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1simple distillation

Simple Distillation

Introduction(Adapted from Mohrig, pp. 109-112.)

The boiling point of a pure organic liquid is a physical property of that liquid. It is defined asthe temperature at which the vapor pressure of the liquid exactly equals the pressure exertedon it. Boiling points can be determined using the technique of simple distillation. Distillation isa technique that is used to purify a mixture of liquids or to obtain a boiling point of a pureliquid (in the case of this course). Essentially, the liquid is heated to boiling and the vaporscondensed above the boiling liquid.

ApparatusThis is a 4.5 minute movie on how to set up a simple distillation apparatus. Please feel free toscroll through it to get to sections that interest you. A complete setup of this apparatus will beon the front bench of lab.

Procedure for Boiling Point Determination

Once you have set up your simple distillation apparatus according to the movie above, placethe liquid to be distilled in the distilling flask. The flask should be no more than two-thirds full

at the start of the distillation because the contents may foam and boil over. Remember toplace one or two boiling stones in the flask to promote even boiling. Ensure that all joints aretight. Turn on the water for the condenser. Only a small stream of water is needed; too much

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water pressure will cause the tubing to pop off. Turn on the Variac and the distillation canbegin. Adjust the Variac until the distillate drops at a regular rate of about one drop persecond. Allow 10 drops of distillate to collect and then discard. Collect the remaining distillate.At the point at which the temperature has become the highest and most stable, record thetemperature. This is your boiling point. Never boil the distilling flask to dryness because, insome cases, high-boiling explosive peroxides can become concentrated. Stop the distillation

when a small amount of liquid is left in the distilling flask. Turn off the Variac and remove theheating mantle to allow the flask to cool more quickly.

Simple distillation is designed to evaporate a volatile liquid from a solution

of non-volatile substances; the vapour is then condensed in the water

condenser and collected in the receiver.

The apparatus consists of a round-bottomed distilling flaskbearing

a stillhead connected to a water condenser (Liebig condenser). This is

attached via a vented delivery bend to thereceiver, also a round-bottomedflask. The stillhead has a thermometer adapter with a thermometer.

Notes:

the bulb of the thermometer is opposite the exit to the condenser. You

want the temperature of the exit vapours since it is these that will

condense. the delivery bend is vented so that when the apparatus is heated the

joints aren't pushed apart by expanding gas. Never draw a closed

apparatus. water goes in at the bottom of the condenser jacket and out at the top.

note the structure of the condenser - the water jacket is separate from

the tube down the middle!

The main difference between fractional distillation and simple distillation is the application in which they areused. Fractional distillation separates component parts when they have a boiling point less than 25 degrees Cof each other. Simple distillation works when the differences in boiling points between the compounds aregreater than each other.Both start out with a heating device and a

still pot. The liquid in the still pot is heated until the first component boils and starts to evaporate. In fractional

distillation, the vapor moves up the fractionating column. Inside this column are several cooling trays. The

vapor will condense as it reaches the trays, and only the vapor with the lowest boiling point will then move intothe condenser where it is converted back to its liquid form.In a simple distillation, since the components have

such varying boiling points there is no need for a fractionating column. The liquid is brought to a boil in the still.

It then moves up the still head and into the condenser where it is cooled back to a liquid."