Warning: Sulfuric acid and sodium bisulfate are corrosive.
Wear gloves when handling them.
Greetings fellow nerds.
In this video we're going to make zinc metal by electrochemistry.
Now previously i showed the alkaline zincate process which is easy, straightforward,
and somewhat forgiving of variations in current and concentration.
The drawback was that it only produced zinc powder.
In this video i'll show the acidic sulfate process which is harder and more complex,
but let's you produced solid deposits of zinc.
But first, i need to crush your expectations:
This is not cheaper than buying zinc directly.
Our objective here is to explore the science.
First we start with 40g of zinc oxide.
This can be bought from pottery supply and soap making companies.
Now to this add 150g of sodium bisulfate monohydrate.
This is sold a pH lowering chemical for swimming pools.
Now add a total of 400 mL of water and stir until dissolved.
This might take awhile.
What we're doing is making a mixture of sodium sulfate and zinc sulfate salts.
Now the later electrochemistry would actually work better with pure zinc sulfate
and if you have that then make a solution of that and skip ahead.
You can also mix zinc oxide and sulfuric acid to make zinc sulfate.
I'm using sodium bisulfate because it's much easier to get
than sulfuric acid for most amateur chemists.
I did try low grade drain cleaner sulfuric acid but the results were absolutely terrible.
The zinc produced was in poor yield and very poor quality.
Clean sodium bisulfate trumps drain cleaner grade sulfuric acid.
I didn't want to use to my lab grade sulfuric acid because if i was going to go that far,
I might as well have bought pure zinc sulfate instead.
Anyway, I digress.
Once we have our solution of sodium and/or zinc sulfates,
we then add in 10 g of zinc powder and continue stirring.
This powder is the same zinc powder i got from
the alkaline zincate method of making zinc powder i showed in a previous video.
What we are doing is displacing all the metals with greater reduction potential than zinc.
Why are we bothering to do this?
Well, the reason is actually quite fascinating if a bit long.
We first have to remember that if we electrolyze a solution containing a mixture of ions
we're first going to reduce the ions with higher reduction potentials at the cathode.
Then when those are depleted we get to the ones with lower reduction potentials.
For example, an ion like sodium is very low at -2.71 volts.
Hydrogen on the other hand is defined as having a reduction potential of 0 volts
and will this get reduced long before the sodium ever will.
At least that's how it works under thermodynamic equilibrium conditions.
Copper on the other hand lies at +0.337 volts
so it is very easily deposited out of solution.
That is actually the basis for my video on making sulfuric acid from copper sulfate.
Now where does zinc lie? It's around -0.76 volts.
Wait a minute, if hydrogen is at 0 volts and zinc is at -0.76 volts
then as we apply progressively more and more negative voltages at the cathode
we should only be generating hydrogen.
Sure we can alter the numbers somewhat by changing the concentrations,
but 0.76 volts is a pretty significant difference in electrochemistry
and we should be doomed right from the start,
never being able to plate out zinc.
Luckily for us, zinc has a special property to help us.
One must first remember these numbers only apply under thermodynamic equilibrium conditions.
Hydrogen is defined as having a potential of zero volts in an idealized standard hydrogen electrode.
A zinc plating cell is not.
On the surface of a zinc metal electrode hydrogen actually has a very difficult time forming
and requires a much stronger potential over that of an idealized electrode.
This overpotential is actually very high for zinc, -0.77 volts, just marginally lower
than the reduction potential of zinc itself at -0.76 volts.
We can further improve these numbers by using very high zinc concentrations
but the main point is that under the right conditions,
we can indeed selectively plate out zinc metal versus hydrogen.
So why are we displacing everything else with zinc metal?
Well our hydrogen overpotential cheat with zinc metal only works with zinc metal,
other metals have different overpotentials for hydrogen.
For example iron has a hydrogen overpotential of just -0.15 volts.
So if we have iron impurities in our solution
they will plate out onto the zinc and serve as sites for generating hydrogen gas.
Since hydrogen comes out on iron at -0.15 volts,
before we can get to the zinc plating potential.
We can preempt this process by stirring in zinc metal powder
and displacing all the metals that have reduction potentials greater than zinc.
If they're all gone and the solution is pure.
They won't plate out during our actual zinc plating process and won't start up hydrogen generation.
Now you might be wondering why i didn't do this
during the alkaline zincate process for making zinc powder we showed in a previous video.
A couple of reasons actually.
First, under the very alkaline conditions of concentrated sodium hydroxide,
most of the problematic metals would just precipitate out.
And second, under extremely alkaline conditions
we push the reduction potential for hydrogen formation into more negative regions.
Okay, i'm going to let my mixture stir overnight
to ensure all impurities are displaced by the zinc powder.
The next day filter off the zinc powder and whatever impurities are deposited on it.
Now for our electrodes.
For our positive electrode, or anode,
we'll need to use a platinum coated titanium electrode.
A lead dioxide electrode also works for this process.
Most other electrodes corrode under acidic conditions and contaminate the solution.
So we have to use platinum or lead dioxide.
You can use zinc metal anodes if your end goal is just to plate zinc rather than isolate it.
Since i'm exploring the science, i'm using platinum.
For our negative electrode or cathode, i'm going to use a strip of nickel metal.
A strip of zinc metal is better but all i had was nickel.
Now we apply the current.
Because we can still generate unwanted hydrogen if we apply too much current,
we'll need to use a very low current density
of less than 30ma per square centimeter of cathode area, preferably 20ma.
What's happening is the zinc ions are being reduced to zinc metal at the cathode.
At the anode the water is being oxidized to oxygen.
Overall we're converting zinc sulfate to sulfuric acid, zinc metal and oxygen.
For those wondering the nickel electrode with its low hydrogen overpotential
does indeed generate some hydrogen when the plating starts.
But the zinc concentration is very high
and the plated zinc covers up most of the nickel and blocks it off.
Nonetheless i still recommend using actual zinc strips if you have them.
Anyway, as we continue depositing zinc metal,
we deplete the solution of zinc and raise the acidity.
Eventually the zinc concentration will become too low to continue depositing
and the concentration of acid will be high enough
that hydrogen generation becomes the dominant reaction.
You can see the cathode starting to bubble here.
Here is the electrolytic cell after leaving it for sixteen hours,
the major bubbling of the cathode indicates the zinc solution is severely depleted
and converted to sodium bisulfate again.
The deposit of zinc is much thinner and more compact
than the alkaline process, but that's because it's solid.
Here is a close up look.
You can see small holes or pits in the deposit where hydrogen bubbles formed
and blocked zinc deposition.
I probably didn't remove all the metal impurities with the zinc powder treatment
or i used too high a current density when i was electroplating.
Nonetheless, we still have a fairly thick deposit of zinc.
The nickel metal strip is somewhat flexible
but the zinc plate is very strong and hard to bend.
Now since we regenerated the sodium bisulfate,
you can add in zinc oxide, repeat the zinc powder treatment,
and put the electrode back in to deposit another layer of zinc metal.
You can build up very thick layers of zinc metal with repeated deposition.
Although it does get rougher and duller as the imperfections build up.
Anyway, that is how you make raw zinc metal by electrochemistry.
Thanks for watching.
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