Greetings fellow nerds
In a previous video we made a divided membrane cell with Nafion, a special type of chemically stable ionic polymer.
In this video we're going to use it and make sodium hydroxide out of sodium bicarbonate.
The concept is rather simple.
If we can take the sodium ion in sodium bicarbonate and separate it from the bicarbonate, then split water into hydroxide and hydronium ions and combine them.
We could produce sodium hydroxide and carbonic acid.
The carbonic acid would break down into water and carbon dioxide, and we'll get useful sodium hydroxide.
Now I know what you're thinking, what kind of voodoo black magic arcane sorcery of the dark arts is this to be able to just swap ions.
That seems about as crazy as swapping organs.
Sure it sounds easy, but the underlying practical complexity must be enormous.
Precipitation can't be used here since all species are soluble.
And you all have been with me long enough to know that separations like this usually require multiple laborious steps.
Thermodynamics also doesn't allow this either, we need a driving force.
And you would be absolutely right, we're not doing this exactly, we're also going to be generating hydrogen and oxygen byproducts because we'll be using electrolysis as the driving force.
But the amazing part is that the ion separation aspect actually does occur in one automatic step.
I'll be honest when I learned of this I thought it was magic.
Fortunately, there exists a type of membrane that does exactly that sort of magic.
I have here a Nafion membrane.
It's an ionic polymer or ionomer that consists of a fluorocarbon backbone and is functionalized with sulfonate groups.
These sulfonate groups inside the polymer facilitate the transfer of cations by allowing them to hop from sulfonate group to sulfonate group inside the membrane.
So overall this makes the membrane permeable to cations but blocks anions.
It's almost like a magic filter that passes through just cations.
Now the process is very slow if you just do it by diffusion.
And you have to balance the charges by somehow replacing them on each side.
Otherwise the electric field will just push the ions back.
Fortunately we can do both by applying an external electric field to power the charge separation.
So let's do that.
First we get ourselves a solution of sodium bicarbonate.
I have here about 200 grams of sodium bicarbonate and 400 ml of water.
I'm just being very crude here and it's not going to all dissolve.
Then we get our nafion divided membrane cell that we built in a previous video.
This will serve as the cathode compartment.
I'm going to use a titanium plate as the cathode but you can use copper or steel since the cathode isn't subject to the same oxidative stresses as the anode.
Now the great thing about a watertight half compartment cell is it's super easy to make two compartment cell, just insert the whole cell into the larger container.
No messing around with screws, gaskets, or tubes in most two compartment setups.
The larger container is the second compartment.
Now we connect a tube to the inner compartment to catch overflow of our product sodium hydroxide.
For our anode i'm going to use this titanium sheet coated in cobalt oxide.
But you can use carbon, platinum, nickel, and many alkaline stable electrodes since we're electrolyzing sodium bicarbonate.
The inner compartment needs electrolyte as well so i'm going to pour in some deionized water to fill it.
Normally deionized water is non-conductive but a small amount of sodium ions will diffuse in through the membrane to jump start our electrolysis.
But if you find the resistance of the whole setup too high at first, you can add in a few grams of sodium hydroxide or carbonate yourself to start it up.
More will flow in as the electrolysis progresses.
Now we hook up our power clips and apply a current.
The cathode is the titanium electrode inside the inner compartment.
And the anode is the cobalt oxide coated titanium electrode in the outer compartment with the sodium bicarbonate electrolyte.
I'm applying a 1 amp current which corresponds to a 40ma per square centimeter current density.
And there it goes.
So what's happening? At the anode we're electrolyzing water and producing oxygen gas and hydronium ions.
These react the bicarbonate and liberate carbon dioxide as well.
So we're releasing oxygen and carbon dioxide at the anode.
At the cathode, we're electrolyzing water and releasing hydrogen gas and producing hydroxide ions.
So far that's all pretty boring.
But the amazing stuff happens at the membrane.
As hydroxide builds up in the cathode compartment, charge balance must be maintained.
You cannot have a significant imbalance of charge.
The anionic hydroxide ions cannot cross the cationic exchange membrane, but cationic sodium ions can.
So they get pulled through by the driving force of the electric field.
The carbonate ions in the anode compartment are anionic so they remain behind.
Overall, we produce sodium hydroxide in the cathode compartment.
If we didn't have the membrane the sodium hydroxide would start reabsorbing the carbon dioxide
and producing sodium carbonate and bicarbonates again, we'll never separate out our sodium hydroxide.
This is why we couldn't simply electrolyze sodium bicarbonate by itself.
Now if we have a high enough current we can actually precipitate it out and gum up the compartment,
but the nafion membrane is also permeable to water.
So it will diffuse in by osmosis producing a concentrated sodium hydroxide solution.
The effect is strong enough that it will overflow the compartment allowing us to collect the sodium hydroxide separately.
In a commercial cell, they use such high currents that they can exceed osmosis rate
and need to directly pump in water themselves to dilute the sodium hydroxide and prevent it from precipitating.
Now you may be wondering if the anode reaction produces hydronium ions, which are cations, why don't they diffuse through as well.
The answer is they do, but they react with the carbonate ions and neutralize to form carbon dioxide and water.
So the pH of the solution remains high and there is never a high enough concentration of hydronium ions to significantly contribute to the ion transfer at the membrane.
It is for that reason that we can't really use a cationic exchange membrane like nafion to produce an acid.
It would be great if we could fill the compartments with sodium sulfate, separate out the sodium ions, and make sulfuric acid.
Unfortunately as the pH decreases as the acid builds up, we get rapidly diminishing returns as hydronium ion transfer becomes the dominant reaction.
We can get as far as maybe sodium bisulfate, but the efficiencies approach zero.
To make acids we need anionic exchange membranes.
Those membranes do exist, and i'm still trying to find amateur accessible sources of them.
Perhaps someday i'll make a video on them.
Although considering it took me ten years to make this video, expect that video in the year 2396.
But cationic exchange membranes are still tremendously useful.
Another salt we could put in is actual sodium chloride.
Instead of oxygen, the anode produces chlorine gas.
This is the basis for most of the chloralkali industry to produce sodium hydroxide and chlorine.
I didn't use sodium chloride here because I didn't want to make a complicated collection system for chlorine gas, but you can use sodium chloride if you want.
We're also not limited to sodium.
If we use potassium carbonate, or potassium chloride, we can make potassium hydroxide in the above setup.
The nafion divided membrane cell opens up a tremendous amount of chemistry.
Now I would be omitting a lot if I didn't mention that nafion is also used in fuel cells.
It's the basis for the proton exchange membrane based fuel cell.
In this case no electrolyte is used, and the nafion itself is the electrolyte and the electrodes are coated directly onto the membrane.
But fuel cells are a super complicated discussion for another time.
For now we'll just focus on making sodium hydroxide.
Okay so i've run the cell for several hours and we have a small amount of sodium hydroxide already collected.
Let me pour it out into two vials.
To test it, i'm going to add hydrochloric acid to one.
I'm doing to prove it's not sodium bicarbonate.
Sodium bicarbonate would bubble when mixed with acid as it releases carbon dioxide.
It's not doing that here.
Now i'm going to test the remaining sample with aluminum foil.
Aluminum foil reacts with strongly concentrated sodium hydroxide to produce hydrogen and sodium aluminate.
And there it goes.
So it looks like we have a decently strong solution of sodium hydroxide.
I don't know the exact concentrations, but since i was able to precipitate sodium hydroxide by putting the current high enough, I guess we can access 50% concentration if we really wanted to.
If you want the dry form, you can boil this down in a steel container.
Don't use glass since hot sodium hydroxide will dissolve glass.
Anyway, that is how you use a nafion divided membrane cell to make sodium hydroxide.
Granted, if you could buy and make a nafion divided membrane cell, then getting sodium hydroxide probably isn't that hard for you.
But this nonetheless opens up a lot of interesting electrochemistry experiments.
Thanks for watching.
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