Monday, March 09, 2009

How to make dry HCl gas

Dry HCl gas is essential for certain reaction types and can come in handy for making various saturated HCl solutions. The easy solution to this problem is to have an HCl cylinder handy. However, I’m sure that some of you have experienced (or have heard of) the horrors of the corroded gas regulator on an HCl cylinder. Regulators on HCl cylinders have a very bad habit off snapping off! One of my good friends had a very close call with a big HCl cylinder. Luckily he was standing right next to the door so the only thing that needed to be replaced was that particular lab. If you must have an HCl cylinder standing around I’d recommend a lecture bottle (see photo). These are (in theory) less likely to go off since they have a relatively short life time and if they go off they are likely to kill fewer chemists. Nonetheless, this particular HCl lecture bottle in my current lab has decided to corrode/fuse. A tech-guy has been by three times over the last 6 months trying to get the regulator off. I just wish he would take the damn thing with him and not store it in the hood next to me. A safer, relatively simple, cheap and convenient way to get hold of some dry HCl gas is to make it yourself. The standard approach that I’m guessing most chemists still use is to add conc. sulfuric acid to sodium chloride or conc. HCl. However, avoiding the use of conc. sulfuric is desirable because it’s nasty and you’ll have to clean up afterwards. So in the interest of safety I would recommend the addition of conc. HCl to calcium chlorid. It’s cheap, the HCl gas that you generate is completely dry, it’s relatively easy to clean things up and the reaction is very easy to control. Here’s the original reference:
A Convenient Way to Generate Hydrogen Chloride in the Freshman Lab, Francisco J. Arnáiz, Journal of Chemical Education, 199572 (16), 1139.
On the photo you can see one of my recent set-ups. Here I am adding conc. HCl to calcium chloride with a pressure equalising addition funnel and bubbling the HCl directly into my reaction flask. Please note the use of a Pasteur pipette for bubbling the gas into the reaction flask. Do not get tempted to use a metal needle. Also I often add a wash bottle between the reaction flask and the gas source in case of unexpected suck backs. D!