Filtration

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Filtration is a technique used to separate a solid from a solution. The two main types of filtration are gravity filtration and vacuum filtration.

Gravity filtration is most useful when the solid is to be discarded and the filtrate (the liquid that flows through the filter paper) kept.

A gravity filtration.

Vacuum filtration is often faster than gravity filtration and is most useful if the solid must be kept or if a filtration must be done rapidly. A vacuum from an aspirator or a vacuum pump is used to pull the liquid through a funnel containing a piece of filter paper, while the solid is left on the paper.

Equipment used in a vacuum filtration.

Your liquid sample contains decolorizing charcoal, a solid material used to remove certain types of impurities. To recover your sample, should you use vacuum filtration or gravity filtration? Answer

Place a funnel onto an Erlenmeyer flask that is large enough to hold the solution to be filtered. Place a paper clip between the funnel and the flask, or elevate the funnel on a ring stand so that the funnel is not sealed to the mouth of the flask.

Next, prepare a piece of filter paper for the funnel. A coarse grade of paper will allow the liquid to flow through the paper faster, but it may allow small solid particles to pass through. A finer grade of paper will filter smaller solid particles, but it causes liquid to flow more slowly and its pores may easily become clogged.

When in the funnel, the paper should rise slightly higher than the funnel's lip.

Video ( 1.40M ) What should the student in the video have done differently when assembling the filtration apparatus? Answer

When the set up is complete, the solution can be filtered.

For a quantitative transfer, rinse the beaker with a small amount of the solvent from which the solid was precipitated and pour the contents of the beaker into the funnel.

Allow the filtration to continue until the solid is dry (about 15 to 30 minutes).

Video ( 1.86M ) What can this student do next? Answer

First choose appropriate equipment. The side-arm flask should be large enough to easily hold all of the filtrate and should be free of chips and cracks. The Buchner funnel should be small enough so that its flat surface will get completely covered with solid but large enough so that the filter cake (the solid that collects on the filter paper) will not get too thick.

Next set up the filter flask and water trap.

Be sure to choose an appropriate piece of filter paper to place on the flat surface of the funnel.

When the apparatus is assembled, moisten the filter paper. Why?

Video ( 976K ) Why will a filtration using the apparatus shown in the video be unsuccessful? Answer

When the filter paper has been moistened, a vacuum from an aspirator or a vacuum pump may be applied to the flask.

Video: Using a vacuum pump ( 6.21M ) Text description

Video: Using an aspirator ( 7.29M ) Text description

If the apparatus does not have a vacuum, check all the connective tubing and rubber stoppers for cracks and leaks. Make certain that you are using thick-walled vacuum tubing.

In the process of setting up the apparatus, a student notices a fracture in the side arm flask. Explain why this student should not turn on the vacuum. Answer

Video ( 1.16M ) What should the student in the video have done before she filtered the solution? Answer

When the apparatus is ready, the solution can be filtered.

The proper procedure for starting vacuum filtration is: a) Turn on vacuum, pour sample in funnel, close pinch clamp b) Pour sample in funnel, turn on vacuum, close pinch clamp c) Close pinch clamp, turn on vacuum, pour sample in funnel d) Turn on vacuum, close pinch clamp, pour sample in funnel Answer

Video: Filtration transfer ( 10.7M ) Text description

When all of the liquid has passed through the funnel, turn off the vacuum and wash the solid.

Video: Turning off the vacuum with the vacuum jack ( 2.63M ) Text description

Video: Turning off the vacuum with the aspirator ( 2.08M ) Text description

The proper procedure for stopping vacuum filtration is: a) Turn off vacuum, remove funnel, open pinch clamp b) Remove funnel, turn off vacuum, open pinch clamp c) Open pinch clamp, turn off vacuum, remove funnel d) Turn off vacuum, open pinch clamp, remove funnel Answer

Video: Washing the solid ( 8.54M ) Text description

After the solid has been washed, turn the vacuum on again and allow the liquid to pass through the funnel.

Repeat this washing process.

Dry the sample thoroughly by leaving the vacuum on so that air passes through the solid for about fifteen minutes after the final wash.

Video ( 791K ) Suggest a solution to this problem. Answer

After the solid has been thoroughly dried, collect the sample. First remove the funnel from the side-arm flask. Then release the edges of the filter paper from the funnel by scraping the edges of the paper with a spatula. Tap the filter cake out onto a watch glass or other clean, dry surface. Scrape the solid off the filter paper with a spatula, using a glass rod to help steady the filter paper. Avoid touching the paper or the solid with your fingers.

If the solid still appears wet, dry it further by crushing it between two large pieces of filter paper with your thumb while wearing gloves.

How can you tell if a solid is not completely dry? Answer

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