Sciencemadness Discussion Board

How does the diffusion tube in a liquid-liquid extractor work?

Halcyon - 15-2-2012 at 04:51

I'm trying to wrap my head around this apparatus




Unfortunately, I'm at a loss as to the construction of the diffusion tube, and how it works. I'm assuming that it lets the light solvent inside pass through without letting the aqueous phase seep in?

Is it basically a molecular sieve of a hole size that only lets through molecules smaller than that of water? Any clues would be awesome.

bfesser - 15-2-2012 at 08:30

In the photo, the yellow solution is assumed to contain the analyte which is extracted into the solvent contained in the boiling flask, C. With the stopcock, D, closed and the 'diffusion tube', B in place, the apparatus is configured for extraction using a solvent immiscible and less dense than the solution to be extracted.

Solvent boils at C, vapors rise through F to A, where a condenser would be attached. The condensed solvent drips into the top of the funnel shaped tube, B, which you've called a 'diffusion tube'. The extraction solvent, being less dense than and imiscible in the solution being extracted begins to fill the top of B, displacing the yellow solution downward. When the yellow solution is pushed to the bottom of the tube, B, additional solvent will pass through the holes in the bottom of B and float to the surface of the yellow solution. As it collects at the surface, it reaches a height necessary to flow back down F into the boiling flask, C. This solvent carries the analyte back into C where its concentration slowly increases as the concentration of the analyte in the yellow solution slowly decreases.

If the 'diffusion tube', B, were removed, and the stopcock D opened, the apparatus could be used with a solvent having higher density than the solution being extracted.

This description is of the basic travel of solvent and analyte through the apparatus, you'll still want to read up on <a href="http://en.wikipedia.org/wiki/Liquid-liquid_extraction">liquid-liquid extraction</a> for a more in-depth understanding of the chemistry involved.

Simply put, there's no molecular seive, just one or more millimeter scale holes in the bottom of the 'diffusion tube'. Which, I believe, is properly called a <strong>dispersion</strong> tube.

[edit]
The whole is called a continuous liquid-liquid extraction apparatus.

[Edited on 2/15/12 by bfesser]

Halcyon - 15-2-2012 at 11:51

Sorry, I should have been clearer - I understand how the whole thing works; just not what how the diffusion tube (as termed by Sigma-Aldrich here)

http://www.sigmaaldrich.com/catalog/ProductDetail.do?D7=0&am...

works and what its precise physical construction is. If there are simply tiny holes in the bottom, what's to stop the analyte from passing through and filling the tube, causing the solvent to sit on top of the analyte within the tube and stopping its passage to the main vessel?

Magpie - 15-2-2012 at 13:45

The diffusion tube is simply a very long-stemmed small funnel with a fritted glass diffusion plug in the end. Some types may not have this plug; in that case the tube end is simply open.

Notice in the graphic that you posted there is a liquid head in the inner (diffusion) tube. That hydrostatic head drives the solvent out through the bottom of the diffusion tube.

The diffusion tube is used for the case where the solvent is less dense than the liquid that contains the analyte.

The solvent with extracted analyte does not sit on top as it is overflowing back into the boiling flask. The condenser above (not shown in your sketch) is continually replenishing the solvent flow.


(Edited on 15-2-2012 by Magpie]

[Edited on 15-2-2012 by Magpie]

watson.fawkes - 15-2-2012 at 14:10

Quote: Originally posted by Halcyon  
If there are simply tiny holes in the bottom, what's to stop the analyte from passing through and filling the tube, causing the solvent to sit on top of the analyte within the tube and stopping its passage to the main vessel?
As with all things involving fluid flow, the answer is &Delta;P. The condensate drips into the center tube. Suppose the inner and outer liquid heights are equal and that a droplet drips down. Now the inner height is a little bit higher than the outer one. Gravity creates more pressure on the inner column, so now, at the hole between the inner and outer columns, there's a &Delta;P whose sign points outward, so fluid flows outward.

Halcyon - 15-2-2012 at 15:26

Ohhhhh... I understand now; the evaporated solvent condenses and drops into the inner tube until the had rises above the solvent return path, at which point it starts forcing the contents of the tube through the holes/frits at the base of the tube.

The solvent rises up through the analyze, collecting solute as it goes, overflows back into the heated flask and is evaporated back up to the condenser.

I'm hoping to get one made to use with my 45/50(?) condenser I have for my soxhlet. Can't explain what I need if I don't understand it completely ;)

Thanks so much for the explanations, guys.

watson.fawkes - 15-2-2012 at 16:42

Quote: Originally posted by Halcyon  
The solvent rises up through the analyze, collecting solute as it goes, overflows back into the heated flask and is evaporated back up to the condenser.
There's a related device, the Gregar extractor, that does not only liquid/liquid extractions with both density relationships, but also solid/liquid ones. Chemglass has technical notes for the one they sell. At the top of that document, there are three illustrations for its three modes of operation; the one on the right is the one analogous to the original apparatus of this thread. Note the position of the three-way valves and the extraction-solvent return path.

Halcyon - 15-2-2012 at 17:46

Oh Watson you classy lady. I found out about them a year ago, forgot the name and have been trying to find them ever since.

I guess my question now becomes this:

For someone who has a soxhlet, would you recommend trying to source a Gregar, or a dispersion tube and continuous liquid-liquid extractor for use with the existing condenser? The Gregar claims improved efficiency for solid-liquid extractions because of the continuous nature of the flow, but could you not operate the liquid-liquid extractor in my original post to do the same thing?

[Edited on 16-2-2012 by Halcyon]

watson.fawkes - 16-2-2012 at 05:41

Quote: Originally posted by Halcyon  
For someone who has a soxhlet, would you recommend trying to source a Gregar, or a dispersion tube and continuous liquid-liquid extractor for use with the existing condenser? The Gregar claims improved efficiency for solid-liquid extractions because of the continuous nature of the flow, but could you not operate the liquid-liquid extractor in my original post to do the same thing?
The liquid-liquid extractor at the top of the thread seems like it could do solid-liquid extraction if it had the right thimble for it; it has basically the same flow pattern as the Gregar in the same mode. The thimble in the Gregar is a glass tube with a frit plate at the bottom. This ensures that solvent exits only at the bottom and thus passes across the entirety of the sample. Using one of these with the L-L extractor posted would be possible, but the diameter at the top is fairly narrow, and you wouldn't be able to use a particularly large thimble. Also, such a thimble would likely be a custom piece of work.

bfesser - 16-2-2012 at 19:33

Glass wool plug?