, for an alternative to sulfuric.
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