Sciencemadness Discussion Board - NaBrO3

NaBrO3

SAM4CH - 17-12-2005 at 02:42

Is any one try to make sodium bromate? How?

garage chemist - 17-12-2005 at 06:23

Once I heard something about KClO3 being able to oxidise bromides to bromates and that the reaction would go to completion. If someone could give details about reaction conditions it would be most helpful.

[Edited on 17-12-2005 by garage chemist]

woelen - 17-12-2005 at 07:09

I made NaBrO3 (in solution) by electrolysis of a solution of NaBr with graphite anodes and swirling the solution every few minutes. This leads to formation of Br2 at the anode and H2 and OH(-) at the cathode.

The hydroxide ions at the anode react with Br2 as follows:

2OH(-) + Br2 --> Br(-) + BrO(-) + H2O

The BrO(-) ion is less stable than hypochlorite and it quickly disproportionates to bromate:

3BrO(-) --> 2Br(-) + BrO3(-)

When electrolysis is done for sufficiently long time, then a lot of the bromide is converted to bromate.

Isolating pure NaBrO3 is not easy. The salt is very soluble in water.
What I did was heating the mix after electrolysis and evaporating part of the water. This also assures that all hypobromite is converted to bromate. Then I added concentrated hot solution of KCl. After some time you get (impure) crystals of KBrO3. KBrO3 is only sparingly soluble. In this way I have made a few grams of KBrO3. I did not bother recrystallizing the solid, but if you want to have it purer, then you could recrystallize it from boiling hot water.

In fact, this reaction is much easier than making chlorate by electrolysis. It is much friendlier to the graphite anode, it does not pulverize as fast as with chlorate production and you do not have the problem that gas-bubbles escape from the liquid at the anode. You get best results if you use a current source instead of a voltage source. I used a constant current of approximately 1 A and during the electrolysis the voltage slowly increases and I stopped when quite some oxygen bubbles were formed at the anode (indicating that the concentration of Br(-) is dropping).

@garage chemist: I did an experiment with my KBrO3, adding it to hydrochloric acid. What I observed is quite vigorous formation of chlorine gas and bromine. Bromate can be formed from bromide, but only at a very strong excess amount of chlorine in aqueous medium, otherwise the reaction proceeds in the opposite direction, as demonstrated by my experiment.

HBrO3(-) + 5HCl <---> 2Cl2 + BrCl + 3H2O

The color of the gas I obtained is not the nice green color of chlorine, but a somewhat green/yellow gas mix with a much stronger color.

http://woelen.scheikunde.net/science/chem/exps/expshow.cgi?i...

[Edited on 17-12-05 by woelen]

garage chemist - 17-12-2005 at 09:22

You're right, electrolysis is probably the best way to obtain bromates.

The bromate is best isolated as KBrO3 by adding KCl (or maybe KBr must be used?) solution and cooling, KBrO3 has similar solubility as KClO3 and precipitates in the cold.

BromicAcid - 17-12-2005 at 09:34

Remember that the order of reactivity is reversed for the oxoacids. Hypochlorite will oxidize bromide to hypobromite and not the other way around, so the following reaction can easily and quickly be carried out with sodium bromide and bleach:

Br-(aq) + ClO<sup<-(aq) ---> BrO-(aq) + Cl-(aq)

This reaction is fairly fast even in the cold and is evidenced by the reaction mixture turning slightly orange. Then as Woelen stated hypobromite is fairly unstable with respect to hypochlorite and with moderate heating it should disproportionate to the bromate and the bromide. The only problem being that bleach has a bunch of crap in it, when you precipitate with KCl you're going to get chlorate too and bromate and maybe some other junk.

Jome - 17-12-2005 at 12:59

Would the same thing happen to the higher oxoacids, like turning chlorates into bromates by adding bromide?

The_Davster - 17-12-2005 at 13:23

All of the halogens dissolve in potassium carbonate solution giving the potasium halate salt. I have made potassium iodate this way.

BromicAcid - 17-12-2005 at 14:19

Bromides should go to bromates with chlorate, normally though chlorate is kinetically slow to react unless it is under acidic conditions though (isn't it?), but really I don't know how fast the reaction would go, the one with bromide and hypochlorite goes almost to completion fairly rapidly, even at low temperatures, maybe the chlorate reaction does the same?

A little side note building off what Rogue Chemist had to say, how does elemental bromine react with solid potassium carbonate?

garage chemist - 18-12-2005 at 03:05

Quote:

[quote="BromicAcid"]Remember that the order of reactivity is reversed for the oxoacids. Hypochlorite will oxidize bromide to hypobromite

Sorry, but I don't get the logic behind this.

Normally, chlorine is more reactive than bromine, meaning that when chlorine is bubbled into a bromide solution oxidation of the bromide occurs and elemental bromine is formed.
Now, when hypochlorite can oxidise bromide to hypobromite, doesn't this mean that hypochlorite is more reactive than hypobromite?
Based on this, I would assume that the order of reactivity of the halogens is THE SAME when it comes to their oxoacids.
Or is there a different logic behind this?

woelen - 18-12-2005 at 05:00

Quote:

Originally posted by BromicAcid
Bromides should go to bromates with chlorate, normally though chlorate is kinetically slow to react unless it is under acidic conditions though (isn't it?), but really I don't know how fast the reaction would go, the one with bromide and hypochlorite goes almost to completion fairly rapidly, even at low temperatures, maybe the chlorate reaction does the same?

A little side note building off what Rogue Chemist had to say, how does elemental bromine react with solid potassium carbonate?

I don't expect any reaction, as long as the compounds are perfectly dry (this would be an interesting experiment though

). When some water comes into the mix, then of course you get CO2 and bromide and hypobromite, which quickly will disproportionate to bromate and bromide.

Purely looking at reaction equations, you can get a reaction between carbonate and bromine:

K2CO3 + Br2 ---> (K2O,CO2) + Br2 ---> KBrO + KBr + CO2

I strongly doubt that this reaction will occur in reality, without the presence of water.

halogen - 18-12-2005 at 05:13

So, NaClO4 + NaBr --> NaCl + NaBrO4
or is it
NaClO4 + NaBr --> NaBrO3 + NaClO

woelen - 18-12-2005 at 05:32

I think none of these. In aqueous solution the perchlorate ion is more or less inert, so under that kind of conditions you will not have any reaction at all.

In the molten state I expect loss of oxygen or nothing at all. NaClO is unstable, when not in solution, so the second equation will not happen.
You certainly won't get NaBrO4. Perbromates can be made by passing F2 through an alkaline solution of NaBrO3 or KBrO3 or by reacting KBrO3 with XeF2 and H2O.

BromicAcid - 18-12-2005 at 06:06

You cannot make perbromate by reaction of perchlorate with bromide, this was one of the first attempted procedures by which to synthesize perbromate and was found to not work. Garage Chemist, although the logic behind it is like you and I said, the hypochlorite oxidizes bromide to hypobromite, so it's not really an inversion of activity, earlier chemists simply thought that bromide displaced chloride in hypochlorite since the overall effect is hard to discern from the oxidation of bromide to hypobromite, or at least that's how I look at it. As I quoted in the 'garage experiments thread'

Quote:

Chlorine will readily react with trace quantities of bromide to form hypobromous acid.

HOCl + Br- ---------> HOBr + Cl- (4.21)

The speed of this reaction is such that it rapidly proceeds until there is very little bromide left. The hypobromous acid formed can then react with organic matter reforming bromide which completes the cycle.....

Therefore, in the presence of bromide, the active halogen shifts from chlorine to bromine. This is significant, because bromine may react differently with organic matter than chlorine does.

This holds true for most other similar conditions, providing the respective oxyacid or salt thereof is stable, perbromates cannot be made and one of the iodine salts is also unstable, other then that it's kind of reliableish this reversal of reactivity. I read it in an older chemistry book like that and repeat it verbatim without explaining it much most of the time. Because that is what it looks like, it looks like the bromine is replacing the chlorine in the hypochlorite and that’s what it looks like in other similar reactions too.

Jome - 19-12-2005 at 01:56

If the electronegativity of the halogen is higher it's desire to be oxidisied is less*, like in standard redox? Is that why the reaction occurs?

*Because it is closer to the electronegativity of O.

Zinc - 11-7-2006 at 13:55

You here have made NaBrO3 by the electrolysis of a solution of NaBr and then added KCl to precipitate KBrO3. Since I haven't any NaBr but I have some KBr and I was thinking to electrolyse a solution of KBr to make KBrO3. Since KBrO3 is not very soluble would it precipitate during the electrolysis at 25 deggres celsius and if it would precipitate would it casue some problems during the electrolysis?

garage chemist - 11-7-2006 at 14:16

Read this thread, especially my replies:
https://sciencemadness.org/talk/viewthread.php?tid=5726#pid6...

Crystallizing KBrO3 will not cause problems- a part of it just crystallizes at the bottom, and electrolysis continues. Nothing wrong with that. After electrolysis, cool to 0°C to precipitate most of the KBrO3 to isolate it.