Filtration

Filtration is the operation through which solid particles are separated from fluids (liquids or gases) by passing the impure fluid through a filter or some other separation body, which allows the fluid to pass while stopping the solid residue.

Types of filtration

Common

Simple

Also known as gravity filtration, it is the most common form of filtration, where the suspension is placed in a filter funnel and allowed to flow under its own weight through the filter.

Vacuum

Used for removing almost all the traces of liquid from a precipitate. Büchner funnels are generally employed and a vacuum pump or an aspirator Venturi tube are used as source of vacuum.

Centrifuge

Uses the centripetal force of a centrifuge to aid in the filtration. Special centrifuges exist for this procedure, though a simple centrifuge filtration can be done by adding a filter paper or a piece of cotton half-way in a closed tube and placing the suspension in the upper part of the tube.

A special form of centrifugal filtration is cyclonic separation, which involves separating impurities from a gas (or liquid) through vortex separation (hydrocyclone for liquids).

Special

Cold filtration

Used for removing a desired compound which is very soluble in the filtrate.

Hot filtration

Mainly used to separate impurities from a hot, supersaturated solution, where the desired material is dissolved in the filtrate.

Filter media

Activated carbon

Activated charcoal is used to remove impurities from organic solvents as well as during distillation.

Cotton

Can be used to remove most impurities, though it is not compatible with strong acids and oxidizing solutions.

Filter paper

Widely used due to simplicity. A simple funnel or a perforated Büchner funnel is used to hold the filter paper, depending on the type of paper used.

Fritted glass

Encountered in Büchner funnels and similar devices, is commonly used for filtering corrosive solutions.

Filter bed

Used in combination with a fritted Büchner funnel, it is commonly used for removing extremely fine impurities from a solution. The fine precipitate is almost never recovered from filter bed, as it's not worth the effort. Common materials used in the filter bed are silica gel (usually crystalline), diatomaceous earth, perlite, or rarely powdered cellulose.

Useful tips

• Suspensions with large particle size are easy to filter as the liquid flows smooth through its particles
• If the suspension is thick, filtering will occur very slowly, and it's recommended to filter small amounts of suspension at a time before changing the filter or filter paper to prevent it from clogging

References

Relevant Sciencemadness threads

• Vacuum filtration without a vacuum flask.
• filter paper
• Vacuum Filtration