Warning this experiment handles boiling sulfuric acid. This must be performed with gloves, outside or in a fume hood.
Greetings fellow nerds.
In this video we’re going to purify sulfuric acid by distillation using the heating mantle we built in a previous video.
Now sulfuric acid can be made in various ways or obtained from some types of drain cleaners.
But often times it’s not very pure and contains contaminants, particularly water.
Now you can boil it down to remove some of the water.
But the most effective way to purify it is to distill it.
So let’s setup our apparatus.
We start with the heating mantle that we built in a previous video. Mine is connected to a variac off camera.
Now we get a condenser. Mine is connected to a water supply also off camera.
And here is a 100 mL round bottom flask filled about halfway with sulfuric acid from drain cleaner.
As you can see it’s quite dirty.
Remember to clamp it into the stand so there is no chance it will fall over and spill.
Now we connect it to the condenser using an elbow joint coated in high temperature grease.
At the other end here we put some beakers to receive our distillate.
And we’re ready to start distilling. Let me zoom in.
First turn on the water flow and then turn on the power.
Start on low power and gradually ramp it up until you can see stuff condensing.
If you look closely at the acid, you can see it expanding as it heats up.
This is why we fill only fill half way and not completely, to allow for thermal expansion.
Eventually volatile organics and water should start condensing.
The distillate will also have small amounts of acid but this is rather dilute and not worth saving.
We need to collect when the acid concentration is fairly high.
A simple way to find out when the acid concentration in the distillate is high enough
is to take a small sample and apply it to a tissue.
If it’s mostly water then nothing will happen and the tissue will just get wet.
You can discard the distillate you’ve collected up to that point.
Keep running this test and eventually the distillate will burn the tissue,
this is a strong indication that we have more acid than water at this point.
Change out the receiver and start collecting your acid.
Here is a close up of the tissue with the acid having burned clean through it.
Now while the acid is very concentrated,
it’s not yet the common 98% concentration that corresponds to azeotropic sulfuric acid.
To know when we’re at that concentration, pay close attention to the distillate in the receiving beaker
Here is a separate run with smaller beakers for clarity.
Anyway, as the acid distills, if the concentration is still increasing then you’ll get these distortions
as light refracts through the changing density of the acid.
It’s best to have a picture or something behind the beaker to easily see this.
Now if you do see these distortions then change out the receiving beaker.
When we’ve reached 98% pure azeotropic sulfuric acid then the distillate will be clear and homogeneous without any distortions.
This is because the density is now uniform.
The acid from here on in will be the highest purity you can get by distillation.
Just keep going collecting the clean acid until you have about 20% of residue remaining of the dirty acid.
I don’t recommending going further as the distillation tends to bump and boil uncontrollably when it gets this low,
spitting dirty acid over the condenser and into your clean acid.
Before it gets that far, turn off the power and let everything cool.
And there we have it, purified 98% sulfuric acid.
The less pure, but still clean acid can be applied to uses where you can tolerate water or where you’re going to dilute it anyway.
Alternatively you can add it to future distillation runs to get higher purity.
Anyway, if you want to test your 98% pure sulfuric acid, a fun thing i like to do is the classic sugar carbon-pillar experiment.
Just mix the acid with an equal amount of sugar and wait.
Concentrated acid will dehydrate the sugar and convert it carbon that should form a pillar that rises out of the beaker.
Anyway, that’s how you purify sulfuric acid by distillation.
Thanks for watching.
In this video we're going to make a heating mantle.