Part 4 of this series looks at different types of funnels for vacuum filtration – e.g. Buchner and Hartley funnels. Read on to see how to choose the right funnel and achieve the perfect seal.
Funnels for Vacuum Filtration
So far in the series we’ve looked at some important considerations for paper filtration, the properties of filter papers, and how to fold filter paper cones for use with conical funnels. Now let’s take a look at other types of funnels that can be used with flat filter circles under suction or vacuum filtration.
There are several types of funnel available for vacuum filtration and it’s important to select the right one for the job to maximize your filtration efficiency.
In this blog, we list the different funnels you can choose from and talk in detail about Büchner and Hartley funnels, and a little advice on Gooch crucibles. We’ve included some tips and advice to help your vacuum filtration run smoothly. So take a break, have a quick read, and see if you can make your filtration a bit easier.
Büchner and Hartley funnels
In the image below, you can see the selection of funnels available for vacuum filtration.
Büchner and Hartley funnels are suitable for most vacuum filtration applications – e.g. filtering bulk solutions, suspended matter, pulp, and separating crystals. In fact, whenever you need rapid filtration or it’s necessary to press filtered material, these are your go-to funnels.
The main advantage of these funnels is speed. Due to their shape, and if used correctly, you can make full use of the filter paper’s surface area, allowing you to filter samples quickly and effectively. Vacuum filtration further accelerates washing of retained material.
Fig 1. Different funnel types are available
What about filter supports?
Both Büchner and Hartley funnels contain filter supports. The type of support can vary from simple perforated discs to highly porous sintered discs.
If you intend to use a perforated disc, we’d generally recommend sticking to flat supports (not domed), and checking the perforations for slightly rounded upper edges, which will minimize damage to a wet filter under vacuum. In addition, using a funnel having a support with a large number of smaller perforations in preferential to a support plate having relatively few, larger openings.
When setting up the apparatus, also make sure you can position the filter to leave a clear, unperforated outer area to support and seal the edge of the filter paper in place.
Some advice if you use Gooch crucibles
If you need to use sintered glass Gooch crucibles as supports for cellulose or glass fiber discs then extra care must be taken for efficient filtration.
Often, there isn’t a flat area of non-porous material around the edge of the sintered disc, meaning that a paper disc may not seal completely. This can lead to leakage or bypass of the filter, so we’d recommend carefully checking that the paper makes a good seal before proceeding.
The importance of the perfect seal (and how to get it)
The most important point when performing vacuum filtration is making sure you have a complete seal between the filter paper and the funnel. Taking care over this step can be the key to successful, safe, and quick filtration.
Making sure that the filter paper is sealed all around the edge will minimize the risk of the paper being blown back by the sudden boiling of hot filtrates due to pressure change as they pass into the flask. This could otherwise result in contamination or loss of the sample.
In cases where the filtrate needs to be recovered, a complete seal also prevents reverse flow of the solvent when the vacuum is released.
Obtaining the perfect seal is not too difficult:
- Apply a small amount of vacuum
- Wet the paper with the appropriate solvent and allow to filter through
Alternatively, you could use a Hartley or Whatman 3-Piece Filter Funnel. These funnels are simple to set up and clean, and further reduce the risk of any leakage.
Lastly, don’t forget that strong filter papers are essential in vacuum filtration, especially if you need to recover the precipitate afterwards.