Technique D: Setup of Vacuum Filtration
SECTION 1: PURPOSE OF TECHNIQUE
Vacuum filtration is a technique for separating a solid product from a reaction mixture. The mixture is passed through filter paper in a Buchner funnel. The solid is trapped by the filter and the liquid is drawn through the funnel into the flask below. A vacuum system is used to make the filtration faster.
Only a moderate vacuum is typically needed. Fluid is NOT to enter the vacuum system. Care should be taken to avoid liquid getting into the vacuum system. After vacuum filtration, the solid may not be completely dry. Filtered solid material is damp or wet on the surface and might still have to be air-dried or oven-dried.
Buchner funnels are typically used for stable solids and liquids (i.e., air and water unreactive material). Special techniques for dealing with air and water reactive materials are beyond the scope of this technique.
SECTION 2: COMPONENTS OF VACUUM FILTRATION
Vacuum tubing (thick tubing that resists compression)
Vacuum system (provided by the building services or a vacuum pump)
Stand and clamp
Set up the vacuum equipment similar to as the photo below. Your setup may not exactly look like the photo due to available equipment, but it should be functional.
Setup of Vacuum Equipment
SECTION 3: SETUP OF VACUUM FILTRATION
Part 1: Inspect the equipment
- Check the equipment (filter flask, Buchner funnel, and vacuum tubing) for broken parts or cracks before use.
- Make sure the filter paper fits in the Buchner funnel. (Folding of the filter paper is fine if it is bigger than the Buchner funnel; do not use filter paper if it is smaller than the Buchner funnel).
- Make sure the Buchner funnel fits snugly on the filter flask.
Part 2: Putting everything together
- Attach the filter flask to the stand using a utility clamp.
- Place the Buchner funnel on the filter flask. Add a filter paper inside the Buchner funnel.
- Secure the tubing to the arm of the filter flask on the side and to the vacuum system. The tubing should be long enough to attach to the vacuum located in the fume hood.
Part 3: Carrying Out the filtration
- For each filtration, use only one filter and one filter funnel. Make sure the system is set up.
- Turn the vacuum on slowly and gradually pour in your solution to the Buchner funnel. The solution you are filtering should consist of both a solid and liquid, not just a solid.
SECTION 4: IMPORTANT THINGS TO REMEMBER
Requirements for Vacuum Filtration
Use filter paper that is the appropriate size for the funnel. It should lie flat on the inside of the Buchner funnel and cover all of the small holes of the funnel.
You may use deionized water to wet the filter paper enough so that it sticks to the Buchner funnel better and prevents solid substances from getting under the filter paper.
Use a utility clamp and stand to hold the filter flask down. Because you are using thick vacuum tubing for the filter flask, the tubing may cause the flask to be top-heavy. Not properly securing the filter flask can cause the filter flask to fall.
Some examples of how to set the vacuum filtration system are provided below:
When you are ready to filter, turn the vacuum on slowly and gradually pour in your solution.
SECTION 5: SPECIAL CIRCUMSTANCES FOR LIQUID/VAPOR TRAPS
In special circumstances, a liquid trap may be connected between the system and the vacuum with vacuum tubing to prevent liquid from being suctioned into the vacuum system. Remember, fluid is NOT to enter the vacuum system. Using this technique serves as a precaution to prevent fluid from entering the vacuum system. Use of a secondary liquid trap is unlikely needed for this class.
An example of a vacuum trap assembly is shown below:
For certain high-vacuum systems, or when significant organic vapors may be present, additional protection for the vacuum system may be required. A cold trap is shown below, unassembled, assembled and clamped in place over an insulated Dewar flask. The Dewar flask is filled with an appropriate cold bath mixture such as ice, dry ice/acetone, or liquid nitrogen. It cools the exterior of the cold trap. Vapors and other contaminants will condense out on the inside of the cold trap, and the vacuum system is protected.
NOTE: Dewar flasks are evacuated double-walled glass vessels. They are fragile and need to be taped to avoid injury by glass in the event it implodes if broken. Use caution.
NOTE: If liquid nitrogen is used as the cryogen, some vacuum must be initially applied to the system to avoid accidental condensation of liquid oxygen and a resulting hazard. Consult with experienced personnel.