Vacuum distillation allows purification of compounds that cannot be readily distilled under normal atmospheric pressure or that may decompose at high temperatures. It works by reducing pressure above the boiling liquid, lowering the boiling point so compounds evaporate and separate based on differences in their boiling points. Key advantages are faster processing, ability to distill higher boiling point solvents without damage, and improved separation, yield, and purity. It finds applications in separating thermolabile substances and preparing extracts while preserving active constituents.

1. Outline:
1- Definition
2- Principle
3- Purpose
4- Construction And Diagram
5- Working
6- Advantages And Disadvantages
7- Uses
8- Conclusion

2. Definition: Vacuum distillation is a method of distillation whereby the pressure
above the liquid mixture to be distilled is reduced to less than its vapor
pressure(usually less than its atmospheric pressure) causing evaporation of the most
volatile liquid(s) ( those with the lowest boiling points).
Principle:
* Liquid boils when vapor pressure equal to the atmospheric pressure, i.e., pressure
on its surface. If the external pressure is reduced by applying vacuum , the boiling
point of liquid is lowered.
* Therefore, the liquid boils at a lower temperature. This principle is illustrated using
an example of water.
Water boils at an 100 degree Celsius at an atmospheric pressure is 101.31 kPa (
760mmHg). At 40 degree Celsius , the vapor pressure of water is approximately
9.33kPa (70 mmHg). Hence , the external pressure is reduced to 9.33kPa (70mmHg)
where water boils at 40 degree Celsius. The net result is the increase in rate of mass
transfer into vapor.

3. Purpose: The purpose of vacuum distillation is that it allows the purification of
compounds not readily distilled at ambient pressures or simply to save time or
energy. This technique separates compounds based on difference in boiling
points. This technique is used when the boiling point of the desired compound is
difficult to achieve or will cause the compound to decompose. A reduced pressure
decreases the boiling point of compounds.
Construction & Diagram:
1- It consists of a double- neck
Distillation flask knows as
“Clasien flask”.
2- Thick walled glass apparatus
with interchangeable standard
glass joints are used for vacuum
distillation.

4. 3- In one of the necks of the Claisen flask, a
thermometer is fitted. The second neck
prevents splashing of the violently agitated
liquid.
4- Bumping occurs readily during vacuum
distillation. Placing a fine capillary tube in the
second neck of the flask can prevent bumping.
5- The capillary tube is dipped in the boiling
liquid, so that a stream of air bubbles is drawn
out.
6- Water bath or oil bath is used for heating.
7- The Claisen flask is connected to a receiver
through a condenser.
8- Vacuum pump is attached through an
adapter to the receiver. A small pressure gauge
(manometer) should be inserted between the
pump and the receiver.

5. Working:
 Liquid to be distilled is filled ½ to 2/3 volume of flask.
 Small piece of porcelain are added to avoid the process of bumping.
 Then capillary tube and thermometer are placed in the flask.
 Required vacuum is applied through vacuum pump.
 The contents are heated and temperature of liquid raises and liquid get vaporized
rapidly due to vacuum.
 Now these vapors pass through condenser and condensate is collected in
receiver.

6. Advantages Disadvantages
Using vacuum is often preferable for
following reasons:
1. Faster processing time.
2. Effective processing of higher
boiling point solvents without
igniting them or causing thermal
breakdown.
3. Vacuum distillation can improve a
separation by increasing capacity,
yield and purity.
Disadvantages of vacuum distillation are
as follows:
1. In vacuum distillation, persistent
foaming occurs. This may be
overcome by adding capryl alcohol
to the liquid or by inserting a fine air
capillary tube in the second neck of
the Claisen flask.
2. The steam of air is drawn in and
breaks the rising foam. The above
method is not suitable for the
preparation of semisolid or solid
extracts by distillation under
vacuum.

7. Applications:
The applications of vacuum distillation are as follows:
1. Preventing degradation of Active constituents like Enzymes, Vitamins,
Glycosides, Alkaloids.
2- Changing physical forms. Example: Cascara Sagrada Tablets at atmospheric pressure
form dense compact residue , but at reduced pressure form light porous mass.
3- This type of distillation is used in oil industry.
4- Separation of impurities.
5- Separation of thermolabile (heat sensitive) substances.
6- Extracts or crude drugs are prepared by this process.
Conclusion:
Some materials breakdown at high temperature. With the use of vacuum
distillation column, the compounds can be separated at lower temperature
without effecting the components. Purification of compounds through vacuum
distillation is more efficient.