Warning: Bromine and sulfur bromides are toxic and volatile. This experiment must be performed with gloves outside or in a fume hood.
Greetings fellow nerds.
In this video we’re going to make sulfuric acid from sulfur and water using a new process i found that i’m calling the electrobromine process.
First we start with about 16 grams of sulfur in a 250 mL beaker. Next we insert a carbon electrode we’ll use for the anode.
I’m using a carbon raft electrode consisting of 5 carbon rods obtained from lantern batteries.
For the cathode we’ll be using some copper wire. It’s bent to be close the surface as we don’t want it submerged too deep. I’ll explain why later.
Now we clip on the power leads. Positive for the anode and negative for the cathode.
Now we add the electrolyte which is 200 mL of a 5 molar solution of hydrobromic acid.
Make sure the carbon electrodes are submerged to at least five centimeters depth and that the copper electrode is under the surface of the electrolyte.
Now we turn on the current. I’m using 2 amps.
This might be too much but this is the first time i’m doing this, gotta start somewhere.
What’s happening is that at the cathode we’re generating hydrogen gas.
At the anode we’re oxidizing the bromide ions to bromine.
Early on in the electrolysis this doesn’t do much as the bromine quickly reacts with additional bromide to form tribromide ions which don’t react with sulfur.
You can see it here as this orange solution that sinks.
We placed the copper wire cathode high the cell because we don’t want it reaching into the tribromide and reducing it.
This converts it back into bromide ions and wastes our electricity. So keep the copper cathode as close to the surface as possible.
Eventually, as the electrolysis progresses and all the bromide is used up.
The tribromide ions get electrolyzed and form bromine.
Without bromide ions to react with the bromine builds up and starts reacting with the sulfur to form disulfur dibromide.
The reaction is actually more complex than this with polysulfanes being produced but I won’t get into that.
Anyway, the disulfur dibromide slowly reacts with water and more bromine to produce sulfuric acid and hydrobromic acid.
The hydrobromic acid goes on for further electrolysis so it’s essentially a catalyst.
The overall reaction is water and sulfur producing hydrogen and sulfuric acid.
Now the sulfur should remain at the bottom so it captures the sinking bromine and reacts with it.
But if the sulfur starts floating then you’ll need to get back down again.
Occasionally remove the electrodes and stir the mixture to get any sulfur that has floated.
The mixture of disulfur dibromide and polysulfanes forms a dense liquid that sinks as you can see here.
When all the floating sulfur has been captured. Put the electrodes back in and continue electrolysis.
You may need to top off the cell with water periodically as it evaporates.
Since i started with 16g of sulfur and used a current of about 2 amps, i ran the electrolysis for a total of 40 hours.
Now the electrodes are removed and after a brief stir to mix everything up the mixture is transferred to an erlenmeyer flask.
I must emphasize again that you do this outside or in a fume hood as you can see bromine vapor coming off the mixture.
Also sulfur bromides have their own unique stench that you don’t want to sample.
Now we construct a reflux set up with a vertical condenser. The idea is that we don’t want to lose any of our chemicals like bromine.
Turn on the cooling, stirring and heating until the mixture just starts to boil.
What’s happening here is that we’re dissociating any remaining tribromide ions into bromine and bromide.
We’re also forcing any sulfur bromides to react with water and produce sulfuric acid.
This reaction is very quick and you should only need to reflux for about half an hour to use up any remaining bromine.
Now turn off the heating and let it cool.
Now we filter the mixture. As you can see it’s much clearer with all the bromine reacted.
The residue is black from the bits of carbon that eroded off the electrodes. It should be yellowish from unreacted sulfur.
Once everything is filtered, transfer the filtrate to an erlenmeyer flask.
Now this time set up a distillation apparatus. Turn on the stirring, the cooling and the heating and distill off the hydrobromic acid.
It is here we are recovering the hydrobromic acid catalyst for reuse.
The solution is cloudy due to small amounts of sulfur being carried over with the distillation.
If you’re reusing the hydrobromic acid to make more sulfuric acid then this isn’t a problem.
But if you really want to get rid of it, just let the acid sit for a few days and the sulfur will settle to the bottom.
Keep distilling until you get this misting occurring in the flask.
Turn off the heating and let it cool. The residue in the flask is a weak and dirty solution of sulfuric acid.
To purify it we use the process involving the high temperature heating mantle i showed in an earlier video.
I let you take a look at that one yourself so i don’t waste your time here.
And there we have it. It took a lot of work, but we made sulfuric acid from sulfur and water using hydrobromic acid as the catalyst.
To test the acid i’m going to use an equal volume of sugar to run the famous carbon pillar experiment. The production of carbon confirms we have a strong concentration of acid.
Now while this method does work, I’m still developing and refining it.
For one it erodes the carbon anodes considerably and has made them weak and easy to break.
Perhaps i need to use a lower current density.
I can’t use titanium based electrodes since the acid concentration is high enough to destroy them.
And I don’t want to use platinum yet since it’s expensive.
The process also covers a lot of the apparatus in sulfur which is a pain to remove.
I also need to devise a better container as some of the bromine inevitably evaporated from the open beaker.
The final yield from 16 g of sulfur was 21 g of sulfuric acid.
While not very good, i’m sure improving the quality of the electrodes and preventing the tribromide ions from reaching with the cathode would help.
Frankly i’m surprised this method even worked at all since i found almost no documentation of it in the literature.
Nonetheless i’m sure i’ll make a new video on this process in the future once i improve it.
So please subscribe, like and comment.