PLEASE VIEW THE RELEVANT VIDEO BEFORE ATTENDING THE LAB CLASS
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Example of a Typical Experiment
FAQ: When using the condenser, which hose connector is water-in and which is water-out?
FAQ: Why does it matter whether water enters the condenser at the top or from the bottom?
FAQ: How fast should the water run through the condenser?
FAQ: How far down the stillhead should the thermometer bulb be placed when setting up the distillation apparatus?
FAQ: How do you know when the distillation is complete?
FAQ: Why are no drops coming over eventhough the distillate is boiling?
FAQ: Can I use a bunsen burner for reflux and distillation?
FAQ: When setting up the distillation apparatus, can I use an Erlenmeyer flask as the receiving vessel?
FAQ: How much does the boiling point vary with atmospheric pressure?
FAQ: If the atmospheric pressure today is 695 torr, does that mean the actual boiling point of this liquid is listed in the literature at a higher value than the temperature I saw, even after I corrected the temperature for my thermometer?
Distillation is a purification technique. It can be used for the separation of a low boiling solvent from a solid or a high boiling liquid. The liquid vaporizes in one vessel (the distillation pot) and then condenses into another vessel (the receiving flask).
Heat Sources:
Steam Bath - Use with flammable liquids that have a boiling point below 100 oC.
Heating Mantle with Powermite - Use with round bottom flasks during distillation or reflux. The Powermite allows heat regulation.
Hot Plate - Use with Erlenmeyer flasks containing non-flammable or high boiling liquids. Can also be used with beakers containing water (for a water bath).
Boiling Point:
Boiling point is the temperature at which the vapor pressure of a liquid is equal to that of the surrounding atmosphere.
Boiling point varies with atmospheric pressure and is corrected to the temperature that would be observed at sea level (760 Torr).
The higher the altitude, the lower the atmospheric pressure and the lower the boiling point.
Boiling point is measured during distillation when the temperature is constant (at the stillhead).
Corrected boiling point can be measured using a nomograph or the following equation:
Boiling Point Correction Factor = [(760 torr - atmospheric pressure) / 10 torr x 0.5 oC]
Corrected Boiling Point = Observed Boiling Point + Correction Factor
Student A left approximately 20mL of a liquid mixture for his partner, Student B, to work up. Student B was to remove and identify the solvent as well as retain the residue and distillate. Student B decided to use simple distillation to remove the solvent. He placed a lab jack in position and set up a heating mantle with a Powermite control. He clamped the distillation flask in place and added boiling stones followed by his unknown liquid. A stillhead was then attached and a thermometer was inserted into the thermometer adapter. The receiving flask was clamped in position and the condenser was connected between the receiving flask and distillation flask. Student B then attached the water tubing to the condenser and turn the water tap on full. The tubing popped off and sprayed the student's TA (who was not impressed). The student reconnected the tubing and this time used the hose clamps and turned on the water tap slowly. The student set the Powermite to 25 and after a few minutes, the liquid in the distillation flask began to boil. The solvent began to boil and the temperature of the distillate was observed to be 38 oC at ~700 torr. When no more solvent distilled over, the heating mantle was slowly lowered and the apparatus allowed to cool before disassembly.
When connecting the water tubing to your condenser, remember that water enters from the bottom of the condenser and exits from the top.
Add boiling stones to your distillation flask BEFORE you start heating.
Distillation is complete when: a) no more liquid collects in the receiving vessel, b) when the temperature of the thermometer starts to drop or c) when the distillation flask is empty.
Q: When using the water condenser, which hose connector is water-in and which is water-out?
Water goes in from the BOTTOM of the condenser.
Q: Why does it matter whether the water enters the condenser at the top or from the bottom?
If water enters from the bottom of the condenser, it will always be completely filled with cold water which ensures efficient cooling. During distillation, vapors are formed in the heated distillation flask. The condenser cools these vapors condensing them back to liquid droplets that flow down the condenser into the receiver flask. A completely filled condenser provides maximum cooling therefore allowing for maximum recovery of the purified liquid during distillation.
Q: How fast should the water run through the condenser?
A continuous flow of water through the condenser is necessary; however, the lower the flow rate, the lower the pressure and the lower the chance of having the tubing pop off (resulting in you spraying yourself or others).
Q: How far down the stillhead should the thermometer bulb be placed when setting up the distillation apparatus?
The mercury bulb of the thermometer should be positioned so that vapors condense readily on it. The tip of the bulb should be in line with the lowest part of the connecting tube of the stillhead.
Q: How do you know when distillation is complete?
Distillation is complete when: a) you notice a drop in temperature, b) no more distillate is being formed or c) the distillation pot is empty.
Q: Why are no drops coming over eventhough the distillate is boiling?
There could be two reasons for this:
If this happens at the beginning of the distillation, there is insufficient energy input to cause adequate vaporization of the liquid. In this case, you should increase the heat.
If this happens at the end of the distillation, almost all of the low boiling liquid has been removed. Solvent vapors trapped in the boiling stone pores will continue to be an ebullition source, causing bubbling.
Q: Can I use a bunsen burner for reflux and distillation?
It is safer to use a heating mantle with a Powermite control for flammable vapors. The Powermite also makes it easier to control the temperature.
Q: When setting up the distillation apparatus, can I use an Erlenmeyer flask for the receiving flask?
Yes, but there are less organic vapors if you use a small round bottom flask.
Q: How much does boiling point vary with atmospheric pressure?
For most organic liquids, there is a decrease of 0.5 oC with every 10 torr difference from 760 torr
Q: If the atmospheric pressure is 695 torr today, does that mean the actual boiling point of this liquid is listed in the literature at a higher value than the temperature I saw, even after I corrected the temperature for my thermometer?
Yes. Boiling points are usually recorded as the temperature at 760 torr. Given that, observed boiling point is lowered by 0.5 oC for every 10 torr difference below 760 torr. This means that at 695 torr, the boiling point of your liquid is actually 3.3 oC higher than the temperature you observed during distillation.