HYDROBROMIC ACID

In this particular article we will learn about hydrobromic acid and the process of preparation of hydrobromic acid.

First a brief introduction to hydrobromic acid. Hydrobromic acid is an inorganic acid also known as a mineral acid. It's composed of hydrogen and bromine and is the bromine analogue of the more famous hydrochloric acid. Now hydrogen bromide is actually a gas but when it’s dissolved in water we call it hydrobromic acid. Azeotropic hydrobromic acid has a boiling point of about 124.3 Celsius at atmospheric pressure  and corresponds to a concentration of 47.6%, although variations of a couple of percent aren't uncommon.

It can be used for many of the same purposes as hydrochloric acid but because it is generally more expensive it is more often used when bromine or bromide itself is specifically needed. It's mostly used to make organobromine compounds and we ourselves used it this way sometime ago to make bromoalkanes for making grignard reagents. Organobromine compounds tend to be more reactive than organochlorine compounds and thus their demand as reagents. They also tend to be more toxic and carcinogenic too. But when you need them, you use them.

Warning: Sulfuric acid and hydrobromic acids are corrosive, wear gloves when handling them. Sulfur dioxide is a toxic gas.

So let's get to making hydrobromic acid. First, we get 400 ml of water. Looking back this was too much but it'll still work, we'll just have to distill longer. Now add to it 250g of sodium bromide. Sodium bromide is commonly available as a bromine source for swimming pools. You can also use premixed sodium bromide solution if that's more available to you. Although you'll have to recalculate the masses. For the commonly available 35% concentration you'll need about 700 grams worth.

Anyway, now we carefully add 200 ml of sulfuric acid and stir. Be careful of excessive heating. If it gets too hot, above 75 degrees celsius, then some of the bromide will reduce the acid and generate some bromine. It's not a lot but can be unsightly. I'm using low grade drain cleaner acid. Now this is a lot of acid, it's a large stoichiometric excess because it gives a better yield than using a stoichiometric amount. I can do this because drain cleaner is very easy to buy for me. You'll need about 350 grams to match the stoichiometry. Now once everything is dissolved, the usual procedure is to cool mixture and crystalize out sodium sulfates and bisulfates. I tried this but it didn't work. I think I might have too much water or my drain cleaner wasn't concentrated enough. Not to worry though, amateur chemistry is all about changing your procedures on the fly to accommodate process variations.

We'll just distill out the excess water first. We pour the mixture into a flask and set up a simple distillation apparatus. We then turn on the heat and begin distilling. So what's happening?

In a stoichiometric reaction the sulfuric acid reacts with the sodium bromide to produce hydrobromic acid and sodium bisulfate. The sodium bisulfate goes onto react with more sodium bromide producing more hydrobromic acid and sodium sulfate. If you use a large stoichiometric excess of sulfuric acid like I did then only the first reaction happens as there is enough sulfuric acid to do so. If you use a substoichiometric amount of sulfuric acid, or use sodium bisulfate straight out, then the second reaction happens. The first reaction is preferable since it's higher yielding. The second reaction works, but the equilibrium can reverse giving a lower yield.

Now as we distill we drive the reaction forward as only hydrobromic acid distills. However we have lots of sodium sulfate and bisulfate salts and they'll crystalize out during distillation, making it much harder to distill further. So after about half the liquid has been distilled, stop the distillation and let it cool to room temperature. Sodium sulfates and bisulfates should start crystallizing out. Filter the crystals and retain the filtrate. Now put the filtrate back on heating and continue distillation.

For very large amounts of hydrobromic acid you may have to repeat the process again after removing another half volume of liquid. For my runs though one crystallization was enough. I kept distilling until nothing came over and the distillate temperature began dropping. And here are my collected runs of hydrobromic acid. I have so much more here because I ran the procedure a few more times to make a very large stock. I don't want to have to do this again for at least a few years.

Now this hydrobromic has a lot of water in it. To remove most of it I combined my runs and we setup for fractional distillation. Now we fractionally distill off the water at 100 celsius and keep collecting until our azeotrope of hydrobromic acid distills at about 124 celsius. Then we change out our receivers and collect the rest. And there is our distilled azeotropic hydrobromic acid. As said before the concentration should be around 47.6% give or take. Now you may run into the situation where your distilled acid is yellow or even orange. That orange tinge is actually elemental bromine in the form of tribromide ions. 

It formed originally in the earlier sulfuric acid reaction mixture as the sulfuric acid was reduced by the hydrobromic acid. This occurs if it gets too hot or there are oxidizing impurities in your chemicals. If you try and distill it though it'll never quite go away. But it's not a lot and for most reactions it will have negligible effect. Nonetheless there are ways to get rid of it if you desire. I still had a bit of yellow color.

According the literature, red phosphorus is commonly used but i don't have that available and most amateurs don't either. Sulfur can be used. So i added about 4 grams of sulfur to my 400 ml of azeotropic hydrobromic and began distilling. And it looks like it worked.

What's happening is that the sulfur reacts with the bromine to form sulfuric acid and hydrobromic acid. Just like I showed in the electrobromine process. Sulfur is much easier and cheaper to get for the amateur than red phosphorus so this a great way to clear bromine from hydrobromic acid. However it looks like it isn't perfect. It appears the hydrobromic acid is now contaminated with colloidal sulfur. This happened because at these temperatures the sulfur is molten and a tiny amount of sulfur vapor distills over, depositing sulfur.

I can see why literature methods suggested using red phosphorus instead. It's a very tiny amount though, milligrams at most but it can be unsightly. I found the sulfur itself can be removed by simply refluxing the hydrobromic acid for an hour. The sulfur deposits in the reflux column and the acid is clear. But you have to reflux for an hour before all the sulfur is removed. Sulfur is nonetheless a very cheap and effective way of removing bromine contamination. Another even better way is to use sodium metabisulfite as a reducing agent. It's available online for preparing dyes, gold refining and home brewing so it's easy to get.

Maybe even easier than sulfur. Just add small amounts, like half a gram or so, to your hydrobromic acid until it's clear. It reacts very quickly so you'll know when it's done.

What happens is the sodium metabisulfite reacts with the bromine to produce sodium bisulfate and hydrobromic acid.

Now a drawback is that if you add too much you'll deplete your hydrobromic acid but this negligible if you go slowly so you don't overshoot. And there we go, that was a pretty dramatic change. Now you'll need to distill again in order to leave behind the sodium bisulfates but at least this time the distillation should proceed smoothly and you won't need another cleaning step like with sulfur.

I'll leave it up to you to decide which one is cheaper and more cost and labor effective for your purposes. Anyway, and there we have our purified hydrobromic acid. A simple test to authenticate it is to take a sample and add a few drops of hydrogen peroxide. The yellow or orange color is bromine being formed from the oxidation reaction. Another test for concentration is to take a sample of known volume and divide the mass by it to find the density. Azeotropic hydrobromic acid at 47% percent has a density of about 1.49 g/ml so we're pretty close into the right range. And that is how you make hydrobromic acid.