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The local hardware stores actually sell chemicals in recycled beer bottles with plastic stoppers as lids. The bottle of hydrochloric acid I bought is clearly losing gas to diffusion through the plastic, or a seal that's not tight enough--it smells bad, and the beer bottle's metallic label is slowly corroding.

How can I best seal this bottle? Some ideas are:

Try to mold a stopper from silicone. Use a (synthetic) cork, whatever's used for homebrewing. Line the stopper with a few layers of thin polyethylene or polypropylene (from consumer materials, such as air-filled plastic bags for packing).

Or could I limit the diffusion by putting a plastic bag around it? This is tough to answer experimentally (without a lab) because resisting physical degradation isn't the same as resisting diffusion.

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it smells bad, and the beer bottle's metallic label is slowly corroding.

That's, unfortunately, very common. At one of my former labs 'acid' cabinet was corroded from within. For this reason it is recommended to keep such bottles under fume hood.

AFAIK, common stoppers are made from polyethylene or polypropylene. At one of my former labs concentrated hydrochloric acid was stored in stock plastic canister, similar to that used to store petrol (on the other hand, both nitric and sulfuric acids were stored in 20l glass bottles, even though the stoppers were made from same polymer). I HIGHLY recommend to by (or order by mail) dedicated chemical bottles with caps, though. They are usually fairly cheap and pretty universal.

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Okay, so no stopper will stop it? Right, then. I came up with an alternative solution that seems to work, but time will tell: I put the stoppered bottle of acid next to an open wide-mouthed bottle of water and baking soda solution. These were both crammed together, within a couple of plastic bags. My idea is that any HCl that diffuses through the stopper will have ample chance to contact the water and be absorbed/neutralized before it makes its way to the air in the room.

I put my nose a foot from the bag and couldn't smell anything after 24 hours, so this homegrown hack seems to be working acceptably.

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