Vacuum Filtration
Introduction to Vacuum Filtration
Vacuum filtration is generally faster than gravity filtration, and is the normal method for collecting a solid product, separating it from unwanted solution. If the solid is to be discarded, and the filtrate is to be collected, it is more usual to use gravity filtration (see above). However, vacuum filtration can be used for this purpose, although there is a risk that the reduced pressure in the flask may induce the filtrate to boil during the filtration, resulting in loss of product if it enters the vacuum system. Even so, the speed and convenience of vacuum filtration means that it may be appropriate to use it when filtering off large amounts of unwanted solid material, such as spent drying agent. This should be done carefully, especially when the solvent has a low boiling point.
A typical set-up for vacuum filtration is shown. In this diagram, a Buchner flask is used to collect the filtrate. This flask is equipped with a side-arm (to connect a hose to the vacuum source), and is made from thick glass so that it can withstand changes in pressure. The flask should be securely clamped. Thick-walled rubber tubing should be used to connect the side-arm to the vacuum source. A rubber gasket is placed on top of the flask, and the filter funnel set inside it, so that it can make an airtight seal.
There are several different designs for filter funnels. The two most common are a Buchner funnel (shown in the set-up picture) and a sintered glass funnel. Sintered glass funnels contain a porous glass disk (also known as a “frit”), upon which the solid is collected. A Buchner funnel requires a flat circular filter paper to be placed in the funnel. The filter paper must lie flat in the funnel, and be large enough to cover all of the holes, otherwise solid material will pass through during the filtration. To ensure the filter paper makes a secure barrier, it should be moistened with solvent just before the filtration takes place.
Filter agents
Problems can arise with vacuum filtration, if the filter paper becomes blocked or clogged by insoluble material. This can sometimes be helped by the use of a filter agent, such as celite. Further information can be found in the section on active filtration.
A note on active and passive vacuum
Sometimes, it may be desirable to vacuum filter a suspension, but using a very weak vacuum: for example, if the filtrate was to be retained, but was a very volatile solvent. In this case, it can be useful to deploy passive vacuum.
Active vacuum is when the vacuum source (see below) is actively and continuously pumping on the system, constantly reducing the pressure in the system. For most applications, this is generally desirable. However, if the vacuum is then switched off, the system does not instantly return to atmospheric pressure; depending on how leak-proof the system is, it may remain at reduced pressure for some seconds, or even minutes. This reduced pressure is called “passive vacuum”, and can be used as a gentler form of vacuum – for instance, to filter a suspension in a volatile solvent, without risk that the solvent will boil or be sucked into the vacuum source.
Source of vacuum
The vacuum may be generated by a water aspirator, a device which attaches to a water tap, and uses the flow of water through it (and into the sink) to generate a partial vacuum. Alternatively, the vacuum may be generated by a vacuum pump, which is significantly more expensive. Both generate vacuum that is adequate for routine laboratory work, although the vacuum pump produces somewhat lower pressures. Both vacuum sources must be used cautiously. With water aspirators, any change in water pressure (such as another tap being turned on or off) may cause water to back up into Buchner flask. This can be avoided by constructing a trap between the aspirator and the Buchner flask – consult a demonstrator for help with this. With vacuum pumps, the effectiveness of the pump is significantly reduced if any non-gaseous material is sucked into the vacuum inlet.