Steam Distillation is the process of distilling a mixture wherein one of the immiscible phases is a steam. It is a mild method for separating and purifying volatile liquid or solid organic compounds that are immiscible or insoluble in water. This technique is not applicable to substances that react with water, decompose on prolonged contact with steam or hot water, or have a vapor pressure of less than about 5 torr at 100°C. One way to perform steam distillation is through the live steam method. It is the more widely used method. It is especially used with high molecular weight (low vapor pressure) substances. Although it requires more complex setup, this method reduces bumping.

Principles Involved Live steam distillation is based on Dalton’s Law of Partial Pressures. In a mixture of volatile and immiscible substances, each type of molecule moves in the container as though the other components (compounds) of the mixture are not there. So, the partial pressures of each component (compound) of the mixture at a given temperature is equal to vapor pressure of the mixture at the same temperature and do not depend on the respective mole fractions of the components in the mixture. Each component of the mixture vaporizes independently of the others.
Ptotal = Pa + Pb + Pc ... With these, we can say that the total vapor pressure of the mixture at any given temperature is always greater that the vapor pressure of even the most volatile component in the mixture at the same temperature. Also, the boiling point of the immiscible mixture is lower than that of its component with the lowest boiling point.

Materials Needed Three-necked flask Condenser Three way adapter Florence Flask (Receiving Flask) Ice bath Glass tubings Bent glass tubing Rubber tubings Water trap Water source Thermometer Thermometer adapter Heat source Erlenmeyer flask (Boiling Flask) Air vent (Glass tubing) Plaster of Paris/ Aluminum foil Cork or Glass stopper Screw clamp Iron stand Iron ring

Illustration of a Complete Setup

Source: http://www.ciens.ucv.ve/quimicaorg/PRACTICAS/practica%206.pdf
Procedure:
1.Set up the extreme distillation in its entirety. This includes having the material you want to distill in the distilling flask, the steam trap already attached, and receiver and condensers are prepared.
2.Thoroughly check the setup before starting.
3.Connect a length of rubber tubing to the bench steam outlet and lead the rubber tubing into a drain.
4.Carefully open the steam stopcock until a stream of steam appears.
5.Close the steam stopcock and wait for the rubber tubing to cool.
6.Carefully attach the rubber tubing from the steam stopcock to the inlet of the steam trap.
7.Open the steam trap drain, then carefully reopen the bench steam stopcock. Let any water out of the trap and then close the drain clamp. There will be steam going through the distillation setup and product will start to come over.
8.Periodically open the steam trap drain to let the condensed steam out.
9.When finished, turn off the steam, let the apparatus cool, and dismantle the setup.
Techniques
1. A bent glass tubing is placed in the distilling flask and serves as a steam inlet tube.
2. The distilling flask should never be filled more than half full.
3. The clamp at the bottom of the water trap is left open until the condensation of steam stops.
4. The clamp is opened at times when condensate gathers in the water trap.
5. The receiving flask, condenser and/or vacuum adapter should remain cool to the touch throughout the whole process.
6. Use an ice bath to increase the efficiency of condensation in the receiving flask.
7. Remove at least a tenth part of the distillate once it is already clear.
8. When distillation is to be stopped, the clamp in the water trap should be opened and the steam inlet tube must be removed from the three-necked flask (distilling flask).
9. Hot steam can produce severe burns so steam inlet tube must be cooled a little first before holding and attaching it to the distilling flask.
10. Cool the whole setup first before dismantling.
11. The thermometer bulb should be below the side-arm of the three-way adapter.
12. When droplets condensate at the thermometer bulb, the temperature reading is the boiling point.
13. When the first drop of liquid falls into the receiving flask, the temperature reading is the distillation temperature.
14. Cold water should constantly run through the lower half of the condenser and then water should go out of the top part of the condenser.
15. When a solid substance is placed in the distilling flask for the live steam method, water is not needed to be placed with it.
16. Cover joints or connections of two apparatuses with aluminum foil or Plaster of Paris to insulate it so steam vapors won’t condensate there.

Reasons for Important Steps
When the set-up is being assembled, make sure that the instruments are fastened securely to the ring stand. If the set-up calls for ground glass joints, lubrication is a need to assure that there would be no space for the steam to escape out. The line from the steam source to the receiving flask should be kept as short as possible in order that the cooling of steam is avoided. Check if everything is secured into its place so that there would be no lose steam which can eventually cause burns and may disrupt the whole experiment.
The flask should contain no more than a half full through the distillation process. The surface area of the boiling liquid should be kept as large as possible but too large flasks must be avoided. If the heat source is a Bunsen burner there is a need for wire gauze so that the heat will be distributed evenly.
Applications
Live steam distillation is more often used for solid substances. It is used for isolation of tars, oils and other liquid compounds that are insoluble or slightly soluble in water at all temperatures.
Essential oils from plant materials are isolated through live steam distillation. These essential oils can be used in flavorings, perfumes and aromatherapy.
The following sample oils can be extracted by live steam distillation: oil of peppermint, oil of cinnamon, oil of clove and oil of turpentine.