I was using hydrogen chloride gas straight out of the cylinder today, first time I've done that in many years. That's a very different substance from regular hydrochloric acid, which is technically a solution of HCl gas in water. The straight stuff will really snap your head back if you get a whiff of it, which you'd better not since it does Bad Stuff to your lungs, as you'd imagine.
You need to rig up a trap for the vented gas, since it's rather bad form just to send it up the fume hood. The standard way is to bubble the excess through aqueous base to neutralize it, preferably rigged up so that the water doesn't have a clear path to go siphoning back into your reaction if the pressure goes haywire. Bubbling the HCl into a solvent like methanol always looks a little odd. You can send a pretty vigorous stream of the gas in one side and have very little coming out through the trap at all, since the methanol is soaking up so much of it.
These gas cylinders are under pressure, and the large ones look just like the helium tanks that non-scientists are familiar with from balloon vendors. The regulator valves on top of them need to be made of rather more robust material for an HCl tank than for helium - which is, after all, totally inert under all conditions short of the interior of the sun. Back in grad school, a corroded regulator (on a whopping big HCl cylinder) gave me a real scare as it threatened to give way and vent all the gas at the full tank-neck pressure of about 1500 psi. I had to go sit down for a while after that one.
But today's work was with a lecture bottle, a much smaller cylinder that holds only a couple of hundred grams of the gas. That's enough to do some damage, true, but not on the scale of the free-standing ones. I saw that happen back in graduate school as well. One day I was sitting in the library, looking up some references, when I noticed the occupants of the third floor research labs pouring out onto the lawn from the rarely-used side stairwells. They were hustling right along, too, which suggested some sort of liveliness upstairs.
As it turned out, it was in the lab next door to mine. One of the guys had another HCl tank, a medium-sized one, which was also corroded and jammed. He went for the cylinder wrench, which he then used for the non-standard purpose of vigorously whanging the valve with strong overhand strokes. One of the other guys in the lab summed up the sound of this process as "PING. . .PING. . .PING. . .hisssssSSSSSSS oh @!?#!"
The hood wasn't enhanced by having a kilo or two of hydrogen chloride vented all over it, that's for sure. It looked as if it had been subjected to some sort of accelerated aging process - if there were a market for antiquing lab equipment, this would be a good way to do it. All the exposed metal was pitted and flecked with green. The stainless steel was hazed with rust, having reached its carrying capacity for corrosion. And everything still had a fine mist of concentrated hydrochloric acid all over it where the gas had sucked the water out of the air and condensed on the nearest surface. Cleaning it up is not the way you want to spend your Friday afternoon.
None of that for me today, though. I ran the stuff in uneventfully, with the reaction turning to a clear yellow, which is nothing compared to the colors I'd turn if you sprayed that much on me. I'll find out tomorrow if things have worked according to plan. One thing's certain: something will have happened. Nothing escapes from HCl gas unchanged.