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Author: Subject: Reaction between Potassium Bromate and sulfuric acid?
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[*] posted on 20-6-2021 at 15:36
Reaction between Potassium Bromate and sulfuric acid?


Can anyone add a small amount of sulfuric acid to a KBrO3 solution and tell me if elemental bromine is produced? I'm hoping some of you have some KBrO3 laying around :)

From what I've read online, you need KBrO3 and KBr together to get elemental bromine. And in NileRed's Belousov–Zhabotinsky reaction video, you can see him add sulfuric acid to KBrO3, and the solution stays clear.

I am trying the Belousov–Zhabotinsky reaction, and I have all of the compounds needed, but it is not working. When I add sulfuric acid to the KBrO3, which is the first step in my procedure, it produces copious amount of bromine, which is not supposed to happen. I'm confident the procedure I have is correct, and that I am following it correctly.

So I think this is what is causing the BZ reaction not to work, but I'm not sure why it the bromine is being produced at this early step. The KBrO3 is from Onyxmet, and I assume it is pure. The sulfuric acid, however, is Rooto drain cleaner. It is fairly clear, but I wonder if the impurities are causing the bromine to be created. I could order pure sulfuric acid and try that out, but I'd only like to go that route if I have some confidence that it will solve the issue.

Thanks in advance!
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[*] posted on 20-6-2021 at 18:01


If you add conc. sulfuric acid directly on to solid KBrO3, HBrO3 is formed which than decompose in to Br2, O2 and H2O (HBrO3 is unstable at concentrations >50%). But if you add it in to dilute solution, than it doesn't form bromine. Try to make some dilute sulfuric acid from your drain cleaner and add small amount of KMnO4 and heat it for a few minutes (don't do it with conc. H2SO4 because of formation of Mn2O7). If solution will decolorize, your sulfuric acid contain some organic impurities which react with KBrO3 to form Br2.

But it will be better to use pure stuff, BZ reaction is very sensitive to impurities and ratios of reagents.




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[*] posted on 21-6-2021 at 01:20


If you add dilute (e.g. 20%) H2SO4 to either solid KBrO3 or to a solution of KBrO3, then you should not get any bromine. If your acid contains reducing agents (e.g. organics), then you might get bromine with such dilute acid.

If you add concentrated H2SO4 to solid KBrO3, then you'll see bubbles of oxygen and fumes of Br2.

If you add concentrated H2SO4 to a solution of KBrO3, then you might get some bromine. At the place where the acid still is concentrated and where it gets in contact with solution of KBrO3, the total acid concentration may be so high that the formed HBrO3 decomposes. For this reason, it is important to first dilute the H2SO4 and then add this to the bromate.

As Bedlasky wrote, use pure acid, any reducing material in the acid will interfere with the BZ-reaction.




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[*] posted on 21-6-2021 at 06:33


Bedlasky, thanks for the reply. I added 6M sulfuric acid to the KBrO3 solution, which is what the procedure called for. I'll try your test and see if the solution decolorized.

I'd be curious to see what happens when anyone else adds 6M H2SO4 to a KBrO3 solution, whether it produces bromine or not, but I supposed I'll end up getting more pure sulfuric acid regardless.

Adding concentrated H2SO4 to solid KBrO3 sounds like a quick, easy way to make some concentrated bromine water.
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[*] posted on 21-6-2021 at 06:39


Quote: Originally posted by woelen  
If you add dilute (e.g. 20%) H2SO4 to either solid KBrO3 or to a solution of KBrO3, then you should not get any bromine. If your acid contains reducing agents (e.g. organics), then you might get bromine with such dilute acid.

If you add concentrated H2SO4 to solid KBrO3, then you'll see bubbles of oxygen and fumes of Br2.

If you add concentrated H2SO4 to a solution of KBrO3, then you might get some bromine. At the place where the acid still is concentrated and where it gets in contact with solution of KBrO3, the total acid concentration may be so high that the formed HBrO3 decomposes. For this reason, it is important to first dilute the H2SO4 and then add this to the bromate.

As Bedlasky wrote, use pure acid, any reducing material in the acid will interfere with the BZ-reaction.


Thanks Woelen, this is helpful. I had diluted the 93% rooto down to 6M (~33%) before adding it. I was adding the 6M H2SO4 (~33%) with a pipette to the KBrO3 solution. It produced a good amount of bromine and I did not see any oxygen bubbles. I think I can do a couple of tests to determine if the issue is the concentration of H2SO4, or the presence of organic impurities in the rooto drain cleaner.

I'll report back!
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[*] posted on 21-6-2021 at 11:26


I can try adding appr. 35% high grade acid to pure KBrO3, but I do not think that this leads to formation of bromine. I'll come back on that, probably tomorrow, now I don't have the time for that.
Do you get so much bromine that you get fumes and pure liquid bromine, which settles at the bottom?




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[*] posted on 21-6-2021 at 14:00


There is no visible reaction of any kind without heating between the practical grade Na salt and technical, battery, or good drain cleaner acid (which, unamerikans will be thrilled to hear, is extremely OTC and $32/gallon). Repeating this with slightly dirty glass gave a very faint yellow.

Perhaps you've found a new colorimetric assay for drain cleaner. A test for oxidizable cpds. in products, which I've mentioned before (Rooto fails; I haven't bought that crap for many years), is to add a minimal amount of Cr+6 and heat for a time, and see what that does to the color.

[Edited on 21-6-2021 by S.C. Wack]




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[*] posted on 22-6-2021 at 12:56


I did the experiment of adding room temperature 37% H2SO4 to solid KBrO3. When this is done, no reaction occurs at all. The solid slowly dissolves, the solution remains colorless. The same is true when the KBrO3 is first dissolved and then the acid is added. In that case I get a perfectly colorless solution. Not even the faintest hint of yellow or orange color can be observed.

On addition of a reductor (some alcohol), there is formation of bromine and the liquid also heats up, so that it becomes quite warm (but not really hot).




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[*] posted on 22-6-2021 at 21:37


Thanks so much for your replies woelen (and everyone else). I appreciate you doing the test at home for me.

So I did a couple more tests. Even when I dilute my H2SO4 to 2M sulfuric acid, I get bromine produced when adding even one drop of the acid to either KBrO3 in solution, or solid KBrO3.

At this point, based on my tests and woelen's most recent comment, I feel fairly certain it is the impurities in the Rooto sulfuric acid causing this. I'll just have to order some "purer" sulfuric acid online, which is easy here in the USA.

I do have 30% H2O2, and could probably try to "purify" the Rooto sulfuric acid, but as this is for the BZ reaction which is quite sensitive, I don't think it is worth the effort.

[Edited on 6/23/2021 by MidLifeChemist]
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[*] posted on 23-6-2021 at 12:49


It could also be the KBrO3. Commercially, there is a 1 : 5 molar ratio mix of KBrO3 and KBr available, which on addition of an acid gives a lot of bromine. Sometimes this mix is used as a convenient source of bromine, without the need to store bromine as an element.

Do you have HNO3? If so, dilute some of that to 2M or so and add that acid to your KBrO3. If this also gives a lot of Br2, then your KBrO3 probably is this bromine-producing mix. You could also try this test with 20% to 30% acetic acid (without any additives added).

Another, somewhat less clear, but still useful test is to add some 20% HCl to solid KBrO3. If this reaction produces a yellow liquid and yellow gas (bubbling, the gas mix being a mix of Cl2 and BrCl in a molar ratio of 2 : 1), then your KBrO3 most likely is pure. If this reaction produces Br2, then almost certainly your KBrO3 is not pure, but a mix of KBrO3 and KBr.




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[*] posted on 23-6-2021 at 17:17


Thanks for these tests!

>> Do you have HNO3
Yes, I recently purchased the good stuff :)

I added some 2M HNO3 to solid KBrO3. It produced a lot of bromine.

I also added some 20% HCL to solid KBrO3. The result was interesting - The solid compound turned a dark bromine brown, bubbles were formed, and the surrounding liquid was brownish-yellow. But more yellow than normal when bromine is formed, but not chlorine yellow. I did add the HCL in excess.

It looks like the KBrO3 might not be pure. I purchased it from Onyxmet, I'll contact them and see what they say.
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[*] posted on 24-6-2021 at 03:03


Can chloride ions with bromate in acidic condition form chlorine and interhalogen BrCl?
5Cl- + BrO3 + 6H+ -> 2 Cl2 + BrCl + 3H2O

The reaction is similar to that which will occur when mixing potassium bromide and bromate in acidic conditions

[Edited on 24-6-2021 by Oxy]
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[*] posted on 24-6-2021 at 06:31


Yes, this is what actually occurs. If you add solid KBrO3 to HCl (e.g. 20% HCl), then you get a lot of fizzling/bubbling and a mix of gases escapes from the liquid. This mix is Cl2 with BrCl and has a fairly strong yellow color (much more colorful than Cl2). The liquid itself becomes like piss, the color of dissolved BrCl, which masks the weak color of Cl2.

A nice experiment is to put 3 ml or so of concentrated HCl in a test tube and add a spatula full of KBrO3 to that. This gives a fairly vigorous reaction, and there also is some heating. Under these conditions, nearly all Cl2 and BrCl escape as gas and you get a nice golden yellow gas mix above the liquid.




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[*] posted on 24-6-2021 at 10:23


Quote: Originally posted by MidLifeChemist  
Adding concentrated H2SO4 to solid KBrO3 sounds like a quick, easy way to make some concentrated bromine water.


Better is to mix KBr and KBrO3 solutions and add some acid. This is often used in bromatometry.




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[*] posted on 26-6-2021 at 07:24


Just to confirm - based on my test of adding 2M HNO3 to both aqueous and solid KBrO3, and getting copious amount of bromine, there should be little doubt that the KBrO3 is contaminated with KBr or something similar.

Do you all agree?
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[*] posted on 26-6-2021 at 08:00


Yes. You can separate them by fractional crystallization. KBr is far more soluble than KBrO3. KBrO3 is very soluble in hot water (49,7 g/100 ml at 100°C), so make hot saturated solution and let it cool down to room tepmterature and than in the fridge. Solubility of KBrO3 at 0°C is 3,1 g/100 ml, so you get most of your KBrO3 from solution. You can get more by evaporation of some water, but for that you need to know KBr:KBrO3 ratio. You can determine KBrO3 content by iodometry (if it isn't on the label). Or you can precipitate rest of the bromate as Sr(BrO3)2 or Ba(BrO3)2. You can use them for flame coloring or you can make from them HBrO3 by double displacement with H2SO4 (30% HBrO3 should be fine). You can use acid for making other bromates or just do some experiments with it. HBrO3 is sensitive to light (so store it in brown glass and in dark place) and tends to slowly decompose to Br2 and O2.

[Edited on 26-6-2021 by Bedlasky]




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