Greetings fellow nerds.
A vacuum pump is an optional but extremely useful thing to have to have in a chemistry lab.
You can use it to for vacuum filtrations, drying chemicals
and for changing the boilings points of solvents in distillations.
This is especially useful if you're boiling chemicals with very high boiling points
or getting around azeotropic mixtures.
Now i've shown previously these cheap six dollar nylon diaphragm pumps.
They're primarily useful for moving gases at atmospheric pressure like in an electrolysis box or for filtration,
but they're ineffective for distillations or chemical drying.
They are cheap however and even if you destroy them with solvent vapors it's not much of a hassle to buy a replacement.
Something a lot more powerful is this aspirator pump.
You can purchase these easily online and I got mine for just ten dollars.
You can also get all glass ones for a bit more but worth it if you're going to do a lot of acid distillations.
So let's open it up. This is the vacuum port.
There is a one way check valve inside this connector to prevent backflow but i'll be honest it doesn't work very well.
This is where the vacuum is actually generated.
It's just a nozzle inside a tube, that's all it is.
First you send high pressure water through the inlet here on the right
were it reaches this nozzle and becomes a high speed jet at lower pressure.
The jet shoots down this tube and thanks to the venturi effect it pulls any gases with it and creates a vacuum.
You access the vacuum through this port on the side.
It's a tremendously simple device that does indeed do a good job of providing vacuum.
It also has the advantage of being very robust to chemicals.
To use one of these you're going to need a high pressure water pump to feed it.
Here is my setup.
Now originally most amateurs and even most professionals just connected the aspirator to their faucet and ran water through it.
Obviously this was tremendously wasteful in terms of water going down the sink
and can be blatantly illegal depending on your local laws since the water is now mixed with possibly very toxic chemicals.
Now we usually have our own water pump and recycle the water as it's being used.
I'm using this 12 volt diaphragm water pump to give me a hundred psi of water pressure to feed the aspirator.
It cost me about twenty dollars online.
Now when this turns on the pump sucks up water through this inlet tube.
It's self-priming so it doesn't have to be below the water level to start.
The pressure is increased to a hundred psi inside the pump and forced into the aspirator here.
The aspirator starts pumping vacuum now
and ejects the waste water through this larger tube and flows back down into the reservoir.
You actually don't need this tube at all and can just let the water eject against the side of the bucket.
But i find using this tube to be cleaner and quieter with less splashing.
Let me turn it on to show you it's operation.
And there you go.
The great thing about this is that whatever gases are sucked in by the vacuum are in turn very well scrubbed by the water.
So if you have any reactive gases you can efficiently remove them from the stream
if you add the appropriate neutralizing chemicals to the water.
I like to use sodium hydroxide for scrubbing most of the gasses i encounter.
We effectively created both a chemically resistant vacuum pump and a gas scrubber in one.
But if you are working with toxic gases or solvents,
I still recommend placing this vacuum pump system outside or in your fume hood and leading the tube to where you need your vacuum.
The solvents can still evaporate off the reservoir and fill your lab.
If the gasses don't dissolve fast enough they'll also end up in your lab.
Anyway let's actually test our vacuum pump.
Now i don't have a chemically resistant vacuum gauge yet
but we can crudely demonstrate as well as measure our vacuum by doing a vacuum distillation.
Here i have just water and it's in a sealed apparatus connected to our aspirator vacuum pump.
Let me turn on the pump as well as turn on light heating on the hot plate.
Now at first we're going to see some bubbles and vapor but that's just because of the change in pressure.
Gases dissolved in the water are now coming out and not actually boiling.
We'll know we're distilling when we see actual drops of water falling into the receiver.
My receiver has a bit of water already because i was running some tests before.
Now all we do is wait.
If this works we should be distilling water at below the normal boiling point of water.
Reducing pressure has the effect of lowering the boiling point for all liquids.
Oh look, there it is.
We're already distilling water under reduced pressure at just 39 degrees Celsius.
That's far below the normal boiling point of water at 100 degrees Celsius.
We can actually calculate the pressure based on the boiling point since the properties of water are well known.
To boil water at 39 degree celsius the pressure inside the apparatus must be at around 52.5 torr or 70 mbar.
For an amatuer setup costing just 50 bucks worth of aspirator pumps, tubing and a container, we've done pretty well.
Now what limits our ultimate vacuum in this case isn't the the design of the system, but the working fluid.
The fluid itself has a vapor pressure that starts delivering pressure back into the system as the system pressure goes down.
While in theory we should reach the vapor pressure of the water in the reservoir,
in practice the pumping rate effectively gets slower and you approach the ultimate vacuum asymptotically.
The most effective way of getting around this is not use water at all
but to use a fluid of even lower vapor pressure like mineral oil or ethylene glycol.
Ultra low vapor pressure oil, called vacuum pump oil, can also be used.
You can also use a gas and avoid the problem completely.
But for that you need a gas compressor with much higher pressure and flow rate to match the performance of a liquid.
Now i'm not going to replace my reservoir with oil or ethylene glycol because those are not nearly as cheap and they're quite messy.
There is however one simple trick for getting even better vapor pressure just with water.
We add lots of ice.
By lowering the temperature we lower the vapor pressure of the water.
Granted, it's only temporary until the ice melts but if you have enough ice it can last you a distillation run.
Let's give it a try. We're pumping under ice water aspirator vacuum.
You'll notice the condenser fogging up because i'm also pumping ice water through it.
You can't distill something below room temperature using a coolant that's at room temperature.
So i'm pumping ice water.
The temperature is going down as we further reduce pressure
but we won't know how well we're doing until the water actually starts distilling.
Now occasionally the amateur chemist might get the idea of using the aspirator pump reservoir as the condenser coolant reservoir as well.
I strongly recommend against this as you'll be pumping whatever contaminated water your aspirator produced through your condenser.
Overall i reccomend separate water systems for you condenser and your aspirator.
Anyway there we go.
We're now distilling at 28 degrees, quite an improvement from before.
And it looks like we're still pumping down and improving the vacuum so we may get even better numbers as we wait.
Now if you go online and look for vacuum pumps you'll find thousands of better vacuum pumps
that can easily reach sub millibar pressures for very reasonable prices.
Most of them are of the rotary vane type.
Unfortunately the major drawback of such pumps is that they can only really handle atmospheric gases like oxygen, nitrogen, argon, and so on.
Water vapor and chemical solvent vapors will condense into the oil inside the pump corroding or wearing the mechanisms.
Professional chemists use liquid nitrogen cooled cold traps to trap,
condense and freeze the gases before they can reach the pump.
Unfortunately the operating costs for such protection is often out of reach for the amateur.
Another approach is to use a diaphragm pump with teflon or PTFE seals.
These seals are chemically inert and thus a PTFE diaphragm pump can take corrosive gases and even pump liquids all the same.
Unfortunately they are rather pricey.
Overall an aspirator pump gives the greatest value if you don't need physics or engineering grade vacuum.
Okay looks like the vacuum has improved considerably.
You can see here the water actually getting trapped in the vacuum adapter.
The pressure is so low it can no longer push the water through.
We've reached 19 degrees Celsius in our distillation.
To put this into perspective it's a hot day and my laboratory is at 30 degrees Celsius.
We are boiling water over ten degrees below ambient temperature.
We were able to get this twenty degree improvement from earlier just by adding ice to the reservoir.
The calculated pressure to boil water at 19 degrees celsius is about 16.5 Torr or 22 mbar.
And that is how you use an aspirator vacuum pump.
I hope this video was useful for you.
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