Sciencemadness Discussion Board

The trouble with Buchner funnels

vmelkon - 25-12-2013 at 07:19

Do buchner funnels work okay for you?

It works ok some of the time but these things leak often.
The liquid goes under the filter paper at times. I have to filter over and over again until by pure chance it works.
Why isn't the buchner funnel conical shaped instead of being flat?

plante1999 - 25-12-2013 at 07:22

You have to moisten the filter paper before use...

Not trying to insult you, but you may want to consider reading a work about laboratory manipulations.

It is flat because it filter faster this way.

[Edited on 25-12-2013 by plante1999]

Hexavalent - 25-12-2013 at 08:46

plante is correct, you must moisten the filter paper with a small amount of your solvent first. Doing this, and using the correct filter paper, I have never had any issues using Buchner setups.

bfesser - 25-12-2013 at 09:03

Use the correct size filter circle; wet the paper; apply vacuum; seat the funnel; slowly pour in mixture to be filtered; suck dry; break the vacuum at the flask opening; turn off vacuum. Büchner's and vacuum filtration are generally used for the recovery of solids, <em>not for clarifying liquids</em>. A photo of your funnel and paper may be helpful in diagnosing your trouble.

vmelkon - 26-12-2013 at 11:53

I did wet it. Sometimes it works well and sometimes it leaks and have to start over.
There isn't anything special about it. It comes from united nuclear
http://unitednuclear.com/index.php?main_page=product_info&am...



100_2273a.jpg - 76kB100_2274a.jpg - 48kB

Aconite - 26-12-2013 at 12:10

Do you pre-wet with the vacuum on? You should.
I like to use sintered glass funnels- they are more expensive but easier to clean and filter paper is generally unnecessary (except for very very fine solids). I use Course (C) and Extra course (EC) frit pore size and it removes even very fine precipitates without needing paper. I'll use paper in them for some materials that generally have trouble filtering though - seems to speed things up.
Cheers~

Hexavalent - 26-12-2013 at 12:16

Quote: Originally posted by Aconite  
Do you pre-wet with the vacuum on? You should.
I like to use sintered glass funnels- they are more expensive but easier to clean and filter paper is generally unnecessary (except for very very fine solids). I use Course (C) and Extra course (EC) frit pore size and it removes even very fine precipitates without needing paper. I'll use paper in them for some materials that generally have trouble filtering though - seems to speed things up.
Cheers~


I find sintered glass funnels more expensive and actually harder to clean than regular Buchner funnels; solids can easily get trapped in the pores, which can sometimes be very difficult to do as mechanical scrubbing is limited. I generally only use them when filtering strongly acidic, basic or oxidizing solutions, which may damage conventional filter paper on Buchner or Hirsch funnels. In this regard, however, strongly basic solutions, such as concentrated hydroxides, will dissolve the frit by converting the glass into soluble silicates, in which case the filtering medium must be considered very carefully. If you need to filter very fine particulates, and you are not concerned with retaining the filtrand, the use of a layer of activated carbon, diatomaceous earth, silica and other aids can be greatly beneficial.

[Edited on 26-12-2013 by Hexavalent]

bfesser - 26-12-2013 at 12:43

vmelkon, what is immediately evident from your photos is that you have a cheap Chinese Büchner. I ordered one of those from eBay once, and got nothing but poor results. The holes are just too big, causing paper to bulge and tear. Try doubling or tripling up on the filter circles. If I were you, I'd just look for a genuine Coors Büchner on eBay, that takes the same size papers (looks like a 70 mm?). Fritted glass funnels are nice, but they just can't replace a good ceramic Büchner for all applications.

Hexavalent - 26-12-2013 at 12:56

<b>bfesser</b>, I ordered one of these last year from Shanghai, and have found it to work perfectly. I routinely use grade 1 and grade 3 Whatman filter papers (for coarse and finer solids, respectively), and have never had any issues with tearing or damage to the filter paper due to abnormal holes.

[Edited on 26-12-2013 by Hexavalent]

Aconite - 26-12-2013 at 15:36

Quote: Originally posted by Hexavalent  
Quote: Originally posted by Aconite  
Do you pre-wet with the vacuum on? You should.
I like to use sintered glass funnels- they are more expensive but easier to clean and filter paper is generally unnecessary (except for very very fine solids). I use Course (C) and Extra course (EC) frit pore size and it removes even very fine precipitates without needing paper. I'll use paper in them for some materials that generally have trouble filtering though - seems to speed things up.
Cheers~


I find sintered glass funnels more expensive and actually harder to clean than regular Buchner funnels; solids can easily get trapped in the pores, which can sometimes be very difficult to do as mechanical scrubbing is limited. I generally only use them when filtering strongly acidic, basic or oxidizing solutions, which may damage conventional filter paper on Buchner or Hirsch funnels. In this regard, however, strongly basic solutions, such as concentrated hydroxides, will dissolve the frit by converting the glass into soluble silicates, in which case the filtering medium must be considered very carefully. If you need to filter very fine particulates, and you are not concerned with retaining the filtrand, the use of a layer of activated carbon, diatomaceous earth, silica and other aids can be greatly beneficial.

[Edited on 26-12-2013 by Hexavalent]


It all really depends on your particular application. If you have a precipitate that is wildly insoluble in everything (like fine mesh silver metal), it would be unwise to use sintered glass. They are easy to clean ONLY if your material can be dissolved in something like acid, base, etc. Also- unless you use a strong base that is very hot over numerous and/or prolonged applications, I wouldn't worry much about the frit. In any case- there are pros/cons for both. I find great utility with sintered glass as well as ceramic buchners.

Also I'll second Hexavalent with using a prefiltrant like celite, charcoal, etc. It's extremely helpful.
Don't forget to pre-wet with the vac on!

Cheers~

[Edited on 26-12-2013 by Aconite]

[Edited on 26-12-2013 by Aconite]

chemrox - 26-12-2013 at 16:43

I have a couple of really good lab manuals (much more information than "surviving O chem labs.."). I'll try and find e-copies for the library. I only knew one guy who could do organic syntheses without having taken a class... he was a contributor at adc in the pre-hive era..

Filter paper choice will be the next hurdle. I like whatman's 1 for getting rid of C. I tried #4 and without celite the damned C came through. Celite is another very cheap material to get. If you pay the freight I'll send a pound.

Dr.Bob - 26-12-2013 at 18:25

The other problem you can get is if your vacuum is too weak, then the filter paper will leak like that. If you are using a water aspirator, then get more flow, if not, you might need a better vacuum pump. Often, if the vacuum is weak, you will get better filtering with a smaller funnel, as then the vacuum will be stronger, and hold the filter paper down better. And the Coors funnels are much better, I have used a few Chinese ones, they are lousy.

WolfPack - 27-12-2013 at 05:39

As they can be unmounted, and hence are really easy to clean using the test tube brushes, I like to use <a href="http://www.sigmaaldrich.com/catalog/product/aldrich/z178136?lang=en&region=US">this kind</a> of Büchner funnel when using solvents that do not attack PP (<a href="http://www.engineeringtoolbox.com/polypropylene-pp-chemical-resistance-d_435.html">you'll find this tables useful</a>;). This is one of the tricks I learnt from the technicians at the analytical chemistry department.

Dr.Bob - 27-12-2013 at 06:02

I have a few of the plastic ones Wolfpack referred to, in addition to the standard porcelain ones, if anyone wants to try them. They pop apart to clean, which is handy. And polypropylene will handle nearly any normal lab solvent or acid, and temps up to 100 C. We use polypropylene funnels all of the time, as glass ones break constantly in real use.

S.C. Wack - 27-12-2013 at 14:29

Quote: Originally posted by vmelkon  
Why isn't the buchner funnel conical shaped instead of being flat?


Filter paper plus vacuum wouldn't work any more.

Large holes can be covered with plastic mesh cut to fit. Kartell makes these ("HDPE Buchner Funnel Mesh Pre Filter Disc Holder"), I do use them.

Panache - 28-12-2013 at 04:37

Filtering lab-scale is generally the shittest bit of lab work. This is because for efficient filtration one needs to match the 'holes' with the 'grains', get it wrong by too much and you either get no filtration or it takes forever.
I hate filtering, generally once you understand you've mismatched your paper to your needs it's too epic to adjust your choice.
That said your problems appear to simply be due to lack of proper instruction, not the general vagaries of filtering.
On a second note, always clamp your filter flask when buchnering, as on the day you don't it's a long way back from Tipperary.

Bronstein - 30-12-2013 at 13:16

I find that when I filter a something which is in a nonpolar solvent, the filter paper will stick very poorly to the büchner, and things will sometimes leak through. When using water I usually have no problems, the filter paper sticks properly. Is there a solution for this?

elementcollector1 - 30-12-2013 at 13:56

I think there's something up with my aspirator/buchner system. When vacuum is off, it filters at the normal slow rate. When vacuum is on (from a garden hose)... the thing doesn't filter at all!

subsecret - 30-12-2013 at 16:42

I have problems with my filter paper tearing. I use a 50 mL Buchner funnel with one circle of filter paper, and vacuum is created with an aspirator connected to the sink. I have to keep the vacuum pretty low, or else the filter paper tears. :mad:

I, honestly, think that an extremely coarse fritted filter would be the best. Many have ground glass joints, which is handy. I don't think that the action of a strong alkali on a coarse filter frit would make a large difference, especially if you are using a filter paper over the frit. The frit supports the paper more evenly, and more of the paper's area is available for liquid to pass through (in a Buchner funnel, this is only over the holes).

Shikimol69 - 11-1-2014 at 22:37

I use both of them, fritted funnels as well as buchners (porcelain or PP). In general, I use fritted (coarse D most of the time) to filter and wash small portions of crystals, and after each use a good portion of methanol to clean the glass.
For the problems usually encountered with buchner funnels when "properly" used :
- or the quality of the buchner is poor (typical for lab discounters provided by asian manufacturers), with a sieve plate that is no flat at all and present a convex surface OR smudges around the holes > good chance to stick your wetted filter on these ones, unless you use celite and such helpers...
- or the nature of the solvent is not adapted to the filter you're using, as written before it is the case for unpolars like xylene or toluene : when wetted, vacuum on, instead of sticking nicely, it can forms waves and as a result leaks. My advice, change the filter type but don't combine 2 or 3 filters, it doesn't help at all...
Usually, you don't meet such problems with PP buchners, they are polyvalent, much more cheaper and NOT prone to breakage!!! I broke so much of these porcelain buchners, RIP 90 & 70 mm - you are now in pieces but gave me so much help all these days & nights at lab!

Kapitan - 17-1-2014 at 06:53

The real problem with buchner funnels is that it's difficult to know if they're clean.

Maybe this wouldn't be a problem if I was a better chemist, and didn't ever need to recover the filtrate.