zaro

How to Use a Suction Flask?

Published in Lab Techniques 3 mins read

Using a suction flask (also known as a Büchner flask or vacuum flask) is a common laboratory technique for filtration and separating solids from liquids. Here's a step-by-step guide:

Steps for Using a Suction Flask

  1. Prepare the Equipment:

    • Gather the necessary equipment: suction flask, Büchner funnel, filter paper, rubber adapter (or Büchner ring), vacuum tubing, vacuum source (e.g., water aspirator or vacuum pump), and the mixture to be filtered.
    • Ensure all glassware is clean and dry.
  2. Assemble the Filtration Apparatus:

    • Place the Büchner funnel into the top of the suction flask. Use a rubber adapter or Büchner ring to create an airtight seal between the funnel and the flask. The rubber adapter should fit snugly.
    • Cut a piece of filter paper to fit the inside of the Büchner funnel. The paper should cover all the holes in the funnel but not extend up the sides.
    • Place the filter paper flat inside the Büchner funnel.
  3. Connect to Vacuum Source:

    • Attach one end of the vacuum tubing to the side arm of the suction flask.
    • Connect the other end of the tubing to your vacuum source (water aspirator or vacuum pump).
  4. Wet the Filter Paper:

    • Turn on the vacuum source.
    • Moisten the filter paper with the same solvent as the liquid you are filtering. This helps the paper adhere to the funnel and creates a better seal for filtration. A small amount of the solvent will be pulled through the paper.
  5. Carefully Pour the Mixture:

    • Decant first: If the mixture has a large amount of solid, gently pour (decant) the liquid portion into the funnel first. This prevents the filter paper from becoming immediately clogged.
    • Add the solid: Then, slowly pour the remaining solid and liquid mixture into the funnel, trying to distribute the solid evenly over the filter paper.
  6. Maintain Vacuum and Wash if Necessary:

    • Allow the vacuum to pull the liquid through the filter paper.
    • If desired, wash the solid residue in the funnel with a small amount of clean solvent to remove any remaining impurities. Simply pour the solvent over the solid and let the vacuum pull it through.
  7. Break the Vacuum:

    • Before turning off the vacuum source, disconnect the vacuum tubing from the suction flask. This prevents backflow of water (if using a water aspirator) into the flask, which could contaminate your filtrate.
    • Then, turn off the vacuum source.
  8. Collect the Filtrate and Solid:

    • Carefully remove the Büchner funnel from the suction flask.
    • The liquid that has passed through the filter paper (the filtrate) is now in the suction flask.
    • Carefully remove the filter paper with the solid residue from the Büchner funnel. You can use a spatula to help with this.
  9. Dispose of Waste Appropriately:

    • Dispose of the filtrate and solid residue according to laboratory safety guidelines.

Important Considerations

  • Filter Paper Size: Ensure the filter paper completely covers all holes in the Büchner funnel but doesn't touch the sides.
  • Vacuum Strength: Adjust the vacuum strength as needed. A strong vacuum can cause fine particles to pass through the filter paper, while a weak vacuum will slow down the filtration process.
  • Safety: Wear appropriate personal protective equipment (PPE), such as gloves and eye protection.
  • Airtight Seal: Ensure a good airtight seal between the Büchner funnel and the suction flask to achieve optimal vacuum pressure.

By following these steps, you can effectively use a suction flask for filtration in your laboratory work.