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Discussion

Vacuum filtration is used to recover solids because using suction is faster than gravity.

The side-arm receiving flask should be much larger than the amount of liquid that it will ultimately contain. Add the volume of the mixture and the volume of any rinses or washes together to approximate the total volume that the flask will have to contain. Then, select a side-arm flask that is much larger than this total volume (if the approximation is 100 mL, select a 250 mL flask).

Inspect the side-arm flask for defects. The flask should be free of chips or cracks that could cause the flask to implode when the interior pressure is reduced (a vacuum is initiated).

A Buchner funnel or fritted-glass crucible should be small enough that the surface of the filter is completely covered, but large enough to contain all of the solid as a thin layer.

Paper and fritted-glass are available with different separation properties called the filter grade. Coarse, medium, and fine grades are typically available. Coarse grades will separate relatively large particles from the liquid. Fine grades will separate all but the smallest particles from the liquid, and medium grades will perform somewhere in between. The finer the grade, the more time it will take for the filtration. Thus, you should select the coarsest grade that will completely separate the solid from the liquid.

If the filtrate contains solid after the filtration, check for leaks or channels in the filter. Barring a leak or channel, select a finer grade of paper or fritted glass for the next experiment.

A trap is placed between the vacuum apparatus and the side-arm receiving flask to prevent contamination of the filtrate and/or prevent the filtrate from reaching the vacuum apparatus. For low-vacuum applications (when a water aspirator or a vacuum pump with an intentional leak in the system is used), the trap can be assembled with an Erlenmeyer flask, a three-hole rubber stopper, glass tubing, rubber vacuum tubing, and a hose clamp (or a valve).

Glass tubing is inserted into each hole on the stopper. To two of the glass tubes, 1-3 foot lengths of rubber vacuum tubing are connected. One length is connected to the receiving flask, and the other length is connected to the vacuum source. To the third glass tube, a 3-4 inch length of rubber vacuum tubing is connected and the tubing clamp is loosely attached. The assembled stopper is then placed in the opening of the flask.

Clamp the side-arm receiving flask to a ring stand. Then, connect the side-arm receiving flask to the trap and place the Buchner funnel or glass crucible in the mouth of the receiving flask.

Narrative

The required equipment includes: a thick-walled Buchner filter flask with a side arm, a Buchner funnel, properly sized filter paper, an adapter, a filter trap, and the mixture to be filtered. The first step is to clamp the Buchner filter flask. In our labs the filter trap apparatus contains a clamping system and the flask only needs to be attached to the heavy tubing.