Talk:Cannula transfer

syphon technique

There's a third, lesser known way to cannula xfer reagents that is useful when a slow, dropwise addition is called for. This might be too technical/obscure for the article, which is why I'm bringing it up here, rather than just adding it. In this technique, you get the xfer started the usual way, but then quickly lower the receiving flask such that the tip of the cannula on the receiving side is lower than the tip on the source side. This sets up syphon. You can then allow the pressure between the receiving and source flasks to equalize. If you've got the syphon started properly, the material should continue to xfer over. The genius part of this technique is that you can control the speed of the addition by changing the relative heights of the flasks. Super useful for slow, dropwise addition, but if you have any clogs, the whole thing can grind to a halt. Yilloslime (t) 07:00, 6 February 2009 (UTC)

I assume once you've primed the syphon, both are open to the nitrogen line, such that they are at atmospheric? Such that the pressure at the inlet end of the cannula will equal h(rho)g + atmospheric + (weight of fluid within tubing)/diameter, which is greater than atmospheric? I'm going to try to phrase it below, please feel free to edit: --Rifleman 82 (talk) 07:19, 6 February 2009 (UTC)

Ya, I think we're on the same page. Assuming you are using positive pressure to push from the source to receiving flask, then once you've got the transfer started & flasks at the appropriate heights, you remove the vent from the receiving flask and open it to nitrogen. Both flasks are now under equal N2 pressure, and the transfer proceeds by syphon. The transfer is much slower than if you just push with N2, but that's the point.Yilloslime (t)

Syphoning

By carefully filling the cannula fully with either above techniques, then allowing the pressures within the vessels to equalize to atmospheric, a syphon may be set up. This arrangement allows the slow addition of a fluid to a reaction vessel; the rate of addition may be controlled by adjusting the relative height of the donor vessel.

This looks great. Yilloslime (t) 07:28, 6 February 2009 (UTC)

Use of the term "vacuum transfer"

I realize there are probably different "cultures" in different labs, but I've only ever heard the term "vacuum transfer" applied the air free technique in which you connect the receiving and source flasks to high vacuum line, pump everything down, immerse your empty, evacuated flask in liquid nitrogen, and open the valve source flask. Solvent vapors from the source then condense in the receiving flask, affecting the transfer. In my experience this is often used to transfer NMR solvents off of sodium benzophenone ketal. So at any rate, I think it's confusing to have two air-free transfer techniques both called "vacuum transfer," and I've never actually heard the term used in this context.

The other problem I have with this section--and again, maybe this is just my experience--is that I would never actually use this technique, nor have I heard of being used, except by mistake. The minute a flask is at less than atmospheric pressure there's an opportunity for air to be sucked into the system, defeating the whole reason behind employing an air free technique. My inclination is to just delete this section, but perhaps this technique and its name are in greater use than I realize. Yilloslime (t) 07:21, 6 February 2009 (UTC)

Maybe it's cultural. I guess if all your fittings were tight, you won't have a problem. I understand the preference for over-pressure rather than under-pressure, but I understand it as more about storage (don't store flasks under vacuum!). The use of vacua is inevitable in Schlenk work and I suppose not much air gets into good, properly assembled glassware. Anyway, I have found this technique helpful for viscous liquids, since the pressure difference is greater than nitrogen pressure. --Rifleman 82 (talk) 07:43, 6 February 2009 (UTC)

I've worked in labs that use Yilloslime's more formal definition of vac transfer. I've worked in another lab that didn't do much formal vac transfer since we had authentic solvent stills used at reflux. In the lab that didn't use formal vac transfers the language was not used as strictly. Actually the section should probably be called "transfer" and have subsections for the different methods of achieving the necessary pressure differential. Notably the vacuum technique uses a static (vs active) vacuum. This is my preferred technique with stuff that isn't super hot (reactive). Also "pressure transfer" is best done when venting through a bubbler but there are all different levels of hack and when I use this technique I usually just vent through a high gauge needle. I actually had a married couple for graduate advisers and they each taught different versions. Nice article I might expand the filter section.--OMCV (talk) 12:08, 6 February 2009 (UTC)

I usually achieved the (static) vacuum by turning the stopcock 180 degrees, from closed to closed, via vacuum. Once or twice was usually enough. That's how we used to fill bubblers too from the heating oil baths, too. The gallery at the bottom shows some ways to vent, while the Errington ref shows the use of a needle, but I always used a stand-alone bubbler.

With regard to the article, I think we should call it "cannula transfer" as opposed to "syringe transfer", the next equal-level header. --Rifleman 82 (talk) 19:16, 6 February 2009 (UTC)