How to Use a Buchner Funnel in a Chemistry Lab

Written by michael judge
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How to Use a Buchner Funnel in a Chemistry Lab
Buchner funnels are much faster than gravity filtration. (Hemera Technologies/ Images)

Although it seems the Buchner funnel must've been used in labs since the beginning of chemical experimentation, it was invented only in 1885. It remains an indispensable tool in the practice of chemistry. Essentially, the Buchner funnel simply speeds up the separation of solid/liquid mixtures by drawing off the liquid under vacuum and catching the solid component on a piece of filter paper. This basic concept can save significant amounts of time compared to the much slower process of gravity filtration.

Skill level:
Moderately Easy

Things you need

  • Buchner funnel
  • One hole rubber stopper
  • Filter paper circles
  • Side arm flask
  • Rubber hose
  • Ring stand and clamp
  • Vacuum source

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  1. 1

    Connect a side arm flask to a ring stand support via a clamp. This will keep the flask stable and prevent it from tipping over due to any pull from the vacuum hose.

  2. 2

    Place the one hole rubber stopper in the neck of the flask. The stopper should be sized so that it fits snugly into the flask neck and makes an airtight seal.

  3. 3

    Connect a rubber hose to the protruding arm of the side arm flask. Connect the other end of the hose to a vacuum source such as a water aspirator or a mechanical vacuum pump.

  4. 4

    Carefully push the tapered lower portion of the Buchner funnel through the stopper hole, so that the tip of the funnel protrudes below the stopper. The tapered section of the funnel should fit airtight against the interior of the stopper hole. The funnel tapered portion can sometimes be moistened first and also rotated as it is inserted to make this step easier.

  5. 5

    Place a correctly sized piece of circular filter paper inside the Buchner funnel, so it lies flat on top of the perforated base of the funnel interior.

  6. 6

    Turn on the vacuum and press the filter paper down so that it makes a seal against the base of the funnel. You can pour a small amount of liquid onto the paper at this point to help it seal. If the paper does not seem to be drawn down flat by the force of the vacuum, try gently pressing down on the Buchner funnel to make sure it has sealed properly where it contacts the stopper.

  7. 7

    Pour the mixture which you wish to separate into the upper cuplike portion of the Buchner funnel, taking care that you don't add too much at once and overflow the walls of the funnel. Make sure that no liquid runs under the filter paper; all the liquid should instead run through the paper. The vacuum will drain the liquid into the flask quite quickly and the solid will be retained on the filter paper.

  8. 8

    Pour some pure liquid over the filtered solid to wash off any residual impurities, if necessary, and allow that liquid to also drain into the flask. The vacuum can be maintained for a few minutes after all the liquid has been pulled through in order to help dry the solid.

  9. 9

    Carefully disconnect the hose from the side arm flask and then shut off the vacuum source.

  10. 10

    Remove the Buchner funnel from the flask (the stopper will normally remain attached to the funnel) and transfer the solid to a suitable container.

Tips and warnings

  • Ensure that the side arm flask is not overfilled during filtration, or the excess liquid will be drawn into the vacuum line. An intermediate trap can be used to prevent this and also to prevent water from backflowing into the side arm flask when using a water aspirator vacuum.
  • Use thick walled hose which will not collapse under vacuum.
  • Make sure that the side arm flask is free from cracks and chips and has not been treated roughly (dropped, for example). These conditions can weaken the flask and cause it to implode when vacuum is applied.

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