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Fractional distillation of binary solvent mixtureBy - 05/25/2006

in

Objective

To separate the binary mixture by simple distillation using fractionating column

Prinicple/Background of the Experiment

Distillation is apurification technique in which compounds with different boiling points

can be separated by controlled heating. Vapors from a sufficiently heated sample can

be recondensed and collected, purer than the initial mixture.The liquid which has not

vaporized is called the residue, and the liquid which is collected in the receiver is called

the distillate.

Since not all chemicals distill the same way, there are several distillation techniques can

be preferred depending on the nature of constituents to be purified or to be separated.

These include simple distillation, fractional distillation, steam distillation and

vacuum distillation .

A simple distillation (figure 2) is for purifying liquids of one component (separating

nonvolatile liquid impurity or to purify a liquid from solid contaminants), multiple liquids

where the differences in boiling points is very large (a low boiling liquid from a high

boiling liquid)(b.p difference around 50 -70C). Simple distillations are not effective in

removing multiple solvents from one another with a high degre e of success.

In fractional distillation (figure 3), a fractionating column is inserted between the

distillation flask and the distillation head. The fractionating column provides a large

surface area in which the mixture can be continuously vaporized and condensed.

The principle of a fractionating column is that, as the vapours ascend the column from

the boiling mixture below, the high boiling components are condensed and returned to

the flask, the ascending column of vapour being thus steadily scrubbed by the

descending column of liquid condensate. The ascending column of the vapour becomes

therefore steadily richer in the lowest boiling component, and the descending column of

condensate steadily richer in the highest boiling component.

Figure 1 represents the typical curve for simple and fractional distillation. In an ideal

fractional distillation, two distinct fractions are obtained. The first corresponds to the

component with the lower boiling point and the second to the high -boiling point

component. What characterizes a good fractional distillation is the sudden increase intemperature between both fractions, or in other words, a very small volume distilled at

temperatures other than the boiling points of the pure liquids. In simple distillation, a

much more gradual increase in temperature is observed, reflecting the impure nature of

the distillate

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Figure 1. Simple and fractional distillation curves

Steam distillation is used for separating mixtures of chemicals such as oils, resins,

hydrocarbons, etc. which are insoluble in water and may decompose at their B.P.

Vacuum distillation is used for separating liquids boiling above 200C

Fractional Distillation Apparatus

The set-up for fractional distillation is shown in Figure 3. It consists of a round -bottomed

distillation flask where the liquid is placed, a fractionalting column, a distillation head that

connects the distillation flask to the condenser; and a distillation adaptor that connects

the condenser to the receiving flask. The condenser is a tube surrounded by a water

jacket to cool and condense vapors. The distillation head holds a thermometer to allow

the temperature of the vapors to be monitored during the distillation.

Figure 2 Simple distillation set -up

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Procedure:

Assemble the apparatus as shown in Figure 3, use graduated cylinder to collect the

distillate. Place the liquid (25 mL of cyclohexane (b.p.80.7C) and 25 mL of

toluene(b.p.111C) or 25 mL of benzene (b.p.80C)or 25 mL of of toluene) to be distilled

into the distillation flaskPlace a boiling chip in the distillation flask to ensure smooth

bubbling and prevent bumping of the liquid up into the distillation head. Heat the liquids

and distil slowly, so that the total distillation occupies about 1 hour and 30 minutes. As

the liquid boils, vapors rise into fractionating column and to the distillation head and

condensed liquid will be seen dripping from the thermometer bulb. Eventually the vapors

enter the side arm of the distillation head and continue into the condenser. Once in the

condenser the vapors are cooled due to the water circulating in the outer jacket; the

vapors condense back to a liquid that runs down the condenser and is collected in the

receiving flask. Increase the heating rate near the midpoint of the distillation otherw ise

the head temperature will drop.

Take temperature readings at the first drop and at each 2 ml increment. Continue until

you have distilled 48 ml(collect the fractions having boiling points (a) 80 85, (b) 85-

107 (c) 107-111, these fractions should have the volumes about 23, 2, 23 mL

respectively). Construct a table in your notebook as given below, to record the

temperature at the distillation "head" as a function of volume distilled.

Volume distilled (mL) 2 4 6 8 10 12 14 16 1820

Temperature without column

Temperature with column

Make a plot of Temperature (C) Vs. volume of distillate Co llected (ml) and determine

the b.p of each of these solvents.

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