Distillation under reduced pressure, Chemistry

Distillation under reduced pressure:

Distillation under reduced pressure is widely used for the purification of liquids which have very high boiling points at atmospheric pressure (760 mm of Hg). Such liquids are always difficult to distil at atmospheric pressure, due to the high temperatures which are required, and they may sometimes decompose at their ‘normal' (760 mm) boiling points. Boiling points in an apparatus in which the pressure has been artificially reduced by attachment to a water-pump are commonly about 100o lower than the values observed at atmospheric pressure. Thus, a liquid boiling at around 250 oC (at 760 mm Hg) will boil at about 150 oC when an an efficient water-pump is used.

An acceptable water-pump should produce a pressure which does not exceed 15 mm of mercury. Pumps normally employ some type of ‘pumping fluid', and cannot generate a vacuum which is less than the saturation vapour pressure of that pumping fluid. Since water, at 100C, has an svp of 9 mm Hg then the best vacuum a water-pump can produce is about 9 mm. (Since svp decreases as temperature decreases, water-pumps give their best vacuums on cold, winter days!) Before you set up the apparatus for a vacuum distillation, it is usually wise to first find a pump which will give you a satisfactory vacuum : you will find that most pumps won't! Test the performance of a pump by attaching to it a manometer of the type that you will be using during your distillation, having first read the precautions for using a manometer, given below.

For this vacuum distillation (referred to colloquially as ‘vac distillation') you will need a tripod/gauze and Bunsen burner as heat source, high-vacuum grease to lubricate all ground-glass joints, a 100 mL RB distillation flask, a Claisen head (lagged with glass-wool), a Quickfit thermometer, a fine air-bleed (You draw this out from a Pasteur pipette. You may need help from a demonstrator for this, as it's trickier than it seems : too fine a hole and the bleed may break, too wide a bore and the ‘gale' of air which results will blast the contents of the flask over into the receiver!), a short path air condenser, a pig (ie an adapter which will enable you to take several fractions during a vacuum distillation, without breaking the vacuum to change receivers), 3 x 25 mL tared collecting flasks, a manometer and a water-trap to prevent flooding by water which (due to, eg, a deterioration in the pump's performance) may flow back from the pump into the glassware. See Diagram 2.

Check the integrity of your apparatus before you start, as defective equipment may implode under reduced pressure : look especially for star cracks in the glassware. Always work behind a safety screen and, as always, wear eye protection.

Ensure that the manometer is kept over a tray to contain the accidental spillage of any mercury. Remember that manometers are delicate pieces of equipment : never admit air rapidly into a manometer, as the sudden change of pressure will propel mercury up the central mercury tube with considerable velocity, resulting in the glass envelope shattering (from ‘mercury hammer').

Posted Date: 2/11/2013 6:39:27 AM | Location : United States

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