DECANTATION: CONCEPT, EXAMPLES AND TYPES - CHEMISTRY - 2025

The decantation is a procedure used for separating heterogeneous mixtures, especially those which are solid-liquid or liquid-liquid. It occurs due to the difference in densities between the components of the mixture, which causes the less dense substance to be located at the top, while the denser substance, at the bottom.

The solid-liquid mixture occurs when the solid is insoluble in the liquid, adhering to the bottom of the container. On the other hand, liquid-liquid mixing originates when two liquids cannot be mixed. This is observed in the appearance of two layers or phases, the lower one being the one that corresponds to the densest liquid.

Decantation process to separate a heterogeneous solid-liquid mixture. Beakers are used

The solid-liquid mixture is decanted with the help of gravity and tilting the container, taking care that the liquid flows out, while the solid remains adhered to the internal walls of the container. For its part, the liquid-liquid mixture is decanted using the separating funnel.

Decanting is an inexpensive separation method, since it does not usually use many reagents, and in many cases it can be done manually, without the need for equipment.

Settling concept

Decantation is a technique for separating two-phase mixtures: solid-liquid or liquid-liquid. In the case of liquid-solid mixtures it is viable only if the solid remains strongly adhered to the internal walls of the container. Simply pour the liquid into another container so that the decantation is ready.

In the case of the liquid-liquid mixture, it can be decanted when both liquids are visibly differentiated. The separatory funnel is used for large volumes.

Decanting is perhaps the most robust separation technique, and it works well for simple mixes.

Types of decantation

Test tubes

Test tubes are used for decanting when the volumes of the mixtures are small. It is recommended that test tubes, placed in a rack, have an angle of inclination of approximately 45 ° during decantation.

The inclination of the tubes facilitates the separation of the liquid phases, since it avoids the interference of the movement of the liquids when they are being separated in the initial stage of the decantation. The less dense liquid can be removed using a Pasteur pipette.

Beakers

In essence the decanting procedure is the same as that followed with the test tubes. But the volume is greater and it does not tilt 45º. Settling time is usually long to achieve the best possible separation of liquids.

The most cumbersome thing is the collection of the less dense liquid. For this, it is recommended to pour the liquid, using a glass rod as a guide, in order to avoid losing part of the liquid. In this illustration you can see how this type of settling is done:

Separating funnel

Settling by funnel

This type of glass funnel is pear-shaped. At the lower end it has a tube equipped with a key that is used to control the exit of liquids through it. Meanwhile, at the other end it has a ground glass mouth that is hermetically coupled with a glass stopper.

Place the liquid mixture in the shaking funnel and stir vigorously. The funnel is then attached to a universal support and allowed to settle for settling to occur.

When the liquids that cannot be mixed have been separated, the tap is opened to collect the denser liquid found in the lower part of the separating funnel. Finally, the tap is closed so that the less dense liquid remains in the funnel, which is removed through the mouth of the funnel.

The following illustration shows how the separatory funnel tap is opened and the denser liquid begins to fall into an Erlenmeyer flask. The tap will remain open until this liquid falls completely, leaving only the upper, less dense liquid in the funnel, which is subsequently emptied:

Centrifugation

Centrifugation is a technique used to shorten the time required for settling to occur. Through it, the force of gravity acting on the particles in suspension increases.

Centrifugation is used to separate suspended particles based on their size and shape, as well as the density and viscosity of the medium, in addition to the speed of centrifugation. The higher the density and size of the particles, the greater the ease of sedimentation.

Centrifugation is used routinely in clinical laboratories to obtain blood plasma by sedimentation of erythrocytes, leukocytes, and platelets.

This illustration shows test tubes with sediments placed in a centrifuge machine:

The machine is then rotated for a few seconds (spinning):

The test tubes are then removed and it is observed that the sediments have fallen to the bottom of the tubes. This is what is known as decantation by centrifugation:

Examples of decantation

Oil and water separation

One of the most common examples of settling is the separation of water and oil. This method is usually used to separate lipids that are suspended on the surface of the water. This process is usually carried out using a separatory funnel.

During the manufacturing and refining process of vinegars from plant material, it is normal to use the decantation process to remove the heavier fats derived from the raw material.

Oil extraction

In the process of extracting oil from marine sources, the decantation process is commonly used.

This takes place when the hydrocarbon mixes with seawater and must be decanted. Oil is denser than water, therefore once it is separated from it, it is stored. While the excess water is returned to the ocean.

Butter making

The non-homogenized whole milk is placed in a covered container and refrigerated for 24 hours so that the formation of the cream or milk cream occurs. This is placed on the surface of the milk because it is of lower density. The heavy cream or cream can be removed by centrifugation.

It can also be obtained by placing a transparent tube to the bottom of the container containing the milk and skim milk is removed by a siphon procedure, leaving the cream at the bottom of the container. Then, the milk cream is processed to make the different types of butter.

Wine decanting

Decantation of wine. Source: suksim

The wine produces a sediment during its fermentation that gives it an unpleasant taste. For this reason, the wine is decanted to remove the sediment, formed by crystals of tartaric acid salts. The wine before serving is decanted in a glass container with a very long and narrow neck.

Juice Preparation

When preparing juices derived from high fiber fruits, it is common to see the solid material sink to the bottom of the container, while the liquid remains suspended at the top.

Folch-Pi method

This is a technique widely used in biochemistry and chemistry laboratories for the extraction of lipids from various types of tissues. The tissues are placed in a solution composed of a mixture of two volumes of chloroform and one of methanol.

The preparation is placed in a separatory funnel and shaken vigorously, allowing it to settle for decantation to occur. Chloroform (apolar) has a density of 1.48 g / cm ³, and methanol (polar) a density of 0.81 g / cm ³, so they are liquids that cannot be mixed.

The chloroform in the settling is located at the bottom of the separating funnel and contains the lipids that dissolve in it, as chloroform is a non-polar liquid.

Removal of mercury from water

In the mining extraction of some metals, such as gold, mercury is used, which becomes a very dangerous contaminant of water. Mercury can be removed from the water by decantation.

Drying of organic liquids

These liquids can contain significant amounts of water that requires their disposal. To do this, a desiccant is added to the organic liquids, then the desiccant, with its water content, is separated from the organic liquid.

Sewage separation

During the water filtration and cleaning process, a decantation process is carried out where the dirty water is separated from the clean.

This is possible thanks to the fact that dirty waters are denser than clean ones, due to the amount of substances they carry in them. Thus, the first step of filtering the wastewater is generally settling.

Glycerin and biodiesel separation

Glycerin is a by-product of obtaining fuels derived from animal or vegetable oils and fats, such as biodiesel.

For biodiesel to be used in its pure state, it is necessary to decant it from the glycerin that is derived from it. This process is simple, since the biodiesel (of lower density) will always rest on the glycerin.

References

1. Helmenstine, Todd. (November 25, 2019). Decantation Definition in Chemistry. Recovered from: thoughtco.com
2. Vedantu. (sf). Decantation. Recovered from: vedantu.com
3. Dra. Ana Haro García. (sf). The butter. Recovered from: lechepuleva.es
4. Thermo Fisher Scientific. (2019). Centrifugation Theory. Recovered from: fishersci.se
5. Wikipedia. (2019). Decantation. Recovered from: en.wikipedia.org