Decantation is a process that separates components of a mixture based on differences in density. You may encounter decantation in everyday life with wine or spirits, but it’s also a powerful technique in chemistry for separating a solid from a liquid or isolating two immiscible liquids.
Decantation is easy, but one disadvantage is that it doesn’t allow for perfect separation of mixture components. A small amount of one component gets lost when collecting the other component, or else the collection goes too far and the collection gets contaminated with the second component.
How Decantation Works
Decantation involves two steps:
- Sedimentation: Sedimentation uses gravity or a centrifuge to separate mixture components based on density.
- Decanting: Decanting is pouring or siphoning off the top component of a mixture or draining the bottom component.
A solid component is called “sediment” (or “pellet” when centrifugation is used). The liquid component that is collected is called “decant.”
The basic principle of decantation is that heavier (denser) substances sink, while lighter (less dense) substances float. In its simplest form, decantation uses gravity to separate a solid and liquid or two immiscible liquids. The lighter component is poured or siphoned off the top of the mixture. Alternatively, a separatory funnel drains the heavier component.
Small volumes are decanted using test tubes inclined at 45-degrees in a test tube rack. The angle lets heavier particles slide down the tube, while lighter particles rise to the top. The angle also makes pouring the lighter component easier. Pouring off the liquid is easier if it’s poured along a stirring rod. The decantation process is slower if the test tubes are kept vertical because the heavier component can form a plug and block lighter particles from rising.
Centrifugation speeds decantation by applying centrifugal and centripetal force. Basically, the artificial gravity separates mixture components more quickly. Centrifugation compacts solid components into a pellet. Pouring the liquid away from the pellet results in less loss than in simple decantation.
A separatory funnel decants components of mixtures of immiscible liquids. One component floats on top of the other. The funnel drains the component at the bottom of the funnel.
Here are examples of mixtures that can be separated using decantation:
- Oil and water: Oil floats on top of water. After letting a mixture of oil and water sit, the oil can be poured off of the water. In a separatory funnel, the water can be drained away from the oil.
- Dirt and water: Decantation is one way to clean muddy water. Soil and debris sinks to the bottom, so clear water is poured off. Decanting water isn’t sufficient to completely purify it because some particles (like bacteria and viruses) are too light to sink with the dirt. Other particles dissolve in water (like heavy metals and pesticides). Even after decantation, other methods are needed to produce drinking water, like filtration or distillation.
- Kerosene and water (or gasoline and water): The kerosene or gasoline floats above the water. Decanting this liquid is dangerous because the fuel evaporates to form flammable, toxic fumes.
- Milk and cream: Decantation separates cream from milk. Cream rises to the top of the liquid and is easily skimmed off.
- Precipitate from supernate: Decantation separates precipitate from supernate in a chemical reaction that yields a solid product. Decanting may be preferable to filtration for separating tiny particles from liquid.
- Blood and plasma: Plasma is separated by blood by decantation using a centrifuge.
A decanter is a piece of glassware used to decant liquid, such as wine. Decanters come in many shapes and sizes, but some part of the glassware is wide enough so that pouring liquid traps sediment. For wine, potassium bitartrate crystals are the usual sediment.
- Bye, Ragnar; Agasöster, Tone; Åsheim, Arne (1993). “A novel and fast extraction technique as an alternative to conventional separatory funnels”. Fresenius’ Journal of Analytical Chemistry. 345 (6): 411–414. doi:10.1007/BF00325616
- Tillisch, H. (August 1918). “Decanting”. The Journal of Industrial and Engineering Chemistry. 10 (8): 631. doi:10.1021/ie50104a027
- Robinson, Jancis (2003). Jancis Robinson’s Wine Course (3rd ed.). Abbeville Press. pp. 20–25. ISBN 0-7892-0883-0.
- Shih, Chih-Hsin. et al. (March 2011). “Supernatant decanting on a centrifugal platform”. Biomicrofluidics. 5 (1): 013414. doi:10.1063/1.3571477