9
$\begingroup$

I just bought a sintered-glass style Büchner funnel for some vacuum filtration tasks. There's two issues I'm having with it:

  1. Filter material, especially activated charcoal, ends up stuck in the glass disc. I did a little bit of research trying to find out how to clean it out and the only thing I found (for the carbon, at least), was piranha solution, but I have neither the equipment nor the experience to use it safely so I'm SOL on cleaning. Fortunately the presence of carbon isn't a huge deal for my uses, though.
  2. For paper filtering; the sintered disc goes all the way to the edge of the funnel, so unless I've got filter paper exactly the size of the funnel (which I don't), I'm worried that the 0.5-1.0mm gap I usually end up with around the edge of the paper gives fluid a path to bypass the paper. If I err on the side of larger paper it's still a poor fit with large channels around the edge.

It seems to me that the perforated-disc style of Büchner funnel would solve both of these problems; since the funnel itself can't trap filter material and also the holes don't go all the way to the edge of the disc:

enter image description here

(Incidentally, the pictured funnel on the left is exactly the one I own.)

My question is, given the two apparent disadvantages I've observed with the sintered-glass style funnel, what are the pros and cons of each type? More specifically, what could I do with a sintered-glass funnel that I couldn't do (as easily) with a perforated-disc funnel?

The context here is "kitchen chemistry"; I'm not a chemist I just do a few DIY projects here and there and all my knowledge comes from YouTube and Google searches. The only lab equipment I own is this funnel, a flask, and a vacuum pump. The only relevant PPE I own is a respirator, various gloves, and some chemical safety goggles.

$\endgroup$
  • 3
    $\begingroup$ For the sintered glass filter, add an inch or two of diatomaceous earth (Celite). Wet it under vacuum with solvent that you are using to make a pad. Now slowly filter your charcoal suspension. This will keep the sintered glass clean. Measure the inside diameter of the Buchner funnel. Use a compass to draw a circle on the filter paper and trim. Put the paper in the funnel with vacuum on. Wet paper then go through the Celite procedure. PS: The right size filter paper should be available for the Buchner funnel. $\endgroup$ – user55119 Aug 15 at 20:39
  • $\begingroup$ Super appreciate the pro tips I've got Celite on order anyways so it's perfect. I am using a compass I guess I just need to play around a little to get it perfect. Are there "standard" funnel sizes? Mine is 94mm but I couldn't find the Whatman papers I wanted (#1, 6, 597, and 602H) in that size, and the lack of sizes matching my funnel makes me wonder if my funnel is strangely sized. I just sent Cytiva (Whatman) a support request to ask. $\endgroup$ – Jason C Aug 15 at 20:52
  • 1
    $\begingroup$ Your Buchner could just be old. Back flushing the sintered funnel may help clean it but use the Celite as suggested above and give it a good stir around in the solvent to break up any lumps and cover the entire filtering surface. $\endgroup$ – Waylander Aug 15 at 21:10
  • $\begingroup$ Sweet thanks. I'm already using sodium bentonite as a filtering step and have some on hand so I just combined it with the charcoal step (paper -> bentonite clay -> paper -> charcoal, bottom to top) and that also seems to keep the charcoal dust out and kills two birds with one stone. $\endgroup$ – Jason C Aug 15 at 21:33
  • 3
    $\begingroup$ The thing on the left is not a called a Büchner funnel, but a frit. Frits are only meant to be used for stuff that can be redissolved afterwards. $\endgroup$ – Karl Aug 15 at 22:53
9
$\begingroup$

The thing on the left is not usually a called a Büchner funnel, but a frit. Frits are generally meant to be used for stuff that can be redissolved afterwards, because they tend to clogg permanently, if very small particles get stuck inside.

You do not normally put a filter paper on it, because that would leak on the edge. The filter grade should be etched onto the glass somewhere. https://en.wikipedia.org/wiki/Fritted_glass Sometimes a frit can be used to hold another, powdery filtering substance, if that has a coarse enough grain size.

Advantage of the frit is the well defined pore size, you can heat it for drying, it can be mounted in a glass tube with two ground glass connectors (for evacuating), it can be used with concentrated and/or oxidising acids or other solutions, ...

Advantage of the Büchner funnel is that it does not get clogged with insoluble stuff, ease of use, cheap, available in large diameters, very mechanically solid, tolerates alkalic solutions (glass frits do too, but only briefly), ...

| improve this answer | |
$\endgroup$
  • 2
    $\begingroup$ Hm... I'm a little confused now, because this funnel (the one I have) is described as a "Buchner filtering funnel" with "coarse frit"; and says "Compatible with StonyLab Qualitative/Quantitive Filter Paper Circles". It does give the pore size like you mentioned (80-120 microns), I didn't notice that before. The "G1" grade marking is indeed printed on the glass. $\endgroup$ – Jason C Aug 16 at 2:47
  • $\begingroup$ (Similar deal here) $\endgroup$ – Jason C Aug 16 at 3:07
  • 2
    $\begingroup$ Well, seems some people have extended the meaning. If that misnomer has reached textbooks already I don't know. But I wouldn't trust a company that sells frits that are "compatible with pape disk filters" on correct naming. ;) You have already pointed out the problem above. $\endgroup$ – Karl Aug 16 at 4:45
  • 2
    $\begingroup$ Depends on whether you are removing unwanted solids from solutions - then you use the sintered funnel, with or without Celite - or collecting a solid that is the material you want - then use the perforated disc (Buchner) with filter paper. $\endgroup$ – Waylander Aug 16 at 7:05
  • 4
    $\begingroup$ It is not necessary that the sintered glass glass filters be used for something which can be dissolved later. We routinely use them for filtering synthesized silica particles or chemically modified stationary phases. The reason for using fritted funnel is have to chemical compatibility with harsh solvents (with the exception of bases, but glass frits survive a brief exposure) $\endgroup$ – M. Farooq Aug 16 at 23:08
8
$\begingroup$

I agree with user55119's suggestions (see the comment I have copy and pasted below):

For the sintered glass filter, add an inch or two of diatomaceous earth (Celite). Wet it under vacuum with solvent that you are using to make a pad. Now slowly filter your charcoal suspension. This will keep the sintered glass clean. Measure the inside diameter of the Büchner funnel. Use a compass to draw a circle on the filter paper and trim. Put the paper in the funnel with vacuum on. Wet paper then go through the Celite procedure. PS: The right size filter paper should be available for the Büchner funnel.

However, the celite bed is only good for your charcoal suspension and if you are filtering just to remove minute impurities (when your target is the filtrate). Make sure you pour your suspension slowly, slow enough not to disturb the celite bed. The filtration with celite would be slower than usual (without celite). You may use a thinner bed to have a faster filtration rate, but have to extra careful not to disturb the celite bed. Nonetheless, if you are patient enough, you can achieve your goal.

If you want to filter out your target crystals or solid, you cannot use celite. For that purpose I'd always use a Büchner funnel because it is easy and could have recovered most of materials. Also cleaning procedure is easy. If you have used a sintered glass filter in place of Büchner funnel, after use, you have to wash it thoroughly with suitable solvent to remove trapped particles (similar to charcoal particles you have experienced) before subjected to usual cleanup. That is an extra step.

One advantage of the sintered glass filter is it has a ground-glass end. Thus you can use filtering flask with ground-glass head, so that the equipment would have airtight connection for ideal vacuum filtration. For Büchner funnel, this is impossible because it needs a rubber filtering adaptor to connect with a filtering flask, which is almost always leaky.

Note: When you buy filter papers for your Büchner funnel, always look for what size it fits. This information is provided by some vendors: For example, the filling filter paper size for $\pu{35 mL}$-Büchner funnels is those with $\pu{4.25 cm}$ in diameter. If you can't find the right filter paper, go with user55119's instructions.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Quick terminology question: Does "Büchner funnel" only refer to the perforated disc type, or does it sometimes refer to both types of funnel? $\endgroup$ – Jason C Aug 15 at 21:35
  • 2
    $\begingroup$ You can find enough info here: en.wikipedia.org/wiki/B%C3%BCchner_funnel $\endgroup$ – Mathew Mahindaratne Aug 15 at 21:43
  • 1
    $\begingroup$ @JasonC That wp article is wrong. The thing on the left is not a Büchner funnel. $\endgroup$ – Karl Aug 15 at 23:09

Your Answer

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

Not the answer you're looking for? Browse other questions tagged or ask your own question.