Cheap Vacuum Source For Working With Dangerous Chemicals

[Nurdrage] puts out a lot of neat videos, mostly about home chemistry. For the home chemist it is occasionally desirable to pull a vacuum. For example, a potentially dangerous chemical can be boiled and distilled at a much lower temperature than at atmospheric pressures.

However, there’s a problem with just going to the local import store and buying the first vacuum pump on the shelf.  They are primarily designed for atmospheric gasses and tend to melt when exposed to solvents. If you’re a big university or a commercial lab this is no problem. You just drop three grand on a Teflon diaphragm pump or a liquid nitrogen trap. For the home chemist who’s already having enough trouble just buying the chemicals needed for neat experiments, this is not an option.

[Nurdrage] demonstrates the proper usage of a much cheaper option: an aspirator vacuum pump. You might remember something similar from high school chemistry. School pumps generally use flowing tap water to produce the vacuum. [Nurdrage] is saving water by using a fluid pump and a reservoir to drive his aspirator.

Aspirator pumps use the Venturi effect to create a vacuum. These devices are cheap because there are no moving parts. We looked it up and the one he is using costs ten US dollars on fleabay. It can pull enough vacuum to boil water below room temperature.

The video is really good and provides a lot of useful information. It also seems like a really useful device for other hacking tasks outside of home chemistry. Video after the break.

47 thoughts on “Cheap Vacuum Source For Working With Dangerous Chemicals

  1. Hey,

    as a generell information: you should not preform a vacuum distillation with an erlenmayer flask. The risk of implosion is to high, please use round bottom flasks. I know lab glassware is expensive and you rather want to use what you can get / already have but spend a extra $ or € in the proper setup.

  2. Aspirators are useful and cheap but it’s not always the right tool for the job. You get a very wet vacuum and when running them off a tap, if the water pressure drops, or you turn the tap off it sucks back water. This is not a good way to work with something dangerous. If your dangerous compound is water sensitive it’ll be ruined by the water vapour. If it’s not water sensitive then part of your dangerous compound will be dissolved or floating in the water loop or flushed down your drain. Some solvents attack the plastic of the aspirator but the ones I used at university looked like that hadn’t been replaced in a long time. Some of them pulled a very crappy vacuum. The chemistry department drains smelled strongly of ether and chloroform.

    Nurdrage is great, but he doesn’t always do things the right way, which in this case could be either an aspirator or it could be any vacuum pump that doesn’t upstream water vapour protected by a cold trap cooled by dry ice. Even liquid nitrogen isn’t that expensive if you can buy it from your local welding store. No 3 grand needed here.

    1. Well you could always use a drying tube to protect against water vapour. Moreover, aspirator pumps I was using were completely made of glass, so there were no vulnerable plastic parts. And they had little back-stop valves built in to stop water from entering the rest of the apparatus if water flow should suddenly stop (saved me at least once, when a piece from the water mains blocked the pump). The older ones often pulled crappy vacuum, but newer ones would provide something like 15 Torr, which is all you can really expect. But I agree that with a closed-loop system your compounds will be floating in the loop, which is sorta why you never see such systems in real use.

      Speaking of hacks, you could make a cold trap using a cooling bath of ice and table salt. When placed in a thermally well insulated container, it can easily maintain −20 °C for quite a while for a penny. Although dry ice/acetone or LN2 would be much better and not really expensive anyway — again, with adequate thermal insulation.

      1. Ice and hydrated calcium chloride is one of the classic trap mixtures. I think that will reach -40C. I’ve seen a few glass aspirators for sale but the specs were terrible. I’d expect a drying tube to saturate quite quickly. I don’t know if the one way valves add a back pressure, I used manual valves for things like vacuum desiccators and everything was fine so long as they were operated in the right order.

        I have never seen an aspirator pull enough vacuum to boil room temperature water but then I’ve never seen someone use ice water before, it’s a nice trick. That said his actual pressure is a little higher than calculated because the water isn’t running into the collector so he could easily be at 20 torr. That’s a wide nozzle on his aspirator. I wonder if his vacuum suffers because of the single stage nature of his water pump. With ice water, you’d expect 5 torr if vapour pressure was limiting.

        1. No, the valve didn’t add any back pressure; it wasn’t spring-loaded, just a loose glass rod with a ground-glass male ball joint trapped inside the air inlet tube, which, in turn, had a matching ground-glass socket. Looking at the Sigma-Aldrich catalogue, I guess they’ve become a standard feature.

    2. Oh i never said an aspirator is the “always the right tool for the job”. I was aiming for a distillation-worthy vacuum at the lowest possible price point. $10 for the aspirator and an extra $40 for the rest of the parts (water pump etc.) if you don’t already have them. I was aiming to help the amateur chemist get started in chemistry. I’m NOT trying to say or even imply that it’s a good approach for everybody.

      If i had to post videos only on perfect solutions for everybody under all conditions… I’d never post a video :)

      1. I think you have a good point. In fact, I would use aspirator pumps all the time as an undergrad, and unless I required super dry or unusually high boiling solvents, these puppies were all I ever needed. But srsly, Erlenmeyer flasks and vacuum don’t mix.

  3. I wonder whether this is a possible approach for Acetone- ABS print smoothing. Sure. if it works it will take longer to alter the print due to the lower temperature, but I never thought that heating up Acetone was a good idea…

      1. Heating acetone raises the vapor pressure. Lowering the pressure in the vessel lowers the temperature at which the acetone vaporizes. Same exact thing.

        It should be more controllable since temperature speeds the solvent’s action, but acetone vapor getting pushed into the part’s voids when you release the vacuum might present some issues. Maybe there’s a way to leverage the effect?

        1. Alas no. Don’t think of a substances boiling point as the point it turns from liquid to gas. Liquid and gas are around anyway, as the temperature increases the partial pressure of the gas phase increases. At the boiling point the vapour pressure exceeds atmospheric pressure and the vapour is able to simply push the atmosphere back as it moves from liquid to gas.

        2. Nope, it’s not the same thing at all; in fact, it’s close to being directly opposite. By heating acetone, you increase the pressure of saturated acetone vapour and therefore its concentration in the air. By lowering the pressure in the vessel, you can bring acetone to a boil precisely because the pressure in the flask drops below the pressure of saturated vapour at ambient temperature. Instead of increasing acetone concentration in the gas phase, you lower it. In fact, it’s a good method of removing solvents from polymer solutions (something I’ve done more than once), which is the opposite of what you’re trying to do in print smoothing; see “rotary evaporator” for an example.

  4. Many aspirators use air instead of water for the vacuum. A friend used one to pull his auto’s A/C down to around -29psi so he could then recharge his system. Seems to have worked since he’s been driving it for over 10 years now.

  5. Use a good vacuum pump to evacuate a sturdy tank. Use the tank to evacuate the project. Air flush the tank (to get out the harmful vapors) and repeat. Voilà! Clean, deep, safely (albeit slowly) obtained vacuum. Am I missing something?

      1. To be honest, it would probably work. Sure, vapours would be continuously generated in the still pot, but they would be also continuously condensed in the condenser, so the only vapour pressure you have to take into account is the saturated vapour pressure at the condenser/receiver temperature (and even then only at t → ∞), which is likely to be negligible.

          1. Sure, because as we all know, distillation only starts after you have removed a house worth of gas from the system. Seriously, you don’t have to worry about vapours, they only exist in significant amounts in the still pot and in (part of the) condenser. In fact, if you build an airtight system, you can just evacuate it, get it running, then seal it and shut down the pump — the distillation will proceed beautifully. I’ve done this quite a few times with a rotary evaporator when I was too lazy to monitor pressure in the system; in an airtight system, it would maintain itself beautifully.

          2. Totally. Not to mention the solvent (in liquid phase) cannot have zero vapor pressure, so you can’t simply evacuate the system and start your distillation. The setup would explode.

          3. @Serge: I don’t think Rodney McKay meant “evacuate” as in “create interstellar-grade vacuum”; I think he meant “reduce pressure”. So no explosions are necessary.

          4. A sealed system doesn’t have to explode; I’ve successfully run many distillations in pre-evacuated sealed systems. As solvent evaporates from the still, it condenses in the condenser, so pressure does not build up. Pretty much the only critical thing is the flow of coolant through the condenser.

          5. It just so happens, Marco, that I not only thought about it (and have the knowledge to do it without making ridiculous errors), but actually performed a very similar process many times. If you, however, wish to persist in your misconceptions, I will make no further effort towards discouraging you.

          6. I’m not arguing you didn’t do it, I’m arguing it did not work the way you think it did. Sealing and evacuating a setup like this, and then performing the distillation, is the same as performing the distillation at atmospheric pressure. The pre-evacuation did nothing.

          7. Except of course I had a thermocouple installed inside the set-up to measure the boiling point (which was a lot lower than it would have been at atmospheric pressure), a pressure gauge to monitor pressure (it was a lot lower than 1 atm) and I could compare boiling intensity to that observed at atmospheric pressure (it was a lot more intense). Marco, sometimes you are just wrong.

          8. Bullshit Max. It’s simple physics. You can’t condense the condensate as fast as the vapor is produced by the boiling, so even if you freeze the condensate in the collection vessel, the pressure always builds to the point of rupturing the containers or blowing out the joints. It is very dangerous to try distillation in a completely sealed system.

          9. I like the way you said “it’s simple physics” followed by a completely unsubstantiated and plainly erroneous claim. I shall call it Mathew’s First Postulate: “thou shalt not condense the condensate as fast as the vapor is produced by the boiling”, because neither actual simple physics nor empirical evidence hints that this might be true. On the contrary, physics — simple or otherwise — strongly suggests that vapour pressure cannot significantly exceed the saturated vapour pressure at the condenser/receiver temperature. Suppose for a moment that it does (for instance, we rapidly inject a lot of solvent vapour); this will lead to two things: 1) boiling rate will drop (actually, boiling will stop for a time, until the still pots heats to the boiling point at the new pressure, and will then proceed at a lower rate, governed by reduced heat transfer from the heating bath), 2) condensation rate will increase, as vapour pressure is now a lot higher that saturated vapour pressure at the condenser temperature. In other words, we now make less vapour but consume more vapour. I smell negative feedback!

            Also, you didn’t do this and I did, so there.

          10. Sure Max, If your condenser has infinite surface area and you maintain it at absolute zero. Then you can keep the internal pressure of the system from going positive. Sure.

          11. A sufficient condition, but not a necessary one. You want to make a valid argument? Nothing can be simpler: 1) demonstrate, using actual physics, that this result cannot be achieved with tap water as a coolant and a reasonably dimensioned condenser; 2) make an experiment that proves you right. Until you do, I’m going to trust 2 L of tetrahydrofuran that would routinely make their way from the still to the receiver in a sealed rotavap on my lab bench more than some guy on the internet (or a bunch of them).

    1. What size? You normally want to pull out a lot of air fairly quickly, then pull down more of a vacuum. Some systems use vacuum tanks with a small vacuum pump to “store” a vacuum to remove air quickly. Then the small vacuum pump only has to remove a bit of air to pull down to a better vacuum.

  6. Would this technique (either using water or air) be worthwhile for degassing silicone during casting? I have a batch to cast and don’t want to spend the 100+ for a cheap vacuum chamber and I’ve seen the pressure cooker hack (here?) but that just seems unwise.

    1. Yes actually. if you’re just after getting the bubbles out then this will do nicely. Just be sure to have check valves in case the water backflows into your chamber and practice a few times with an empty chamber so you can get used to the process and know what to do if you see a backflow starting.

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