Since lowering the pressure dramatically reduces the temperature at which a liquid will distill.
Distillation under reduced pressure always carries a slight risk of the apparatus imploding.
For this reason, such distillation should be carried out in a fume cupboard or behind a toughened safety glass screen.
Always examine flasks very carefully for star cracks before using them under reduced pressure.
One of the significant changes from simple distillation is the need for the 2 neck claisen head in place of the distillation head.
As with simple distillation, insert a thermometer in a neck nearest the outlet.
And attached a water cooled condenser to the outlet joint.
Insert a capillary bleed into the other neck.
This may have either a rigid or a screw cap quick fit joint. But the later is easier to adjust to the correct neck. Do this before assembling the apparatus.
Lightly grease all the joints to prevent air leaking into the apparatus.
Attach a plastic clip to secure the joint between the claisen head and the condenser
There is no need to clip the thermometer or the capillary bleed.
Since the major cause of the decomposition of hot liquids is oxidation, it’s usual to draw oxygen free Nitrogen through the bleed.
The simplest way to do this is to use a Nitrogen filled balloon.
Attach this to the end of the bleed.
The fine bore of the capillary will prevent a rapid escape of the gas even under reduced pressure.
Use a multi-flask receiver adaptor to allow the receiving flask to be changed without letting air into the apparatus.
Carefully grease the upper part of the condenser cone so that the joint will rotate even when there is reduced pressure in the system.
A plastic clip securing this joint will still allow it to rotate easily.
The receiving flask should be held in place by clips and only very lightly greased.
For many applications, a water pump gives an adequate reduction in pressure such as down to 20-25 Hgmm.
If a lower pressure is required, use a rotary oil pump which easily gives pressures down to few mm or less.
When using an oil pump, always insert a trap cooled with dry iced acetone or a similar coolant between the pump and the distillation apparatus.
This is to prevent pump damage caused by solvent filling the pump lubrication oil.
Attach suitably thick walled tubing from the trap to the outlet on the receiver manifold.
In this case, the apparatus with reduced pressure distillation has been assembled in a fume cupboard to give the necessary protection in case of implosion.
Heating is best supplied by a heated oil bath mounted on a lab jack.
At the start, make sure the liquid in the bath is leveled with that in the flask.
Suspend the thermometer in the oil to monitor the bath temperature.
Position the safety screen to provide adequate protection but leave a small space for access to the apparatus.
With the pump on, close the vent to the suck-back trap.
Then open the Nitrogen bleed. Bubble should appear immediately.
Any traces of low boiling components in the liquid will rapidly be drawn off into the water pump of the cold trap.
As the bath temperature rises, distillation of the first few drops of liquid begins.
Collect distillate in the first receiver until the distillation temperature becomes steady.
Then rotate the receivers to collect the main fraction in the larger flask located on the central arm of the manifold.
Collect the third fraction in the remaining flask if the temperature changes again while there still liquid in the distillation flask.
When the level of liquid in the distillation flaks is near the bottom of the bleed, or a foaming is observed in the flask, lower the oil bath.
Continue to draw Nitrogen through the apparatus for 5 -10mins while it cools.
Then release the vacuum by letting air into the system through the vent tap on the trap.
Then switch off the pump.
Finally, unclip and stopper the flask and if necessary, weigh them and record their weights.
Dismantle the apparatus for cleaning before it cools completely.
No comments:
Post a Comment