Filtration

Concept:

Filtration is a technique used for two main purposes. The first is to remove solid impurities from a liquid. The second is to collect a desired solid from the solution from which it was precipitated or crystallized. Several different kinds of filtration are commonly used: two general methods include gravity filtration and vacuum (or suction) filtration. 

Theory:

Gravity Filtration:

As it is called, this method uses gravity to reach the goal of separation through filtration. The most familiar technique is probably filtration of a solution through a paper filter held in a funnel, allowing gravity to draw the liquid through the paper. Because even a small piece of filter paper will absorb a significant volume of liquid, in most microscale procedure requiring filtration, this technique is useful only when the volume of mixture to be filtered is greater than 10 mL. For many microscale procedures a more suitable technique, which also makes use of gravity, is to use a Pasteur (or disposable) pipet with a cotton or glass wool plug (called a filtering pipet). 

Vacuum (or suction) Filtration:

Vacuum, or suction, filtration is more rapid than gravity filtration and is most often used to collect solid products resulting from precipitation or crystallization. This technique is used primarily when the volume of liquid being filtered is more than 1-2 mL. In a vacuum filtration, a receiver flask with a sidearm, a filter flask, is used.  The sidearm is connected by heavy-walled rubber tubing to a source of vacuum.  Thin-walled tubing (such as the tygon tube you use to connect cooling water) will collapse under vacuum, due to atmospheric pressure on its outside walls, and will seal the vacuum source from the flask.

How do I know when to use what kind of filtration and which equipment?

Method Application
Gravity Filtration  
          Filter cones The volume of liquid to be filtered is about 10 mL or greater, and the solid collected in the filter is saved.
          Fluted Filters The volume of liquid to be filtered is greater than 10 mL, and solid impurities are removed from a solution; often used in crystallization procedures.
          Filtering pipets Used with small volumes, less than about 10 mL, to remove solid impurities from a liquid.
Vacuum filtration  
          Hirsch funnels Primarily used to collect a desired solid from a relatively small volume of liquid (1-10 mL); used frequently to collect the crystals obtained from crystallizations.
          Buchner funnels Used in the same way as Hirsch funnels, except the volume of liquid is usually greater.
Filtering Media Used to remove finely divided impurities.

 Techniques:

Gravity Filtration:

Filter cones: This filtration technique is most useful when the solid material being filtered from a mixture is to be collected and used later. The filter cone is likely to be used in a microscale experiments only when a relatively large volume (greater than 10 mL) is being filtered and when a Hirsch funnel is not appropriate. 

How to set up a gravity filtration?

1. Prepare a filter cone:

 

2. Gravity filtration with a filter cone:

With filtrations using a simple filter cone, the solvent may form a seal between the filter and the funnel and between the funnel and the lip of the receiving flask. When a seal forms, the filtration stops because the displaced air has no possibility of escaping. To avoid the solvent seal, you can insert a small piece of paper, a paper clip, or some other bent wire between the funnel and the lip of the flask to let the displaced air escape. 

 

Fluted filters: Fluted filters are also most useful when filtering a relatively large amount of liquid. They are often used to filter a hot solution saturated with a solute during a crystallization procedure. The major advantage of a fluted filter is that it increases the speed of filtration for two reasons: first, it increases the surface area of the filter paper through which the solvent seeps; second, it allows air to enter the flask along its sides to permit rapid pressure equalization. 

How do I to use a fluted filter?

1. Prepare a fluted cone:

Following are separate steps:

2. When doing hot filtration, it is important to ensure that the filter does not become clogged by solid material accumulated in the stem of the funnel or in the filter paper. When the hot saturated solution comes in contact with a relatively cold funnel, the solution is cooled and may become supersaturated. If crystallization occurs in the filter, either the crystals will clog the filter paper or they will clog the stem of the funnel.

Click here to invite one student to show you how to do a hot filtration step by step!

Four ways to keep the filter from clogging:

Filtering pipets: A filtering pipet is a microscale technique most often used to remove solid impurities from a liquid with a volume less than 10 mL. It is important that the mixture being filtered be at or near room temperature because it is difficult to prevent premature crystallization in a hot solution saturated with a solute. 

1. Prepare a filtering pipet: A small piece of cotton or glass wool is inserted into the top of a Pasteur (disposable) pipet and pushed down to the beginning of the lower constriction in the pipet. Enough cotton or glass wool should be used to collect all the solid being filtered. However, if too much cotton or glass wool is used or if it is packed too tightly, the flow rate through the pipet will be significantly restricted.

2. Conducting a filtration using fitering pipet: The filtering pipet should be clamped so that the filtrate will drain into an appropriate container. The mixture to be filtered is transferred using another pipet. It is a good idea to rinse the filter and plug with small amount of solvent if the amount of liquid filtering is very small (less than 1-2 mL).

Vacuum (suction) Filtration:

Normal Vacuum (suction) filtration setup:

Filtering Media: It is occasionally necessary to use specially prepared filter beds to separate fine particles when using vacuum filtration. Often, very fine particles either pass right through a paper filter or they clog it so completely that the filtering stops. This is avoided by using a substance called Filter Aid, or Celite. Filter aid will not clog the fiber pores of filter paper. It is slurried, mixed with a solvent to form a rather thin paste, and filtered through a Hirsch or Buchner funnel until a layer of about 2-3 mm thick is formed on top of the filter paper. The solvent in which the Filter Aid was slurried, is removed from the filter flask, and, if necessary, the filter flask is cleaned before the actual filtration is begun. Finely divided particles can now be suction-filtered through this layer and will be caught by the Filter Aid. This technique is used for removing impurities, not for collecting a product.

How to set up a vacuum filtration?

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How to do a vacuum filtering?