# Acetone - Can it already be anhydrous when purchased?

If I have 99.9% Reagent ACS, Lab grade Acetone, is it safe to say that it's free of moisture/Anhydrous?

• No, that will have some water. I'd guess that you can buy "dry" acetone but it would be expense. Once you open the container and expose it to air then some air will fill the bottle which has at least a small amount of moisture in it and that moisture will migrate into the acetone. You can lower the water content of 99.9% reagent grade acetone by adding molecular sieves to the bottle. That will help keep the acetone "relatively" dry. – MaxW Sep 22 '16 at 18:10
• An important thing to keep in mind is that "dry" isn't necessarily a single thing. Your downstream application determines how much residual water is acceptable. Some applications require much "dryer" solvents than others. Your acetone may be fine for some applications but not others. - Most solvents pick up water from the air, though, in amounts depending heavily on storage conditions. That's why most people re-dry solvents according to their needs, unless they purchase special solvent grades which are certified low moisture and are packaged and stored to limit reintroduction. – R.M. Sep 22 '16 at 18:52

The acetone you have ($99.9~\%$) may be pretty pure but is certainly not dry (anhydrous). The dry solvents we have in the lab are typically marked as $< 50~\mathrm{ppm}\ \ce{H2O}$, which in percent would be $< 0.00005~\%$. In your acetone, however, there are $0.1~\%$ impurities, likely a large amount of which would be water.
If you buy dry acetone (and note that it is much more expensive than the already expensive $99.9~\%$ stuff), remember that it will come in an argon or nitrogen atmosphere and beneath a septum to prevent moisture coming in. Try and keep a positive pressure of whichever inert gas you have in that bottle so it will actually stay dry. Otherwise you will waste a lot of money.