Elutriation

The word “Elutriation” is derived from the Latin word “elutriare” meaning “to wash out”.

Definition:
It is the separation, purification or removal of something from a mixture by decanting, straining or washing.

Process of elutriation:
In the process of elutriation, the movement of the fluid, generally water or air, is in the opposite direction to that of the sedimentation process.

Types of elutriation:
According to direction:

Vertical elutriation:
In the gravitational process, the larger particles present in water (or any other liquid) will move vertically downwards with the affect of gravity while the small particles in the fluid travels straight up with the fluid. This is a type of vertical elutriation.

Horizontal elutriation:
If a water current of suspended particles is flowed through a settling chamber. The particles that move out of the water current are collected in the bottom of the chamber. This is a type of horizontal elutriation.

Centrifugal elutriation:
In this case the water current is caused to spin with some force resulting in the large centrifugal force on the suspended particles. The heavier particles will settle to the walls of the elutriator or to the bottom.

The DorrClone is an example of a centrifugal-type of classifier.

According to the type of fluid:

• Air elutriation

• Water elutriation

If the velocity of the fluid is smaller than the velocity of setting down of the particles then the particles will settle downwards. On the other hand, if the velocity of the fluid is larger than the velocity of setting down of the particles then the particles will be carried up along with the fluid.

Air elutriation will give precise separation of the particles and in less time than water elutriation.

Factors affecting elutriation:

Elutriation is affected by the

• velocity of the fluid

• the particle size : As the small sized particles will flow (upward) along the fluid while the large sized particles will move downwards (against the velocity of the fluid).

• position of the particle in the (tube containing) fluid

• density of the particle

In a tube, there exist different velocities i.e. the velocity is largest in the centre and is smallest along the walls of the tube. So the small sized particles move upward, when in the centre and in the meantime they are also pushed towards the wall of the tube. Where the velocity is smaller and here the small sized particles start to move downwards.

Process of removal of particles:

If the upward flow of the water (or any other liquid) is slightly increased, the small sized particles (which move down slowly) will move along the movement of the water (i.e. upward) and will be removed from the water. In this process, the medium sized particles will remain immobile and the heavier particles will continue to move downward.

The upward flow of water will then again be increased and the next smallest size particles will be removed. And in this way, particles of different sizes will be separated and obtained.

Centrifugation

It refers to the process of sedimentation by using centrifuge machine.

Basic idea behind centrifugation:
Centrifugation is based on the widely known idea of sedimentation by the use of centrifugal force, which represents a force that apparently moves a spinning or rotating object away from the axis of rotation in a curved path.

Centrifugal effect:
The processes using centrifugal force (F) can be described by the equations involving the gravitational constant (G). In this case, it is easy to determine the centrifugal force in the terms of the ratio of the centrifugal force to the gravitational force. In addition, this ratio represents the centrifugal effect (C).

Centrifugal effect (C) shows that how many times the centrifugal force is larger than gravitational force.

C = 2.013 dn^2

Where
d = diameter of rotation
n = speed of rotation

Here in this equation, “n” has the value in “s-1” and “d” has the value in “m”.


This equation shows that centrifugal effect is directly proportional to the diameter and to the square of the speed of the rotation i.e. greater will be the diameter of the tube or container more will be centrifugation and similarly for the speed of rotation.

Factors affecting centrifugation:

Centrifugation is basically affected by centrifugal effect. Moreover, nature of the liquid medium in which the particles are placed also affects the centrifugation.

Apparatus for centrifugation (Centrifuges):
Container is the most important part of centrifugation apparatus i.e. centrifuges. This container is used for the placement of a mixture or solution of solid and liquid or of a solution of two liquids.

This container is then rotated at greater speed resulting in the separation of the ingredients of the mixture takes place by the use of centrifugal force.

Mechanism for the separation in the apparatus of centrifugation:
A mixture of liquid or solid in a liquid of low density can be separated as the material of larger density is thrown in the outward direction to the bottom of the tube or container with a larger force. This results in the separation of pure, low-density liquid as a transparent or purified supernatant liquid which forms upper layer.

Types of centrifuges:

There are two basic types of centrifuges:

1. Sedimentation

2. Filtration

Sedimentation centrifuges:

The basic principle, in the sedimentation type of centrifuges, is difference in the densities of the ingredients of the mixture. In these types of centrifuges, the particles are settled to the wall by the action of the centrifugal force.

These types of centrifuges are used for the separation of ingredients of the mixture of solid in liquid as well as liquid in liquid.

Two types of centrifuges are based on the principle of sedimentation:

1. Bottle centrifuge

2. Disc type centrifuge

Bottle centrifuge:
It is mostly used centrifuge machine in the laboratories. It consists of a vertical rotating rod that causes the “bottle-type” containers or test tubes, which are fixed symmetrically, to be rotated in a horizontal plane resulting in the separation of the materials of varying densities. The vertical rod is rotated usually by means of electric motor. Sometimes, gas turbines can also be used for the rotation.

Disk type centrifuge:
It consists of vertical pile of thin conical disks, which are arranged in the manner of one on the top of another. The sedimentation of the particles takes place in the space between neighbouring cones. In this way, settling distance is greatly reduced, thereby increasing the rate at which the particles in the material are separated. The cones are adjusted in such a manner that heavier material moves down the surface easily upon reaching the inner surface of the cone.

Filtration centrifuges:

These types of centrifuges are used for the separation of the mixture of solid in liquid only performing the operation similar to the filtration process. These are also sometimes referred to as clarifiers.

It is same in the basic operation to the sedimentation types of centrifuges but instead of solid containers or tubes, it contains a porous wall or perforated containers or baskets, which causes the liquid phase to pass through it but keeps the solid phase on it.

Centrifuge based on the principle of filtration is “Basket centrifuge”.

Basket centrifuge:
Basket centrifuge consists of a porous wall and rotor which is cylindrical and tubular in structure. The porous wall is some times replaced by one or more of the fine mesh screens. The fluid go through the screen where as the particles larger in size are left on the screen.

Application of centrifugation:

Centrifugation is used for the separation of ingredients of a mixture of solid in liquid or liquid in liquid as the degree of separation achieved by centrifugation is of greater amplitude than the action due to gravity.

It is important specifically when the separation by normal filtration methods is difficult such as in the separation of fluids of highly viscous nature.

In the pharmaceutical research, it is considered as an important tool in determining the stability of emulsions.

Bottle centrifuge can be used for:

1. Finding the sediments present in crude vegetable and mineral oils
2. Determination of the butterfat content in the milk
3. Various clinical trials and tests

Disk type centrifuge can be used for refining of vegetable oils by removing soap stock

Basket centrifuge can be used for:

1. Drying and washing of several different kinds of crystals and fibrous materials
2. The preparation of cane sugar.

Bougies

Introduction:
Its name came from the town namely “bougie” in Algeria, which was famous for trading in wax. Its original meaning is “A candle made by wax”. These are the types of suppositories intended for insertion into the urethra, nostrils or ears.

It is also a medical instrument in the shape of cylinder, made up of a flexible tube.

Forms of bougie:
Bulbous bougie:
It is a form of bougie with a bulb shaped tip.

Eder-pustow bougie:
It is a form of metal bougie which resembles olive in shape. It may be used in esophageal stricture.

Elastic bougie:
A bougie made of rubber or latex.

Elbowed bougie:
A bougie with a bent near its tip.

Use:
It is used for opening of constricted areas in tubular organs such as urethra or esophagus and rectum, so, that medicines for local application or another instruments can easily be inserted in that part.

Types of properties of solutions

There are following three types of properties of solution:
1. Additive properties
2. Constitutive properties
3. Colligative properties

Additive properties:
These are the properties which are due to sum of corresponding properties of individual atoms or functional groups within the molecules e.g. molecular weight.

Constitutive property:
These are the properties which depend upon the structural arrangement of atoms within the molecules for example optical properties and surface and interfacial properties.

Colligative property:
These are the properties which depend upon the number of molecules present in solution.
Following are colligative properties of dilute solution:

1. Lowering of vapor pressure
2. Elevation of boiling point
3. Depression of freezing point
4. Osmotic pressure