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Author: Subject: How useful is a soxhlet ?
Sulaiman
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[*] posted on 29-8-2017 at 12:47
How useful is a soxhlet ?


If I am not mistaken, the 'benefit' of a soxhlet extractor is that it continuously uses freshly distilled solvent for the extraction,
rather than just boiling the source material in the solvent,
which will become progressively more saturated as extraction continues.
Simple, an obvious benefit.
but
given that every extraction that I've come across uses a massive excess of solvent,
the benefit of a soxhlet is lost on me.

What am I missing ?




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[*] posted on 29-8-2017 at 14:36


With a soxhlet you can get away with a lot less solvent in total. This is because you are constantly extracting with pure, freshly-distilled solvent and not something that has stuff already dissolved in it.
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[*] posted on 29-8-2017 at 15:59


I understand that part, but I've not seen an example of using a Soxhlet with a minimal quantity of solvent
(just saturated in the boiling flask + Soxhlet capacity)
people normally use far in excess of this quantity.

e.g. NileRed "How to extract Capsaicinoids from Chili Peppers "
https://www.youtube.com/watch?v=cQIMLEwQWL0

250 ml solvent used for extraction, evaporated down to <10 ml prior to final solvent evaporation.
so at least a 25x excess of solvent was used,
equivalently, the solvent in the pot must have been <4% saturated.
I doubt that there is a significant difference in dissolution rate between 0% and <4% saturated.
In fact, the minimum quantity of solvent required in this case is less than the Soxhlet capacity.

I suspect that Franz Ritter von Soxhlet had a good use for his invention,
but it seems that folk use a Soxhlet where none is required, other than for watching the cool siphoning cycle :P




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[*] posted on 29-8-2017 at 17:06


You can go beyond saturation with a soxhlet. Your boiling flask can build up a precipitate and still your extraction is meeting fresh solvent.
It is possible to recharge a soxhlet with fresh material without touching the solvent. This is messier with a one-pot or reflux setup.
There is also a set and forget kind of approach with a soxhlet. Yes I know that you could achieve the same with a reflux but the soxhlet is designed for numerous cycles unattended.
With a soxhlet extraction your material is not stewing in the flask -- the extraction takes place at temperatures below the boiling point. This may or may not be an advantage depending on what you are doing. It does have some implications for solvent choice.
A soxhlet can be used as an absorption device instead of an extraction device. Nurdrage did a tidy video where he improved the yield of a Fischer esterification. Molecular sieves were used in the soxhlet to remove water from the reaction solvent and drive the equilibrium forward.
And, as said before, it is possible to get away with a lesser volume of solvent with a soxhlet extraction -- considerably less for substances with very low solubility.
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[*] posted on 29-8-2017 at 22:12


I think the set-and-forget operation is one of the more important reasons. At a lab I worked (we did analytical work, often on samples of unknown composition and concentration), we had 6 soxhlet units in a fume hood ready for use. It took 5 minutes to start an extraction and it would run all day without intervention.

The ability to remove water (it forms an azeotrope with toluene) is also quite beneficial. Air drying comes with the risk of product loss and heat/air degradation and can take absolute ages in some cases.

Usually the amount of solvent used is one of the less important aspects of an extraction. Knowing that you will get a complete extraction in one try regardless of what you're working with can be much more important.




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[*] posted on 29-8-2017 at 23:58


OK, I think that I understand some of the benefits of a Soxhlet, thanks.
I think that for most extractions I will not need a Soxhlet but I can see occasions where I may.
____________________________________________
PART 2 ... how long is a piece of string ?

I have standardised on 500 ml 24/29 or 24/40 flasks for boiling pots,
so I would use up to 400 ml of solvent.

I assume that a 500 ml Soxhlet would be too large to work,
a 250 ml Soxhlet could work but there would be very little solvent in the pot at times.

What would be the maximum recommended capacity of a Soxhlet to match 500 ml boiling pots ?

OR

would it be a good idea to get a 500 mL Soxhlet/Allihn pair (cheap via eBay) and fill any unused capacity with e.g. 3mm glass balls,
allowing me to extract from small quantities of source material 'filled' with glass balls,
or large quantities of solids for extraction ?

AND

For a similar cost I can have a 500 ml Soxhlet extractor with either a
.............................. Graham .............................. or .............................. Allihn condenser
Soxhlet_Graham.jpg - 212kB Soxhlet_Allihn.jpg - 131kB

I guess that the Graham would be more efficient but prone to fouling or choking and more difficult to clean, so
Which is likely to be the more useful of the two ?
Why ?




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[*] posted on 30-8-2017 at 16:48


I've never extracted natural products. So, I was never a big fan of the Soxhlet, until I saw it used, to drive a reaction to completion by water removal, via loading molecular sieves in the thimble.

Same reason we like the Dean-Stark trap. Drives reactions to completion by removing H2O as formed.

Don't need that function, every time you run a reaction, but when you do, the Soxhlet seems to do it really well.

https://www.youtube.com/watch?v=Ah5ds_3s5BI



[Edited on 31-8-2017 by zed]

[Edited on 31-8-2017 by zed]
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[*] posted on 30-8-2017 at 18:34


i used the condeser on the right with a different style soxhlet to extract fat from solids using 30-60C pet ether

i had the solids in a cellulose thimble i didnt need glass or cotton wool as my sloids didnt float but that would be a must if your solids floated

my soxhelts looked like this one https://i.ytimg.com/vi/6hY4u9t8bEQ/hqdefault.jpg

it worked well as long as you had more solvent in your flask than the soxhlet can hold others wise it had a tendancy to catch fire

I even had these in series about 14 they managed to condense the pet ether really well with no losess

I used much bigger ground joints for the flask and soxhlet base

[Edited on 31-8-2017 by feacetech]

[Edited on 31-8-2017 by feacetech]
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[*] posted on 31-8-2017 at 11:36


For some extractions- especially from natural products- there is more to it that just "adding enough solvent to dissolve the material you want to extract".
Quite often the material is strongly absorbed on the cellulose/ lignins etc. So it's not an issue of dissolving it, it's an equilibrium where the ratio (stuff in solution) / (stuff stuck to the substrate) is more or less constant
So shaking it with solvent won't dissolve all of it- perhaps just a tenth, or a hundredth.
You could keep doing that then recombine all the 100 or so extracts and distil off the solvent.
But' it's easier and quicker to let a soxhlet extractor automate the process.
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[*] posted on 31-8-2017 at 14:03


There was a reduction I attempted to emulate from a piece of literature ( I cant remember what that was now) but it called for the addition of LiAlH in THF added to the reaction zone under reflux. The setup used a Soxhlet to dissolve the LiAlH and add it to the reaction. Sounded great in theory but the reaction was way too vigorous with the first addition and boiled the reaction zone straight up the extractor and threatened to flood the LiAlH left in the thimble and cause further mischief.

I yanked off the soxhlet after pulling the power from the wall and had to watch the foamy mass errupt out of the flask and into the mantle while I disconnected the condenser and got it connected to the flask. Thankfully the condenser was the same joint size as the soxhlet and the swap over was quick. The reaction was ruined and the mantle took ages to clean...

One application of a Soxhlet I was not in a hurry to repeat.

The soxhlet's habit of filling then siphoning was replaced by a thimble held under the reflux stream of the condenser. Excess solvent flowed over the thimble while the frit allowed concentrated LiAlH THF solution to percolate into the reaction more constantly.
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[*] posted on 1-9-2017 at 16:54


Yup, my buddy used to run that reaction with Diethyl Ether. It gave me the heebie-jeebies watching him do it, but he had no mishaps. At least no mishaps with that part of the reaction.

Did blow up a few building, eventually.

[Edited on 2-9-2017 by zed]

[Edited on 2-9-2017 by zed]
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