Sciencemadness Discussion Board

Buchner funnels: which is best for what?

Funkerman23 - 30-4-2012 at 07:04

after looking around on the Laboy site I noticed they mentioned an alternative to a glass fritted buchner funnel. I was under the impression that the glass fritted ones where the go to but to cut my waffling short:
When is it best to use a ceramic Bucher funnel, fritted glass one or a perforated glass plate buchner funnel? Opinions greatly appreciated.

Hexavalent - 30-4-2012 at 08:03

All have their pros and cons, it really depends on what kind of stuff you'll be filtering on the most part.

Dr.Bob - 30-4-2012 at 08:51

Buchner's are great when you want to collect the solids, as there will be a piece of filter paper under the product, so it is easy to get most of it out from the funnel once it dries a bit. Also, there is frequently some small amount of solid that goes through or around the filter paper, so it works best when the product is cheap or you will be concentrating the filtrate anyway to recover more product, since you won't mind losing a bit. They are also much cheaper and easier to clean one you are done with them. If you have something goey, they are good, as it is easier to clean them. You can also get various types of filter paper, but 90% of the people I have seen use Whatman #1 paper, which is cheap and easy to get.

Glass fritted funnels are very nice, there are several frit porosity choices available, usually course, medium and fine. The course ones are best for removing a drying agent like sodium sulfate, filtering through a bed of celite or silica gel to remove polar or particulate impurities, or quick filtrations. The medium are often used to collect products or remove finer byproduct solids from reactions (like K2CO3 or KBr or other salts). If I am trying to collect the solid product, I sometimes put a piece of filter paper on top of the frit to make it easier to get the product off of the frit. And fine ones are for collecting products that are themselves very fine solids. Be aware that if you use a course frit, it is hard to scrap a solid ff of it without loosing a lot into the porous frit itself, which looks like a fused piece of fiberglass insulation, with lots of empty space inside it.

That is the downside of frits, if that they hold solids well, and are very hard to get clean once you get an insoluble solid in them. I have lots of them which are dirty with something that is hard to remove. But if you use a med or fine one, there is little chance of solids getting through them, so I use them most often when I want to keep the filtrate and I don't want it contaminated with the solids. And if the filtrate is a salt, they are not so hard to clean out. But get some activated carbon, Pd black, or other fine solids in them, and they are black or dirty forever, unless you resort to very hazardous cleaning solutions.

I don't use perforated plates much.

My favorite other choice is a Whatman type membrane filter holder, typically the 47 mm type, which has a bottom piece with a glass frit, but is designed to hold a membrane type filter (finer than filter paper) and a top reservoir, so it gets the best of both worlds, a glass frit for stopping fines, and a membrane filter that is easy to remove product from, and it can be taken apart for cleaning. Plus you can get membranes of several types of materials and in different porosities, for a lower cost than buying a bunch of different glass frits.

I will shamelessly note that I have many of these types available for purchase. Please contact me for more info, or look at the thread:
http://www.sciencemadness.org/talk/viewthread.php?tid=15667

[Edited on 30-4-2012 by Dr.Bob]

Endimion17 - 30-4-2012 at 09:32

I'll say - go with the Büchner funnel, which is the one made out of ceramic, with holes inside. You don't really need more than that.
I used several glass fritted funnels at home, and as Dr. Bob mentioned, they're so easy to get dirty, usually with something insoluble and inert, making them look like they were dipped in mud after a while. Then you resort in washing them with chromic acid or even piranha solution because you want them to look perfect... oh. :S

One thing that comes to my mind is Willstätter nail (about which I've made a local Wiki entry few years ago; it never got translated to the English version, though). It's laughably easy to make, and does the job well.
You get an ordinary glass funnel and a short glass rod that fits into the funnel's neck easily. Then you heat up the top of the rod until it softens and then you press it lightly against a suitable smooth surface (I used asbestos) until the squished part is wider than the neck of the funnel. If all done well, the edge of the nail will be very smooth.

Now, if you put the nail in the funnel, there's a tiny space between them, large enough for the liquid to pass under a lowered pressure, but small enough to stop even tiny crystals. Here's a sketch.

It's very efficient and a nice piece of DIY equipment letting you filter out small amounts of solids, and it doesn't gulp up some of it, like fritted funnels do. You essentially recover all of the solid.
As usual, you can even put a piece of rubber glove over the funnel while it's being vacuumed, to help drive out the solute.

[Edited on 30-4-2012 by Endimion17]

Funkerman23 - 1-5-2012 at 19:01

Pardon me if I sound...well dumb but if a little gets past the ceramic buchner funnel wouldn't it be easier(assuming the reaction/synthesis allows this) to take the filtrate and let it settle in a sep funnel? If I need to shell out the cash for a fritted funnel I will but if a plain old ceramic one is acceptable then I'd rather go that route. although Dr Bob: do you have any of those rubber funnel gaskets? The last time I used a bored out Rubber stopper the sodding thing hardened!!( although slowly I still don't have a stopper boring machine just yet to replace it)

[Edited on 2-5-2012 by Funkerman23]

jamit - 1-5-2012 at 20:03

For the home chemists there are three choices for filtration: 1. gravity filtration using a cone funnel, 2. buchner funnel (ceramic or plastic), or 3. glass fritted buchner funnel. I have all three and from my experience, the best choice is the ceramic buchner... hands down!

The glass fritted buchner funnel is a nice addition to the home chemist but like dr. bob and edimonian noted, it is so easy to get it dirty and it's a pain to clean it.... I had to use pirahna solutions many times to get rid of the brown and carbon stains on them, and they do a good job but I've learn to use them selectively. Some fitration will blow a hole if you use regular filter paper and for those situation, the glass frit is ideal (for example, making hydrobromic acid using sulfuric and sodium bromide).

Most of my filtration, especially for larger synthesis of chemicals, requires me to use the 9cm and 11cm ceramic buchner funnel and they're great!

My advice to you, Funkerman23, is that you shouldn't just buy anything until you know what you want to use it for. And the glass fritted buchner is nice but for the most part, not necessary. I have 5 of them, which I won on ebay, but I don't use them very much. Ceramic buchner, that's the best choice, I think, for the home chemists! Good luck.

fledarmus - 2-5-2012 at 03:58

Glass-fritted funnels are best for speed if you have a nice solid, preferably soluble in a convenient solvent, that is larger than the pore size in the frit. It takes no time to set up, you don't have to be careful about adding material to avoid floating the filter paper (since there isn't any), you don't even have to be particularly careful about removing the solid from the filter paper (since there isn't any). You can rubber-band a piece of Kim-wipe over the top and pop them directly into your vacuum desiccator if you like, without having to worry about losing your product. They are especially useful for systems where you are collecting the mother liquor rather than the solid - filtering out drying agents, polymer-bound reagents, or filtering agents like celite, that you intend to just toss when you are finished.

The problems - they are much more expensive, they are easier to break, and there is a strong temptation to try to filter something which is really a little too fine, and you end up plugging the frit. In short, the glass-fritted funnels let you move faster and work sloppier while still getting good results.

If you are a careful experimentalist, the buchner funnels work just fine. There are systems you can't filter through a buchner (because they eat filter paper), and systems you can't filter through glass (because they plug the pores), so it certainly helps to have both. Which ones you use really depends on your willingness to trade cash for speed.

Dr.Bob - 2-5-2012 at 05:03

Quote: Originally posted by fledarmus  
polymer-bound reagents,


There is a concept I haven't seen in a while. Do you still use them?

Does anyone else on here ever use polymer bound reagents or scavengers? I used to do a lot of that work, so I would be interested to know what people are using them for still.

Magpie - 2-5-2012 at 09:10

Quote: Originally posted by jamit  
For the home chemists there are three choices for filtration: 1. gravity filtration using a cone funnel, 2. buchner funnel (ceramic or plastic), or 3. glass fritted buchner funnel. I have all three and from my experience, the best choice is the ceramic buchner... hands down!


I agree.

I have 4.5cm, 7cm, and 15cm Buchner funnels and use them often, especially the larger ones. I also often just use gravity filtration using a folded 15cm paper in a glass long-stemmed funnel.

I bought 2 ea 6cm fritted glass funnels for next to nothing at a university excess sale. Otherwise I would have deemed them too expensive. I haven't used them yet but have tested them with water under vacuum. One filters free but the other is partially plugged with something I can't even see. I've tried running acids and H2O2 through it for cleaning but nothing has worked so far. I don't think I've tried piranha yet.

Funkerman23 - 2-5-2012 at 14:12

Wait a minute.. don't they sell fritted glass inserts for ceramic buchner funnels? ehhhh I think I am mistaken on that..hmmm

Poppy - 25-6-2013 at 11:12

"The filtros plate for filtration can be very conveniently sealed into the Büchner funnel with a paste of water-glass and amorphous silica which is then hardened with concentrated sulfuric acid."

source - Organic Syntheses, Coll. Vol. 1, p.10 (1941); Vol. 5, p.5 (1925).

DraconicAcid - 25-6-2013 at 11:23

We've got plastic ones. They work just as well as the ceramic ones, but the students don't break nearly as many of them. And they come apart for easier cleaning.

crazyboy - 25-6-2013 at 17:47

Personally I prefer fritted glass funnels because you never need to buy filter papers, they come in a range of porosities and you don't have the issue of scraping up the filter paper along with your product. The downsides are they are more expensive, you have to get different funnels for different porosities, and they can get dirtied and have to be cleaned with sulfuric acid or piranha solution. Personally I don't think I'll be switching back to standard buchners any time soon.