Warning: This experiment uses toxic nitrogen dioxide gas.
Work outside or in a fume hood.
Nitric acid is corrosive, wear gloves when handling it.
Greetings fellow nerds.
I've been making nitric acid in various ways these past weeks and so far I've used another acid in all of them, like sulfuric acid.
The idea is that we mix a nitrate salt with an acid to produce nitric acid and the complementary salt.
Nitric acid is volatile so it's easily distilled off with strong heating.
This is a very simple process, but very robust and high yielding.
Various acids can be used with sulfuric acid being the most popular.
And it's still the standard method to make nitric acid for the amateur.
However as you know i sometimes attempt crazy processes just for the challenge.
In this video we're going to try and make nitric acid without using any other starting acids at all.
To do that we're going to use a very interesting chemical, copper nitrate.
Copper nitrate decomposes with moderate heating to form copious amounts of nitrogen dioxide and oxygen.
If these are bubbled into water then this can be used to make nitric acid without using any other acid.
And that's it, it's that simple.
Except for the fact we need to get copper nitrate itself.
If you can buy it directly then you can skip this part, but for most of us we're going to make it.
First I'm going to start with 20g calcium hydroxide and mix it with 131g of calcium ammonium nitrate and 200mL of water.
Calcium hydroxide is available as slaked lime or pickling lime as a food additive.
Calcium ammonium nitrate is a fertilizer.
A reflux column is set up on top of the mixture and the mixture is refluxed for a few hours.
What we're doing here is removing any ammonia from the calcium ammonium nitrate and turning it into calcium nitrate.
If you're using pure calcium nitrate then you can skip this step.
It's very important that we remove the ammonia because later when we make the copper nitrate we want to avoid any formation of tetraamminecopper nitrate, which is explosive.
While the danger is very low, especially since it's going to be mixed with a large amount of copper nitrate,
it's still good practice to eliminate any links in an event chain that could lead to a lab accident.
After a few hours we stop heating and let it cool.
In a separate container we get 166g of copper sulfate pentahydrate and dissolve it in 500mL of water.
Copper sulfate pentahydrate is available as a root killer.
After it's dissolved, we add in the calcium nitrate solution from earlier.
The copper sulfate reacts with the calcium nitrate to form copper nitrate and insoluble calcium sulfate.
Stir the mixture to ensure the reaction proceeds.
If it's too thick to stir, you can add more water to thin the mixture out.
I actually had to dilute the total volume to 1 liter before it could flow easily.
After letting it react for ten minutes, we then filter the mixture.
I strongly recommend using a disposable paper or coffee filter.
The precipitate is calcium sulfate and difficult to remove from reusable filters like glass frits.
So i recommend a disposable filter since it's going to be ruined anyway.
I also recommend washing the precipitate with copious amounts of water to flush out any copper nitrate soaked inside.
Anyway, we have a solution of copper nitrate.
It's dissolved in a lot of water that we don't need.
So we first set up a simple distillation apparatus and distill off the water.
The copper nitrate decomposes at a temperature higher than the boiling of water so we don't have to worry about our reaction starting too soon.
But we need to remove a lot of water.
Now as it boils, some calcium sulfate will start precipitating out.
Calcium sulfate actually has a very low, but not zero, solubility in water.
So even though our filtrate seemed clear, it will precipitate small amounts of calcium sulfate as we reduce the volume.
We're just going to leave it in since the residue will be waste anyway but be aware this is normal to see.
To know when we're done, look at the color.
At first the copper nitrate will have the classic deep blue color of the copper (II) hexaaqua complex.
But eventually the color will change to a dark blue green color.
At this point there is not enough water to form the copper hexaaqua complex and this is the best time to stop heating and let it cool.
Do not try and dry it completely.
There is still a little water left that hydrates the crystals and we'll need that later so don't push it all the way to dryness.
Now we rearrange our apparatus and set it up to collect the nitrogen dioxide and convert it to nitric acid.
The easiest and simplest method is to simply directly distill the water and nitrogen dioxide into a single receiver.
This works, but the yield will be bad since a lot of the nitrogen dioxide will be lost.
You can use two receivers where the gases that aren't absorbed in the first receiver will get absorbed in the second.
And extra water is usually added to the second receiver for better absorption.
This works better but has the drawback that if the water in the second receiver backflows into the first, then any new nitrogen dioxide is at risk of being lost.
An improved version uses an inverted funnel to prevent backflow by not allowing the water to rise high enough before it starts sucking in air.
This works best and I recommend it for most amateurs.
However it needs decent sized funnels that are wide enough to expose a large surface for gas absorption.
Mine are too small for this so i'm going to use three receivers with extra water in the last one, and a deep tube in the second and third receivers.
In this setup water can backflow and forward flow in the second and third receivers, but will never flow in a way as to allow the nitrogen dioxide to escape without mixing with the water first.
It's more complicated than using inverted funnels though.
Anyway, I've connected it up and immersed the last two flasks in an ice bath.
There is 100mL of water in the third flask to absorb any nitrogen dioxide gas.
The whole system should be air tight with only the outlet of the last flask allowing the extra pressure to escape.
Now we turn on heating to maximum and start decomposing the copper nitrate.
While it's optional, it's helpful to also pump ice water through the condenser rather than just room temperature water.
Nitrogen dioxide condenses at 21 celsius so if we chill the condenser down to zero celsius,
we can liquify the nitrogen dioxide and have it fall directly into the receiver water giving it more time to react.
As we wait the copper nitrate will remelt and the remaining water in the copper nitrate will condense over.
It's actually very difficult to get perfectly dry copper nitrate because it will decompose before all the water is removed.
We're taking advantage of the leftover water to produce our nitric acid and provide some of the water we need.
Eventually, when the temperature gets high enough, it will start to decompose.
At first, we actually produce basic copper nitrate and nitric acid directly.
As we continue heating, the decomposition proceeds further and produces copper oxide, nitrogen dioxide, and oxygen.
The nitrogen dioxide then reacts with water to produce nitric acid and nitrogen monoxide.
Interestingly enough, the oxygen produced in the decomposition reaction helps here to convert the nitrogen monoxide into more nitrogen dioxide which in turn makes more nitric acid.
The stoichiometry is perfect, so ideally you should get a very high yield if you efficiently decompose all the copper nitrate and capture all the gases in the receiver.
As you can see hydrated copper nitrate is a very interesting reagent in that it directly produces all the chemicals necessary to make nitric acid.
Water, nitrogen dioxide, and oxygen.
Anyway, you can see our reaction is proceeding nicely and most of the nitric acid is condensed into the first flask.
It's only unreacted nitrogen dioxide gas that escapes into the second and third flasks where the extra water absorbs them.
Decomposition is pretty quick and I was done in just an hour and half, but I left it heating for a total of three hours to ensure a complete reaction.
And here is our flask containing the nitric acid.
To measure the yield I first mixed them all.
I then attached a fractional distillation column and boiled off the excess water to reduce the volume down to around 100mL.
This is done to drive off any unreacted gases that would interfere with the yield measurements.
And to increase the concentration as dilute nitric acid of lower density gives inaccurate results when using density to help determine yield.
At this point you can see a persistent blue color.
That's actually small amounts of copper.
The decomposition aerosolized some copper oxide particles as the copper nitrate decomposed, sometimes violently, so to purify the nitric acid I distilled it.
Meanwhile, during the whole synthesis, I was also running a parallel experiment with the exact same quantities.
But this time I used the crudest receiver setup of having the nitric acid and nitrogen dioxide directly fall into a beaker of water.
I wanted to compare yields and see if the extra effort of a complicated receiver setup was worth it.
Again I ran ice water through the condenser and used an ice bath to keep the receiver cold.
I also purified the acid to obtain a yield.
After all that work, total yield for the first method with three receivers came out to 97%,
while the second method with the simpler receivers came out to 59%
I'll be honest, I'm pretty sure the first yield is blatantly wrong.
I'm good, but I don't think i'm that good.
I must have measured the wrong quantities somewhere to end up with such an anomalously high yield.
I'm certain there is an error somewhere because during my filtering step, a fair amount of copper nitrate remained stuck in the calcium sulfate.
Certainly more than 3%.
So the yield has to be lower.
Normally I would run the experiment again but I didn't for a simple reason.
This method of making nitric acid is terribly labor intensive.
The most annoying thing about this is filtering the calcium sulfate.
Since I cannot use suction, and must use a gravity filtration, it takes a whole day or so.
I've spent too much time on a method that no chemist should do except for the challenge.
Fortunately i'm not lost, my friends that also tried this synthesis reported 82% yield.
So i'm going to go with their numbers.
Granted, they used the inverted funnel method but the results should be similar.
Having an efficient gas capture setup for your receivers goes a long way into improving the yields for making nitric acid.
At this point you might be asking how to clean out the copper oxide and calcium sulfate residue.
If you're doing this experiment as a challenge and have sulfuric acid or sodium bisulfate anyway, then you can dissolve the copper oxide as copper sulfate salts and get it out that way.
But if you're doing this experiment because you don't have access to strong acids then the only thing i can recommend is using a weaker acid like acetic acid or vinegar.
It might take over night or even longer, but weaker acids should dissolve it out as well.
Okay so that was using copper nitrate to make nitric acid.
Overall it's a very interesting experiment in making an acid without using any other acids.
And surprisingly had very good yields for what is essentially a thermal decomposition and gas liquid reaction.
Two reactions that tend to have highly variable yields.
The only problem is making copper nitrate itself which is incredibly labor intensive
so i don't recommend doing this and instead going the extra mile to find a source of sulfuric acid or sodium bisulfate.
Personally I vow to never do this again for the rest of my life.
Nonetheless a good experiment.
Thanks for watching.
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