Warning: Nitric acid and sulfuric acid are corrosive.
Wear gloves when handling them.
This experiment also produces acidic fumes and should be performed outside or in a fume hood.
Greetings fellow nerds.
In a previous video I demonstrated how to use copper nitrate to generate nitric acid without needing sulfuric acid by thermal decomposition.
Interestingly enough, I got a large number of comments from precious metal refiners who had large amounts of copper nitrate waste
and wanted to know if the process could be used to recover their nitric acid and if there are other processes than just thermal decomposition.
There is actually, the overall idea is to react copper nitrate with sulfuric acid to produce nitric acid and copper sulfate.
Then electrolyze the copper sulfate to recover our copper metal and sulfuric acid again.
The whole process is very efficient and much better than having to deal with toxic nitrogen dioxide gas as in the thermal decomposition method.
You can not only recover the nitric acid, but also the copper as well.
You just need a platinum electrode and some sulfuric acid but both are reused.
So let's get started.
I have here copper nitrate waste after I dissolved impure copper in nitric acid.
As you can see it has some insoluble contaminants so the first thing we need to do is filter it out.
At this point I must stress that you shouldn't have any precious metals like silver in solution alongside the copper.
So if this copper nitrate is a waste product of precious metal refining then leave it to react with excess copper metal to precipitate out precious metals first.
Precious metals in solution will contaminate subsequent steps.
Anyway, here is our filtered copper nitrate solution.
To recover our nitric acid we need to react it with sulfuric acid.
And if you know exactly how much copper is in solution you can add a slight stoichiometric excess and be done with it.
But I'm going to assume the amount of copper in your solution is variable since it's a waste product of metal refining
or you have sulfuric acid of an unknown concentration, so you don't really know how much to add.
Fortunately we can crudely test for our endpoint.
First, just make a guess and add in some sulfuric acid that you think will be enough to react with all the copper and stir.
If you precipitate a large amount of copper sulfate, as i have here, then add in some water and stir to dissolve it.
Some precipitate is fine, about 10% by volume.
But more than that and you should add in water to prevent trapping of the copper nitrate.
Anyway, after stirring take out a small sample of a few drops in a vial and heat it on the hotplate to about 300 celsius.
If there is insufficient sulfuric acid then as the copper solution dries, it will turn black as the unreacted copper nitrate decomposes.
If this happens, add some more sulfuric acid to your copper nitrate solution and test it again.
Don't worry if you use too much sulfuric acid, an excess is fine.
Eventually it should turn white when heated.
This is dehydrated copper sulfate and indicates that we have excess sulfuric acid.
At this point we take our copper nitrate solution and set up a distillation apparatus.
Then we turn on heating and distill off the nitric acid.
What's happening is the sulfuric acid reacts with the copper nitrate to produce copper sulfate and nitric acid.
This process is much more efficient than thermal decomposition of copper nitrate and you can routinely get yields better than 90%.
The reason why i never presented this as a viable way to make nitric acid is because i honestly thought copper nitrate was far too expensive to buy
and if you could buy it, you would also be able to buy cheaper nitrate salts that would have been better.
I never realized copper nitrate was a waste product of precious metal refining, and thus essentially free.
Thank you viewers for letting me know.
Anyway, as you distill, the excess sulfuric acid will eventually start forming a mist in the boiling flask.
At this point the nitric acid will have completely distilled over so you can stop the distillation.
And here is our distilled and dilute nitric acid.
You can purify and concentrate it using fractional distillation as i've shown in a previous video.
Although I recommend accumulating a large quantity first to save on labor.
As for the residue, this is a mixture of copper sulfate and sulfuric acid.
It's white, rather than the classic blue color, because when you boil copper sulfate with excess sulfuric acid,
the anhydrous white version precipitates out rather than the blue pentahydrate version.
My copper sulfate is dirty grey because it also has other contaminants.
Anyway we can recover copper metal and the sulfuric acid from this corrosive slurry using electrochemistry.
First weigh the slurry so we know how much water to use later.
We need to have twice as much mass in water as we have in sulfuric acid and copper sulfate.
I have about 300 grams so i'll need 600 grams, or 600mL of water.
Now we need to set up our electrolysis cell.
Get a large beaker that can contain both the water and slurry and get a large copper electrode.
I'm using copper foil but you can also use copper sheets or copper wire.
Just as long as it has a large surface area it will be fine.
You'll also notice I made sure the copper electrode was on the bottom of the beaker.
This is optional but it lets us do the electrodeposition without stirring.
If you don't have the electrode on the bottom, you'll need to stir later.
I'll explain why in a bit.
Now we get our 600mL of water that we measured earlier and first we use a bit to wash out our various apparatus of the copper sulfate and sulfuric acid.
At this point the numerous transfers between flasks and beakers has produced a lot of sticky residues.
We pour that water into our electrolysis cell and then we pour in our slurry of copper sulfate and sulfuric acid.
You'll notice I poured in the water first and then the sulfuric acid slurry because if you distilled it correctly,
this should be fairly concentrated sulfuric acid and will heat up considerably when added to water.
The old rule of adding acid to water rather than water to acid applies here.
We use the rest of our water to wash out the beaker.
Stir the solution to dissolve as much of the copper sulfate as you can but you won't dissolve all of it.
We now lower in a platinum anode and suspend it so it's equidistant away from most of the copper.
I'm using a platinum coated titanium anode.
You don't want it too close to one piece of copper electrode because that will cause uneven current distribution.
Platinum electrodes are not too hard to find and platinum coated titanium electrodes can be bought online.
Just be aware there are a lot of fakes out there so you'll need to go with a reputable seller and not some cheap knockoff on ebay.
Anyway, once it's secure, we hook up power leads and connect the electrolytic cell to a variable power supply.
The copper electrode is given a negative charge and is our cathode.
The platinum electrode is given a positive charge and is our anode.
Set a maximum voltage of about 3 volts and a maximum current corresponding to 30 milliamps per square centimeter of exposed copper,
or 100 milliamps per square centimeter of exposed platinum, whichever one is smaller.
Since my platinum surface area is the limiting factor in my particular cell,
I'm using about one amp for my platinum electrode that has about 10 square centimeters of effective surface area.
Now these numbers are very conservative and you can actually use three times more current density with little harm,
but i wanted to minimize long term corrosion of my valuable platinum electrode.
Anyway i'm going to remove the stir bar and zoom in.
Now all we do is wait.
At first you may see large amounts of copper sulfate crystalize out as the solution cools from our earlier mixing but that will stop in a few hours.
What we're really trying to do is electrolyze the copper sulfate solution.
The copper ions deposit as copper metal at the cathode while water is electrolyzed at the anode to produce oxygen and sulfuric acid.
This is how we recover both our copper and sulfuric acid for reuse.
I said earlier that we should have the copper cathode laid out on the bottom.
It's not critically important but it does help.
This is because as the solution is depleted of copper sulfate the solid copper sulfate will dissolve.
And the dissolved copper sulfate needs to reach the cathode to electroplate out.
But the freshly dissolved copper sulfate solution is denser than the existing solution and will sink,
forming a pool at the bottom of the container and only slowly diffusing to deposit onto the electrodes.
This produces very uneven, loose and spongy copper deposits which are undesirable.
It needs to reach the electrode faster.
However, if we have the electrode laid out on the bottom then it will already be there when copper sulfate dissolves and sinks.
So it gets plated out very quickly and efficiently, producing strong deposits.
If you don't have this, you'll need to have stirring in your cell to keep everything mixed.
Some amateurs will instead have the platinum anode sitting on the bottom so the bubbles of oxygen stir up the mixture or even continually bubble air into solution and mix it that way.
And others will simply use very low current.
If the electrodeposition is slow enough, then just the natural diffusion of solutions will be enough.
But you have to go extremely slowly for that.
Regardless the process is very slow and could take days to finish.
To speed it up you can use more electrodes to increase the surface area and thus allow you to apply more current.
I'm just using one small platinum electrode,
but if i was doing a large scale operation then i would use very large or even multiple electrodes for both the anode and the cathode and apply large currents of many amps.
I would also use direct stirring like vigorously bubbling air into the mixture to ensure high speed mixing.
I could be done in mere hours or process kilograms of metal.
Another thing I should mention is that the bubbling of oxygen gas will make the solution spit out tiny droplets of electrolyte.
I'm not bothering to contain it here since I want my apparatus to look good on camera.
But if you do this often then the area around the electrolysis cell will be covered in a dirty layer of electrolyte residue.
So you may want to put a cover over the cell to contain the spray, or even put your electrolysis cell into an electrolysis box as I showed in an earlier video.
Anyway, as we progress the copper sulfate will be consumed as it's converted to copper and sulfuric acid and the sulfuric acid concentration will increase.
Eventually most of the copper sulfate will be consumed and the copper cathode will start electrolyzing water and generating hydrogen rather than deposit more copper.
I actually recommend stopping early when most of the copper sulfate has been dissolved but not all of it.
This is because if we continue to electrolyze when there is very little copper in solution, the deposited copper will be very loose and spongy rather than a solid mass.
Now in the interest of science i'm going to keep electrolyzing until i see hydrogen formation, so i can then show you what happens to the copper.
And later on I'll show another run where I stopped early.
And there we go, we're now generating hydrogen as there is too little metal left to deposit.
We have to stop as there is nothing more happening.
Now we carefully decant our solution to a flask for boiling.
It also helps to wash the copper with some water to clean off the residues.
You can see here the copper powder sloughing off the electrode.
It's not all like this, the bulk core of the copper that was deposited early is solid and strong,
but this last coating is weak enough that just shaking the beaker causes it to fall off into solution.
As said before this is because at the end of the run, the solution became severely depleted of copper so the last deposit is weak.
Anyway, we decant the wash water as well.
The supernatant mostly contains sulfuric acid so to recover that we distill off water.
The green color is from small amounts of copper as well as other contaminants in my impure copper metal.
I think I have tiny amounts of chromium, iron, nickel and maybe some zinc in there.
And once again when the residue starts misting like this then most of the water has been removed and you have fairly concentrated sulfuric acid.
Turn off the heating and let it cool.
And there is our recovered sulfuric acid and you can reuse it for more copper recovery from copper nitrate wastes.
Don't worry if it has leftover copper sulfate, that will feed forward into our next run.
But it will accumulate contaminant metals like iron, nickel and zinc until it's more metal sulfate than sulfuric acid.
At that point you'll have to dispose of it.
You can recover the base metals from it but that's a very complicated topic for another time.
Anyway, let's get back to the copper.
And there is our recovered copper from the copper nitrate wastes.
Recovery is near perfect as I had about 83 grams of copper in my metal wastes and we can see here that the mass as well as the mass of the copper foil electrodes is accounted for.
This can be directly used for more metal displacement reactions if you're recovering precious metals like silver from silver nitrate.
You can also melt this down to make larger copper ingots if desired.
At this point i like to draw your attention to the fact that it is indeed very rough and crumbly because the last layer of copper deposited came from a very weak and depleted solution.
But it's not nearly as bad as this looks as it's just the surface.
But if you want a more solid appearing electrode, stop the electrodeposition before the copper sulfate is depleted.
Here is another electrode where i did so and as you can see, it's more solid and doesn't come apart in my hands.
So stop your electrodeposition before the copper sulfate is depleted if you want results like this.
So that is how we can recover nitric acid and copper metal from copper nitrate wastes with very high efficiency using some sulfuric acid, a platinum electrode, and electricity.
And the platinum electrode and sulfuric acid are both reusable for numerous cycles.
At this point you might be asking can you electrolyze the copper nitrate directly and produce copper and nitric acid that way.
Unfortunately no as electrolysis of nitric acid tends to destroy the nitric acid.
And any amount that is produced will immediately begin dissolving the copper again that was just deposited.
This is why we have to use sulfuric acid as our intermediate as it does not itself dissolve copper nor does it decompose when electrolyzed.
Now I know a few of you have already gone ahead and decomposed your copper nitrate using the thermal decomposition method to get nitric acid
and have already produced a fair amount of copper oxide waste.
You might be wondering if you screwed up and can no longer recover the copper.
Don't worry, you can still use the electrolytic process to do so.
Just get your copper oxide residue and titrate in a 10% solution of sulfuric acid until it's completely dissolved.
This will produce copper sulfate.
You may need to boil it to drive the reaction forward.
You can then proceed as normal and distill off the excess water and residual nitric acid and then perform the electrodeposition as shown in this video.
So there you go my precious metal refining friends.
I hope this video was useful for you to save yourself some money and maybe produce less of an impact on the environment.
While there are other ways to deal with copper nitrate waste like precipitation with sodium bicarbonate.
The method I described in this video has the advantage of fully recovering the nitric acid and copper metal as well as minimizing waste chemicals produced.
While the sulfuric acid does accumulate contaminants, these were already present in your copper nitrate solution and so are not an inherent waste product of the process itself.
Thanks for watching, and thanks for letting me know about this issue.
I actually had no idea such a problem was out there.