# Does storing a hygroscopic reagent in a bottle makes it prone to absorb humidity from the air trapped inside?

I've got a full bottle of 99,5% acetone, which I'd really like to keep it as dry as possible, though without using molecular sieves or other dessicants.

Once I open the bottle for the first time, use part of the reagent (say 25% volume) and then close the bottle again, will the remaining 75% acetone begin to absorb water from the 25% trapped air inside the bottle?

Also, does vapor pressure plays a role on this?

• Due to acetones low boiling point it can easily be distilled again prior to use. The significant difference between the b.p of acetone and water makes them easy to seperate in this way and produces a pure distillate if done correctly. – Technetium Oct 30 '15 at 12:02
• Yes over time the acetone will absorb moisture from the air (humidity) inside the bottle. – Technetium Oct 30 '15 at 12:16

Does storing a hygroscopic reagent in a bottle makes it prone to absorb humidity from the air trapped inside?

Yes, there is that tendency. I'll make a couple of points which I'll number for possible further discussion.

(1) 99.5% acetone is probably 0.5% water. So the "extra" water from the air above the acetone in the bottle (from one opening and closing of the cap) would be negligible compared to the amount of water already in the acetone. Obviously when using the acetone, uncap it and recap it ASAP to keep it as dry as possible. The "problem" is opening and closing the bottle many times.

(2) I looked quickly at Wikipedia, and do a quick a google search and didn't find any water-acetone azeotrope. So the distillate would be "pure".

(3) I'm not sure how pure a simple distillation in air of the acetone would be in removing water. I'd guess that the resulting purity would be like your 99.5% starting purity.

(4) Vapor Pressure would play a role in that the acetone would have a partial pressure in the bottle. So not all of the void in the bottle would be air. Acetone doesn't "boil" at room temperature though so the partial pressure won't be enough to keep some air from seeping in and out of the bottle. In other words the void in the bottle is mostly air with just some acetone.

(5) The !@#$%^&*( bottles "breath" as the air pressure changes if they are not absolutely sealed. • I did some more research and also got some numbers: density of water in air at room temp. and 100% relative humidity is around 20mg/L. That'd mean, for my example, that 250ml of air "contains" around 5mg of water, which is ~0.0008% in weight of the remaining acetone, totally negligible compared to the starting 99.5% purity. You could add this information to your first point for completeness. Thanks! ;) – emi Oct 31 '15 at 20:50 • That of course is a "one time exposure" as you open and close the bottle. The other thing is that the !@#$%^&*( bottles "breath" as the air pressure changes if they are not absolutely sealed. – MaxW Oct 31 '15 at 21:14