Talk:Air-free technique

Suggested additions

I'm honored that Rifleman 82 asked for my comments about this article, given that I'm a theoretical chemist and don't know that much about the topic, except anecdotally. ;-) The two things I could think could be worth adding are: 1) some information about the (vacuum) pressures involved; 2) some mention of ketyls and other techniques used to monitor the dryness and degassing of the solvents (maybe even the oxygen detector in the box). --Itub 16:30, 18 October 2007 (UTC)[reply]

Ah, we've corresponded thus I have your talk on my watchlist. I noticed you gave someone advise on GA/FA-ness of some articles, and so.

Thank you for your suggestions. I'll see what I can do. Though I wonder if they are more suited to the glovebox/schlenk line main articles. Thanks! --Rifleman 82 16:55, 18 October 2007 (UTC)[reply]

The myth of the myth that Na/Bzp doesn't dry solvents.

Coming across this I read the paper. The authors of the paper have made a fundamental error, and in fact sodium above it's melting point (or aggressively stirred) is one of the most powerful desiccants available. Given wikipedias policies is it best to quote all the contrary papers? I normally wouldn't bother, but in this case I'm worried about less experienced chemists taking a conclusion derived incorrectly from a poorly designed experiment. Monodisperse (talk) 12:22, 3 March 2014 (UTC)[reply]

Well according to the reference and the present article, Na/Bzp does dry solvents but not nearly as well as sieves. Organometallickers like Na/Bzp for its dramatic colors (and the machismo aspect), but the measurements suggest that it is does not measure up to good ole sieves. We would certainly welcome a source correcting the one we have. --Smokefoot (talk) 13:24, 3 March 2014 (UTC)[reply]

Molten sodium? That would imply fairly high-boiling solvents. I except that usage will vary from lab-to-lab but 'normally' the solvents you want to dry are THF or diethyl ether, both of which boil well below this. Also, aggressively stirring a still just sounds really dangerous. Regarding the whole Na/Bzp vs mol sieves debate, my understanding is that mol sieves are ultimately better but take much longer, which can be a problem as there's no inbuilt colour change to let you know when they're done. I've always used both, distillation to remove most the the water, with the solvent then being stored in a Schlenk flask part filled with mol sieves. Older chemists have always advised me to activate my sieves in a vac-oven overnight before use but to be honest I've never found that it makes a difference. Project Osprey (talk) 10:32, 4 March 2014 (UTC)[reply]

For THF the best desiccant is potassium, it melts just below the boiling point, and destroys all water in less than a minute. For diethtyl ether you need to employ the rather hairy eutectic sodium/potassium alloy to get the same effect. If however you can stir the solvent than solid alkali metals are acceptable, as the formed oxide/hydroxide layer gets knocked off. Static drying over sodium wire is much less effective. The benzophenone is only really useful as a rough indicator, and ultimately in a perfectly dry system forms the polymeric dianion complex (brick red) assuming excess sodium. The purple colour observed in very dry THF is a mixture of the radical anion (blue) and the dianion (red).

MS like all desiccants that work by adsorbtion have an equilibrium between bound and free water, and it's highly temperature dependent. At -78oC (dry ice sublimation) MS are ridiculously effective at dry gas streams (liquid air is better, but rarely used now). At room temperature they're reasonable effective, and can certainly keep water down in the 10^-4 mol/dm^3 range, sometimes even 10^-5 depending on the size and polarity of the solvent, and the pore size of the MS. MS are completely ineffective if the solvent itself can fit in the pore. It's also worth noting that any solvent dried over MS is contaminated with MS dust, which can act as a catalyst for some reactions.Monodisperse (talk) 11:02, 4 March 2014 (UTC)[reply]

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