3
$\begingroup$

Most separation funnels have a stem around 50 to 70 mm long (often less if fitted with a ground taper joint) that is used to direct the output into a receiving flask.

While the stop-cock has a small bore matching the small end of the funnel to reduce dead volume the stem tube is generally short and has an internal diameter around 6 to 10 mm to prevent a valuable liquid column being retained in the stem.

Having received a separation funnel c.1970 with a stem of around 200 mm (below the stop cock) I was curious what benefits it could offer over the disadvantages of more difficult clearing of the liquid column in the stem and possible much more likely mechanical mishaps when waving the thing around. I went looking and found that long stem variants are available from numerous vendors but without any explanation of what their purpose might be. I have not seen any demonstrations or descriptions that use or call for a long stem separation funnel so I am curious how they are used.

Could the long stem relate to the occasional presence of long stems in regular 60 degree filter funnels and thistle funnels. Could it have some application for faster draining with the addition of the weight of the liquid column?

P.S. The funnel in question is not the Gilson type with an intermediate diameter cylindrical section between the bulb and stopcock. The stem has a larger bore than the stopcock so not a capillary. It has a typical ground glass stopper.

$\endgroup$
  • 1
    $\begingroup$ The long stem would speed up draining (esp. at the end), but without a filter paper, that's not a big issue anyway. I find a long stem helps against splattering (as does an undamaged end of the stem). But then you have to be a bit more careful with it. $\endgroup$ – Karl May 15 '18 at 5:35
  • $\begingroup$ @Karl While Hmm, if the top layer was viscous and you wanted to remove it you could pour it out the top. I mention this capillary feature of filter funnels as it was new to me and novel but the funnel I acquired has a stopper (ground glass joint) and no way to use as a filter. Perhaps it is still useful for draining viscous liquids faster but how to break the liquid column after closing the tap, or does one leave the top product in the stem? Risky I think. I would be interested in a catalogue or experimental procedure that calls for such a separatory funnel and perhaps mentions the reason. $\endgroup$ – KalleMP May 18 '18 at 21:28
  • $\begingroup$ If the stem has the right dimensions, it empties when the switch is closed. Otherwise I'd want my money back. $\endgroup$ – Karl May 18 '18 at 21:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.