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Vacuum Filtration- How do I secure the funnel to the flask?

Vacuum filtration, sometimes known as Buchner filtration is used to isolate a solid product or impurity from a liquid filtrate. Any solid product or impurity can be left on the filter paper present within the funnel and easily removed, whilst the filtrate passes through the funnel into the flask at the bottom, to be used further or discarded. Whilst the set-up can be considered simple in itself, it is necessary to ensure all of the items such as the tubing, flask and pump fit together and this can sometimes prove tricky and time consuming. We have assembled a few purchase bundles to make choosing compatible equipment easy -available with a 250ml flask and 1000ml flask. You can check out our previous post on the complete vacuum filtration set-up here.

Another type of filtration is gravimetric filtration. If you would like to see more about the differences between these two methods, then you can check out our previous blog post here.

A complete vacuum filtration apparatus setup
A complete vacuum filtration apparatus setup


An important part of completing the set-up is securing your Buchner funnel to your flask. The product used to achieve this is known by a number of different names which can make it difficult to track down exactly what you need. Known as the Buchner/vacuum/filter ring, seal or cone, this rubber ring acts as a seal, to ensure the vacuum is kept. Sometimes a rubber bung with a hole is used to attach the base of the funnel into the flask. Whilst this can be a perfectly satisfactory solution, issues can arise with trying to separate the very tightly connected equipment once you have run your filtration. The positive effect of using a specifically designed Buchner ring is that it has a thinner edge making it easy to break the vacuum by hand.


Set of 7 filter cones

Set of filter cones

We sell a set of 7 filter cones can be used to provide fitting of a funnel to a flask. Simply choose the cones that best fits, or use a combination of cones to provide the seal. This ability to use a combination of cones means that this set can fit almost any standard flask and funnel. Available here.


Rubber Buchner Rings

Rubber Buchner rings available as a set or individually

These green rubber Buchner rings are made of a more heavy duty rubber and are available separately in 5 sizes (to fit a flask neck inner diameter of ~24, 35, 45, 60 and 68 mm) as well as in a pack containing one of each size, brilliant for kitting out your lab. Available here.


If you have any questions regarding this, please feel free to contact us using the contact form below. Alternatively, our email address is support@camlab.co.uk