Learning Objective: Vacuum filtration is a procedure that enables the separation of a solid from a liquid. In this procedure you will learn how to use the equipment involved in a vacuum filtration.

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Students may wish to wear gloves when working with chemicals.

Vacuum filtration is generally used after you have crystallized a solid from a solvent. Usually, the flask containing the dissolved solid product is placed in an ice bath made from small pieces of ice and water. Recall from general chemistry that lower temperatures decrease the solubility of a solute in a solvent. The decrease in solubility induces crystallization. The flask sits in the ice bath for 10-15 minutes to allow the solid to completely crystallize from the solvent.

   
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You will need the following equipment for the vacuum filtration setup: a ring clamp, a ring stand, a side-arm flask, a Buchner funnel or Hirsch funnel, an adaptor, a check valve, two pieces of thick-walled hose and an aspirator. The aspirator is a piece of plumbing that is part of the tap. Attach the ring clamp to the ring stand and then place the side-arm filter flask in the ring. Place the Buchner funnel (in this case) in the top of the flask. Attach the thick-walled hoses, separated by the check valve to the side-arm flask and the aspirator. Make sure the check valve's o-ring is oriented towards the aspirator. Turn the water on full force and place your hand over the funnel to check for suction. Then turn the water off.

   
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Add a piece of filter paper to the Buchner funnel and then turn on the water so the vacuum is working. Wet the filter paper with the same ice-cold solvent used to crystallize the solid. The filter paper is sucked tight against the funnel. Quickly but carefully pour the solvent and solid into the funnel; watch the solvent get sucked to the bottom of the filter flask while the solid sits on top of the filter paper. To maximize yield, add the ice-cold solvent to the flask that contained your solvent and solid, swirl the mixture and pour it into the Buchner funnel.

   
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With the vacuum still on, take another mL of solvent and rinse the crystals in the Buchner funnel. Cover the solid with a piece of filter paper and then press the filter paper with a small cork to dry the crystals. Turn the vacuum off. Remove the filter papers from the Buchner funnel to a watch glass and carefully scrape the crystals off the paper. Be sure not to get bits of filter paper in your solid. Allow the solid to air dry on the watch glass.



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