You need access to vacuum if you’re going to work at the bench in chemistry. In fact, you need more than one kind. Reasonably hard vacuum (well, by our standards, which is laughable by the standards of the physicists) is down in the single Torr or below – that is, less than about 1% of normal air pressure. We use that for pulling out residues of water or organic solvents from our compounds. You can’t usually see it happening from the solid ones, but the syrupy liquids will foam up or blow a long series of thick bubbles when the vacuum is applied. The foam can be an irritating problem at times; some things will fill your flask with sticky bubbles and go right on up into the vacuum line if you’re not watching them.
The lesser vacuum lines are used for bulk evaporation of solvent (on your rotavap) and for filtering things off. We do an awful lot of both of those, too, and a full vacuum-pump pull is too vigorous for them in most cases. Evaporating down reactions is a constant task in an organic chemistry lab; I’d rather not think about how much of it I’ve done over the years. As for filtration, there are many cases where a solid product can be filtered out of the bulk liquid (which is good) or where some undesired solid by-product has to be filtered out before you can go on (not as good).
The low-tech way to get the sort of pull-it-though vacuum you need for these things is a water aspirator. You don’t see these as much any more, and you don’t see them at all in industry, since they necessarily pull solvent vapors into the water stream. But they work. An aspirator is basically a narrowing tube that hooks up to a hard-spraying water tap and has a sidearm fitting. The accelerating blast of water pulls the air in the tube along with it as it goes, creating a useful vacuum. If you wanted to make one rather more environmentally friendly, you’d keep a well-stocked dry ice condenser in line with it to trap out the solvent vapors before they go down the drain (which is what your rota-vap should have on it, anyway), but even with that, you’re always going to be turning the water flow into a waste stream. As I say, you don’t see them as much these days.
But we used them back when I was in grad school, that’s for sure, mostly for the rotavaps. If you wanted to keep things from splashing around back in your hood, you attached some rubber tubing to the other end of the thing and ran it further down the drain a bit.
Well, one day, one of the guys in the lab next door to me was shocked to see water blasting around in his hood. It was a real fountain, just geysering out full blast from what must have been a cracked water line or something in the back. He ran over and immediately shut off every tap – but to no avail. Roaring, showering water everywhere. Getting a look at the source, he realized, to his consternation, that the water was coming up out of the drain in the back of his hood. I remember standing there with him, staring at this in disbelief. It looked like a special effect. How on earth could you get water blasting up out of a drain pipe?
Suddenly it hit me. I ran around to the other side of the lab, where a new Japanese post-doc had taken up residence. “Masa”, I asked him, “Did you just put that rota-vap in your hood today?” “Yes, yes, just started it today”. There was a water aspirator flooshing away back in the back of his hood. “Did you put some rubber tubing on that thing?” “Tubing? Oh, yes” “How much?!” “Whoaaa. . .” He spread his arms to indicate the mighty extent of the rubber tubing he’d added.
Mighty, indeed. He’d run the stuff down his drain, through a horizontal pipe and right through a T joint, and back up out of the drain of the other guy’s hood, which backed on to his. So when he turned his water on full throttle, he immediately started irrigating his labmate’s space. We finally go thing turned off, and trimmed back the rubber tubing to a more reasonable length (like, not seven feet), and order was restored. For a while.
Note: if you want to see How Not To Do It to a really expensive vacuum rig, try here.