Sedimentation, Decantation, and Centrifugation

Sedimentation is the settling of heavier solid particles to the bottom of a liquid, decantation is carefully pouring off the liquid above the settled solid, and centrifugation is spinning a mixture fast to make the particles settle more quickly. In brief: muddy water left standing settles into clear water over a layer of mud, and a laboratory centrifuge separates blood cells from plasma in a few minutes.

These three techniques use the effect of gravity, or of a much stronger force in a spinning machine, on particles of different density. They are often combined: first let the solid settle, then pour off the liquid, and use a centrifuge when the particles are too small to settle in reasonable time. This article explains each method, compares them, works through an example calculation and lists uses and mistakes.

What is sedimentation?

Sedimentation takes place when a suspension stands undisturbed. Particles that are denser than the liquid sink and form a layer at the bottom called the sediment. The larger and denser the particles, the faster they settle. Sand settles within seconds, fine clay may take hours or days, and the smallest particles of a colloid may never settle at all.

Sedimentation can also separate two liquids that do not mix, such as oil and water. The denser liquid forms the lower layer, and the lighter liquid forms the upper layer.

What is decantation?

Decantation is the pouring off of the upper liquid, leaving the sediment behind. It is a simple, fast way to separate a liquid from a solid that has settled, or two immiscible liquids from each other. The cleaner the layers, the better the separation. Decantation is never complete, since a little liquid stays with the solid and a little solid may be carried away.

To separate two liquids, a separating funnel (also called a separatory funnel) is used. The denser liquid is drained out through the bottom tap, and the lighter liquid is poured out of the top.

What is centrifugation?

A centrifuge spins tubes around an axis at high speed. The spinning produces an outward force many times greater than gravity, which drives denser particles toward the bottom of the tube. The solid collects as a compact pellet, and the clear liquid above it is called the supernatant. The supernatant can then be decanted or pipetted off.

Centrifugation is useful for fine particles and small volumes that would take too long to settle under gravity alone.

The three methods compared

How do you decant correctly?

1. Let the mixture stand until the solid has settled and the liquid is as clear as possible.
2. Hold a glass rod against the lip of the container to guide the flow.
3. Tilt the container slowly and pour the clear liquid into a second container.
4. Stop pouring when the sediment begins to move toward the lip.

Worked example: relative centrifugal force

The relative centrifugal force (RCF) compares the force in a centrifuge with gravity. It is calculated as RCF = 1.118 × 10⁻⁵ × r × N², where r is the radius in centimeters and N is the speed in revolutions per minute (rpm).

1. Centrifuge at 3,000 rpm with a radius of 10 cm. N² = 9,000,000. RCF = 1.118 × 10⁻⁵ × 10 × 9,000,000 = 1,006 ≈ 1,000 × g.
2. Interpretation. The particles experience a force roughly 1,000 times stronger than gravity.
3. Same speed, smaller radius of 5 cm. RCF = 1.118 × 10⁻⁵ × 5 × 9,000,000 = 503 ≈ 500 × g. The force is proportional to the radius.
4. Doubling the speed to 6,000 rpm at 10 cm. N² = 36,000,000, so RCF = 4,025 ≈ 4,000 × g, which is four times as much.

The force depends on the square of the speed, so a small increase in rpm gives a large increase in separating power.

Where are these methods used?

• Water treatment: large settling tanks let particles sink before filtration.
• Medicine: centrifuges separate blood into red cells, white cells and plasma.
• Food: cream is separated from milk in a centrifuge, and wine is decanted to remove sediment.
• Laundry: the spin cycle of a washing machine throws water from the clothes.
• Industry: uranium enrichment and oil refining use centrifuges.

How did centrifuges develop?

The first practical centrifuges for separating cream from milk appeared in the 1870s and 1880s, with the continuous dairy separator of Gustaf de Laval from 1878 as a well-known example. Early in the 20th century, the Swedish chemist Theodor Svedberg developed the ultracentrifuge to study very large molecules such as proteins, and received the Nobel Prize in Chemistry in 1926.

Which mistakes are common?

Mistake 1: Using a centrifuge with unbalanced tubes

Tubes must be placed in opposite positions with equal mass. An unbalanced rotor can vibrate dangerously.

Mistake 2: Expecting decantation to remove all the solid

Some solid is always left in the liquid or some liquid remains in the sediment. Combine decantation with filtration for a cleaner result.

Mistake 3: Expecting dissolved substances to settle

Dissolved particles do not settle, regardless of the time given or the speed of the centrifuge in ordinary laboratory conditions.

Mistake 4: Stirring while the particles settle

Moving the container resuspends the sediment. Leave the mixture undisturbed.