# Chemistry - Sintered vs perforated plate Büchner funnels

## Solution 1:

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), ...

## Solution 2:

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.