What is the difference between gravity and vacuum filtration?

There are two types of filtration which are commonly used in the laboratory and it is important to select the correct type for the work you are doing. But what is the difference between gravity and vacuum filtration? How do you choose what type of filtration you need to do? And what equipment do you need for each type?

Vacuum Filtration - also known as Buchner filtration - for when you need to isolate the precipitate (the solid)

Filtration under vacuum using a Buchner funnel is used when you want to isolate the precipitate (the solid) for further work or analysis. The basic setup is as pictured below; [caption id="attachment_6315" align="aligncenter" width="283"] Vacuum Filtration setup[/caption] The apparatus needed is;

• A Buchner funnel
• A vacuum filtration flask (also known as a Buchner flask)
• A filter paper (select the grade with the right material and  pore size for your sample and the right size for your Buchner funnel)
• Adjoining cones (these are used in place of a rubber bung as they are adjustable to fit any flask to any funnel)
• A retort stand set - used to hold the flask firmly in place
• Tubing - to join the flask to the vacuum pump, thick walled type for work under vacuum
• A vacuum pump - either an electrical type or a water stream vacuum pump

All of this apparatus is available together in a basic vacuum filtration kit. This saves you finding parts that will fit together and is a good option for starting out in vacuum filtering. You may also use a spatula to agitate the precipitate on the filter paper for thorough drying, or to remove the precipitate after the filtration is finished.

Gravity Filtration - also known as simple filtration - for when you need to isolate the filtrate (the liquid)

A basic filtration under gravity is used when you are removing the solid precipitate to isolate the filtrate, the liquid, for further work or testing. The setup is shown here; [caption id="attachment_6317" align="aligncenter" width="300"] Gravity filtration setup[/caption] The apparatus needed is;

• Filter paper - remember to choose the right size paper for the funnel, see our guide here
• Filter funnel
• Retort stand set - to hold the funnel
• Conical flask - to collect the filtrate

To read more about the apparatus just click on the links above, or contact us to ask more about filtering samples or choosing a filter paper.