co27
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Ask a solution of Buchner funnel
Can someone give advise to solve this problem, There is little gap between the filter and funnel bottom so that liquid is sucked up through vacuum
hose. it is impossible no gap here, but it might be dangerous or wear out the pump. filter paper or something else can avoid the problem ? thanks
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Funkerman23
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If you are trying to prevent anything from getting sucked into the vacuum hose you might trying reducing the vacuum strength via inserting a valve
between the funnel and your pump( though to be fair, it is never a good idea to use a vacuum pump in the lab without having some sort of trap system
to prevent pump damage).
If you are trying to prevent your frit from getting too dirty, I myself would look for a paper filter( assuming your product& reactants won't
destroy paper or be contaminated by paper) to place in the funnel. Some members here have the means to clean a glass frit properly, while others do
not. You need to know which of these two groups apply to you and plan accordingly.
If your funnel has a gap between the glass frit and the side walls of the funnel then trying to work around that is going to be difficult. In short
that means your funnel is defective. The glass frit should be bonded to the inner walls without any gaps.
if you happen to have any PTFE tubing that will fit over the inner drip tubes diameter then a short length of that fitted to the drip tube might help
as well.This may work or it may be a terrible idea. Your reactions/ procedures are what determines this. Be careful and proceed at your judgement. You
alone are responsible for your safety.
Could you get a picture of the hole itself? That would help and be better than a wall of text.
" the Modern Chemist is inundated with literature"-Unknown
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aga
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When looking at that type of vac setup i chose the plastic cup and erlenmeyer-with-a-side-arm setup instead.
It might help to add a short length of plastic pipe to the glass pipe coming out of your glass filter cup.
Choice of plastic entirely up to you and your experiments.
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Praxichys
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If you're getting solids past the frit, you're either using the wrong frit density or your funnel has some defect. That picture is a big mess.
Foaming can be an issue in the vacuum flask sometimes but you can add a few drops of silicone oil to the empty flask to help with this before the
filtration. That, or pre-charging the filter flask with acid will help break soaps.
Anyway, you should always be using a trap between your vacuum pump and whatever you're sucking on.
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Dr.Bob
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It looks like the filtrate is being sucked from the drip tube back into the vacuum inlet, due to the short length of the drip tube. All of the
suggestions are good, Chemglass even provides a short piece of Teflon tubing for some of their frits, just for that reason, but usually just weakening
the vacuum a bit will work. A trap is also a good idea in general. But you can also just look to only buy frits with longer tubes, but they break
so easily that they often don't last long before the drip tube is broke off entirely, which is why Chemglass went with the short tube and longer tube
extension. That does work pretty well.
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ave369
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It is always safer to use the Buchner flask (thick-walled Erlenmeyer with side arm) than any kind of side-armed funnel.
Smells like ammonia....
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chemrox
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I believe you're getting saturated solvent through. Crystallization is incomplete! Your mixture still has saturated solvent in it. Then it
crystallizes in the lower parts of the system. Try a different solvent or solvent mixture and lower the temperature before running the filtration.
Crystallizations have given me fits over the years. Right now I'm trying to crystallize an amine citrate and there's excess citric acid which wants to
make a syrup instead of a powder. I'm trying to find a way to get it out without starting over. I think the HCl salt will be insoluble in the solvents
so there's a possible way through. But that would in essence be starting over with three more procedures to complete!
[Edited on 30-9-2015 by chemrox]
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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