What Is Decanting?

Decanting or Decantation in Chemistry

This is the setup for decantation in a test tube. The test tube is kept near a 45 degree angle to allow free flow of the liquid past the solid or denser liquid. © Todd Helmenstine

The term 'decant' is usually associated with wine. Decanting is also a chemical laboratory process used to separate mixtures.

Decanting is a process to separate mixtures. Decanting is just allowing a mixture of solid and liquid or two immiscible liquids to settle and separate by gravity. This process can be slow and tedious without the aid of a centrifuge. Once the mixture components have separated, the lighter liquid is poured off leaving the heavier liquid or solid behind.

Typically, a small amount of the lighter liquid is left behind.

In laboratory conditions, small volumes of mixtures are decanted in test tubes. If time is not a concern, the test tube is kept at a 45° angle in a test tube rack. This allows the heavier particles to slide down the side of the test tube while allowing the lighter liquid a path to rise to the top. If the test tube were held vertically, the heavier mixture component could block the test tube and not allow the lighter liquid to pass as it rises.

A centrifuge can greatly increase the rate of separation by simulating a great increase in the force of gravity.

Some mixtures that can be decanted:

  • Oil and water - oil floats on top of water. Decanting the mixture allows the oil to be poured off the water.
  • Gasoline or kerosene and water - this mixture is an example often cited as a safety hazard. Decanting a mixture containing flammable solvents can be dangerous as the flammable material evaporates and forms dangerous fumes.
  • Dirt and water - muddy water can be cleared up by decanting. The soil will sink to the bottom of the tube allowing the clear water to be poured off.
  • Wine - Sediment from the fermentation process can produce an undesirable taste. Wine is decanted to separate the wine from these sediments.
  • Cream and milk - Cream is separated from milk by decantation. Cream rises to the top of the milk mixture and is easily skimmed off.
  • Blood and plasma: A centrifuge is necessary for this decantation. Plasma can be removed from blood by decantation.