Decantation

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1. Upper phase
2. Lower phase
An example of decanter.

Decantation is a process for the separation of mixtures, by removing a top layer of liquid from which a precipitate has settled. Usually a small amount of solution must be left in the container, and care must be taken to prevent a small amount of precipitate from flowing with the solution out of the container. It is frequently used to purify a liquid by separating it from a suspension of insoluble particles (e.g. in red wine, where the wine is decanted from the potassium bitartrate crystals). For example, to obtain a sample of clear water from muddy water, muddy water is left in a container until the mud settles, and then the clear water is poured into another container.

A mixture of two immiscible liquids can also be separated by decantation. For example, the oil and water extracted from fish may be decanted to obtain the oil. A mixture of kerosene and water can also be separated through decantation.

A centrifuge may be useful in successfully decanting a solution. The centrifuge causes the precipitate to be forced to the bottom of the container; if the force is high enough, the precipitate may form a compact solid. Then the liquid can be more easily poured away, as the precipitate will likely remain in its compressed form. A mixture of an insoluble solid in liquid is allowed to stand. The solid is insoluble and settles at the bottom if kept undisturbed for some time. This process is called sedimentation. The clear liquid is then poured off carefully. This process is called decantation.

Another example of decantation is the regeneration of used chiral stationary phase (CSP). The CSP to be decanted is gently mixed in a container with a compatible solvent to form a suspension. The suspension is allowed to rest for a period of time, after which the supernatant is carefully poured off. The supernatant contains the undesirable constituents of the former suspension, while the leftover sediment in the container is clean, reusable CSP. Decantation should be done directly after sedimentation. This process is also used in extracting pure iron from iron ore.

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