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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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