Experiment 3
Fractional Distillation
Background
Distillation is a common method for purifying liquids. The technique consists of heating a liquid to
its boiling point in a container (called the "pot") and condensing the vapors into another container
(called the "receiver"). Because liquids differ in their vapor pressures the liquid of high vapor
pressure is more volatile and can be separated from another liquid of lower vapor pressure. The
vapor pressure of a liquid is a measure of the tendency of molecules on the surface of the liquid to
pass into the vapor state. At a given temperature the vapor pressure is a constant independent of the
atmospheric pressure. The vapor pressure is, however, a function of temperature as the following
data for methylene chloride and methanol illustrate:
Vapor Pressure (torr)
CH2Cl2 (oC)
CH3OH (oC)
1
-70
-44
10
-43
-16
40
-22
5
100
-6
21
400
24
50
(Source: CRC Handbook, p. D-192, 56th Ed. 1976)
800
A plot of this data is:
41oC
65oC
760
CH2Cl2
torr
600
400
200
CH3OH
0
-60
-40
-20
0
20
Temperature
53
(oC)
40
60
80
760
41
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The temperature at which the vapor pressure equals 760 torr (one standard atmosphere) is called the
normal boiling point. A liquid boils when its vapor pressure equals the pressure above the liquid.
For example, methylene chloride boils at 41oC when the pressure above it is one atmosphere as
shown in the graph. This is called the normal boiling point. When the pressure above methylene
chloride is reduced to 400 torr it will boil at 24oC.
The graph also shows that at every temperature methylene chloride has a higher vapor pressure and
is therefore more volatile than methanol. Because of this, the vapors above a mixture of methylene
chloride and methanol will always have a higher concentration of methylene chloride compared to
its concentration in the liquid phase. This relationship is shown in the following graph of the
boiling point versus the composition of various mixtures of methylene chloride and methanol. The
top line corresponds to the vapor composition above the boiling mixture and the bottom line
corresponds to the composition of the liquid.
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65
Temperature
60
50
2
4
50
1
t1
t2
60
vapor
solution
3
5
40
CH2Cl2 100
80
60
CH3OH 0
20
40
40
40
20
0
60
80
100
Mole %
For example, consider a 50:50 mole percent methylene chloride/methanol mixture. What is the
boiling point of the mixture? Begin at point 1 and read the temperature off the temperature scale
(t1). This is approximately 48 oC. What is the composition of the vapor above the boiling liquid?
This value is obtained by going from point 1 to point 2, both at the same temperature, to connect
with the vapor composition line. The line from point 1 to point 2 is called a “tie line”. The
composition of the vapor is read from point 2 by going down to the mole % axis. The composition
is approximately 82:18 mole percent methylene chloride/methanol. If the vapors above the boiling
liquid (82:18 mole percent methylene chloride/methanol) are condensed by cooling the resulting
liquid will have the same composition, 82:18 mole percent methylene chloride/methanol. Overall,
vaporizing a liquid and recondensing the vapors results in a new liquid that is enriched in the more
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volatile (lower boiling point) component.
This process of one vaporization-condensation cycle, going from point 1 to point 2, can be though
of as a simple distillation. One must remember, however, that the composition of the liquid in the
pot and vapor above the boiling liquid continually changes during the distillation process. Initially
the condensed vapors will be enriched in the more volatile component, methylene chloride. As the
methylene chloride is gradually removed the vapors and condensate will be enriched in the less
volatile component, methanol. It should be noted that increasingly higher temperatures are required
to maintain the liquid at its boiling point. This is because the lower boiling component is being
removed during a distillation.
To conduct a simple distillation the condensate is collected in fractions, each collected over a small
temperature range. Initially fractions are enriched in the lower boiling component but as that
component is removed from the distillation pot the fractions will become enriched in the higher
boiling component. The apparatus for a simple distillation is shown below. The set-up shows the
proper placement of the thermometer bulb in the distillation head for accurately recording the
temperature of each fraction of distillate. The inefficient separation of components in simple
distillation is largely due to the short path between the pot and the condenser.
Simple Distillation Apparatus
thermometer
thermometer adapter
condenser
water out
water in
vacuum adapter
clamp
clamp
small round bottom
with stirring bar
(the "pot")
heating mantle
stirring motor
lab jack, extended
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places to use
blue clamps
Fractional Distillation
The boiling point-mole % composition graph on the previous page shows that when the vapors at
point 2 are condensed the liquid, point 3, will have a composition of 82:18 mole percent methylene
chloride/methanol. This is a simple distillation. If the liquid corresponding to point 3 is vaporized
there will again be an enrichment of the more volatile component in the vapors, point 4. The
boiling point, t2, is about 43oC, much closer now to the boiling point of pure dichloromethane,
41oC. Repeated vaporization and condensation cycles (point 4 to point 5 and beyond) will
theoretically produce pure methylene chloride. This is called a fractional distillation. The repeated
volatilization-condensation steps of a fractional distillation are done in a fractionating column
shown below in the fractional distillation apparatus. The tube between the pot and the distillation
head, called a fractionating column, is where the condensation-revaporizations take place. A
variety of fractionating column may be used. Some are packed with stainless steel wool or small
glass beads that provide a large surface area on which the vapors coming from the pot can
condense. Others, such as a vigereaux column have glass indentations.
Fractional Distillation Apparatus
fractionating column
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Azeotropic Mixtures
Non-ideal behavior (Raoult's law is not followed) is observed in the distillation of mixtures of some
compounds. When this occurs a mixture of two components that has a constant boiling point may
be obtained. The constant boiling point component is called an azeotrope and in the distillation it
behaves as if it were a pure compound. For example, distilling a 50% ethanol water mixture will
begin boiling at 78.2oC, the boiling point of the azeotrope. The composition of the azeotrope is
95.6% ethanol and 4.8 % water. After the azeotrope is distilled the next component to distill is the
water. At no time in the distillation is it possible to obtain 100% ethanol. This is one reason why
the ethanol commonly used in the organic lab states "95 %" ethanol. There are many applications
of azeotropic distillation. Handbooks such as CRC Handbook contain tables of azeotropes.
Distillation Curves
The efficiency of separating mixtures can be seen by preparing a plot of mL distillate versus
boiling point, as done in this experiment. The data is obtained by measuring the head
temperature corresponding to the volume of distillate. In the case of a simple distillation, where
there are few volatilization-condensation steps, there is a gradual increase in boiling point as the
distillation takes place. This is shown by the circle data points in the graph below. During the
distillation the composition of distillate slowly goes from the more volatile component to the less
volatile component. When a fractional distillation is done there are more volatilizationcondensation steps which results in a better separation of the liquids as shown with the star data
points. Diamond data points represent a longer fractionating column that has even more
volatilization-condensation steps. The choice of fractionating column depends on the difference
in boiling points between two liquids and the degree of purification desired.
b.p. of B
Head Temperature
b.p. of A
u
u
u
Volume of Distillate
Data points for simple distillation
Data points for fractional distillation
Data points for fractional distillation with a longer fractionating column
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Refractometry: The Refractive Index
In this experiment refractometry, along with other methods, will be used to determine the
composition of the mixture to be distilled.
The refractive index or R.I. is a ratio of the velocity of light in air to that in a transparent
medium (for our intentions, a liquid). The refractive index depends upon the wavelength of light
used and the temperature (usually set at 20 or 25 oC) of the liquid. Thus, a refractive index
reading must be accompanied by both variables. For example, if a sodium vapor lamp is used,
the "D line" is the most likely wavelength to report. This line occurs at 589 nm. The Celsius
temperature is also designated. Most handbooks report R.I. values as shown below:
20
25
n D or
nD
Values at other temperatures can be calculated since refractive index is inversely proportional to
temperature. The amount of change is approximately 0.00045/oC. For example, the R.I. of a
compound is 1.3667 at 25.2oC. The R.I. at 20oC is calculated as follows:
20
nD =
20
nD =
25.2
nD
+
(0.00045)(25.2 - 20)
1.3690
If the compound is an unknown and one of the compounds to choose from had a R.I. of 1.3691, it
would be good evidence for our choice being the unknown. A word of caution is in order,
however. R.I. values for the vast majority of organic liquids vary between 1.3 and 1.7. Thus,
these values cannot lead to an absolute conclusion in identifying an unknown, much like melting
points. Furthermore, R.I. values vary with the purity of the sample. Nevertheless, the R.I. is
useful because it helps to eliminate possible choices for an unknown.
Students often can’t remember whether or not to add or subtract a correction. It is easy to
remember if you realize the density of the liquid determines the refractive index, in part. A less
dense sample has a lower R.I. than the same sample at a higher density. Since the density of a
liquid varies inversely with temperature, correcting to a higher temperature (e.g. 22 oC to 25
o
C) means you subtract the correction, obtaining a lower R.I. Conversely, correcting to a lower
temperature means you add the correction, obtaining a higher R.I. Notice in the example
above, the correction was from a higher temperature (1.3667 @ 25.2oC) to a lower temperature
(1.3690 @ 20 oC) and added the correction (5.2oC x 0.00045/oC) because the density of the
sample increases going from 25.2 oC to 20 oC.
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The refractive index is determined on the Bausch and Lomb Abbe refractometer as described
below:
1.
Close the hinged prism at A. If the system is to be operated at other than room
temperature turn on the water cooling/heating system and set it for the desired
temperature of circulating water.
2.
Pivot lamp B into the position shown and turn on the light. Adjust the eyepiece C until
the crosshairs on lens D are in focus. Open the prisms and introduce two drops of liquid
onto the prism face. Reclose the prism faces. DO NOT TOUCH THE PRISM FACES
WITH THE GLASS DROPPER OR ANY OTHER HARD OBJECT BECAUSE
YOU MAY SCRATCH THE FACES! For more volatile liquids, the drops may be
introduced with the prisms closed by touching the dropper to the right front junction of
the two prisms (try it, it works!)
3.
Look into the eyepiece C and rotate knob F until you
observe a split light image. If the image is blurred or
diffuse, sharpen it by moving the lamp up or down or
by rotating the chromatic adjustment collar G.
4.
When a sharp line is obtained, rotate knob F till the
sharp line is at the center of the crosshairs (see
diagrams below).
5.
Press down the toggle switch, located on the lower
left side of the instrument. This illuminates the index
of refraction scale; you can view the scale through lens C. Read the refractive index to
FOUR decimal places and record the temperature from the thermometer adjacent to
the prisms.
6.
Open the hinged prisms and clean off the sample with the cleaning fluid provided
(usually 95% ethanol), blotting the prisms only with the soft paper provided.
7.
When you are through taking measurements, place a clean, folded piece of soft lens paper
between the prisms and close them.
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The Experiment
WASTE-the water-methanol distillate should be placed in the waste container in hood Z. Water
remaining in the distillation flask may be poured down the drain.
In this experiment you will distill a mixture of methanol and water. A graph of head temperature vs
mL of distillate will be prepared from the laboratory data. From this and the amount of material
collected you will estimate the composition of the original mixture. Refractometry will also be used
to calculate the composition and the results compared with those from the graph.
Procedure
Sign out a fractionating column at the organic prep room door. Return it when finished.
Obtain a 40 mL sample of a methanol/water mixture by bringing a 50 mL round bottomed flask to
the prep room. Record the unknown number in your notebook. Remove about ½ mL of the mixture
to a vial and secure the cap. The refractive index of this will be determined later. Add the football
magnetic spin bar to the flask and set up the apparatus for fractional distillation as shown on the
previous page. Use a 50 mL heating mantle. Extend the lab jack a several inches if it is not already
partially extended. Always build the apparatus from the bottom up and make sure that all pieces are
firmly clamped. Pay attention to the placement of glassware and clamps. No stopcock grease is
necessary. Do not use a round-bottomed flask that improperly fits the heating mantle. The heating
mantle may be turned on with a voltage of 40-50V while you are setting up the apparatus. This will
speed up the process.
With the apparatus fully assembled, start the stirring motor, turn on the condenser water (a trickle),
and adjust the variac to a voltage that gives steady but not rapid boiling (85-95 V). If the mixture
boils too rapidly and the distillation is too fast you may not achieve a good separation. When the
first drop of distillate is collected record the head temperature. Then record the head temperature
after each 1 mL fraction, collecting the fractions in a 10 mL graduated cylinder. When the 10 mL
graduated cylinder reaches 10 mL quickly pour it into a beaker and continue collecting and
recording the temperature. The variac setting and therefore the temperature must be increased as
the distillation progresses. When the temperature reaches ~75 oC wrap the fractionating column
with aluminum foil including the joint above it. This will insulate the fractionating column so that
the vapors will remain sufficiently hot to travel to the top of the column. Collect the distillate until
the head temperature remains steady, near 100oC, for 2-4 mL, indicating that only water remains.
Allow the apparatus to cool before disassembling.
Calculation of the Original Composition by R.I.
In many situations the composition of a two component liquid mixture may be determined from the R.I.
of the mixture and the R.I. of each pure component. This assumes that R.I. is directly proportional to
mole fraction, which it is, and that the volumes of two liquids are additive, which may not be the case.
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For example, if pure A has an R.I. of 1.3667, pure B has an R.I. of 1.4362, and a mixture of the two has
an R.I. of 1.3992 then the original mixture has a mole fraction of .53 for A and .47 for B. The
calculation is shown below. This assumes that the volumes are additive.
x = mole fraction of A
x + y = 1 and
y = mole fraction of B
1.3992 = 1.3667x + 1.4362y
solve for x and y
1.3992 = 1.3667x + 1.4362(1-x)
1.3992 = 1.3667x + 1.4362 - 1.4362x
then x = 0.5323741
-.037 = -.0695x
This must be rounded to two significant figures
x = 0.53
In this experiment, the volume of mixtures of methanol-water is not additive. The volume of a
mixture of methanol and water is slightly less than predicted. Since the volume is less the density
will be greater. An increase in density translates into an increase in R.I. A plot of R.I. vs
composition for methanol-water is therefore not linear. In spite of this, the composition of a
methanol-water sample may be determined from a standard curve prepared by mixing known
amounts of methanol-water. The experimental data for this is shown to the right and the graph is
shown below. The data is obtained at 20.5oC.
In this experiment, the approximate composition of methanol-water may be obtained from the
temperature vs volume of distillate graph. From that value the standard curve (below) may be used
to estimate the original composition of the methanol-water mixture. Before using the graph it is
necessary to calculate your R.I. to 20.5oC. If your refractive index has a value above the curve on
the graph you will not be able to use refractometry to determine your composition. This may be
attributed to an improperly calibrated refractometer.
Experimental Data
mL
water
mL
methanol
RI
10
0
1.3323
9
1
1.3341
8
2
1.3365
7
3
1.3386
6
4
1.3406
5
5
1.3417
4
6
1.3417
3
7
1.3404
2
8
1.3381
1
9
1.3340
0
10
1.3280
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1.3420
1.3410
1.3400
Refractive Index
1.3390
1.3380
1.3370
1.3360
20.5 oC
1.3350
1.3340
1.3330
1.3320
1.3310
1.3300
1.3290
1.3280
0
10
20
30
40
50
60
70
Volume % methanol
Volume % water
The Notebook report should contain the following:
1.
2.
3.
4.
Title of the Experiment
Purpose of the experiment
Procedure (reference and outline form)
Data
Refractive Index
Unknown number
mixture
at ____________oC
Table of mL of distillate vs head temperature.
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80
90
100
5. Observations
6. Results/Calculations
(A) Prepare a graph (proper title, labeled axes, large data points) by plotting the average head
temperature, corresponding to each 2 mL fraction, vs. the total volume of distillate.
Composition of mixture determined from bp vs mL graph.
From the graph estimate (1) the amount (mL) of pure methanol and (2) the mixture of
methanol/water (where the head temperature is changing). The amount of pure water may be
estimated by subtracting (1) and (2) from 40 mL.
mL
mL
pure methanol
mixture
pure water
Calculate the % volume methanol and water in the mixture from the data above. Comment on the
reliability of this.
volume %
volume %
water
methanol
(B) Measure the refractive index of the original mixture (record the temperature). Calculate the R.I.
at 20.5oC and estimate the mole fraction of water/methanol from the graph. Show your calculations.
Refractive Index
mixture
at 20.5oC
Determine the following from the refractive index graph.
7. Conclusions
volume % methanol
volume % water
mole % methanol
mole % water
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ASA Questions:
1.
What is the relationship between vapor pressure and temperature?
2.
What takes place in a fractionating column?
3.
What is the heat source in today’s experiment and how will it be controlled?
4.
For what purpose is the refractometer used in this experiment?
5.
How do you know when the distillation you are going to do is finished?
6.
How would a plot of head temperature vs mL of distillate differ in a simple distillation vs a
fractional distillation?
7.
Corrections in refractive index must be made because of changes in
(a) barometric pressure.
(b) temperature.
(c) boiling points.
(d) molar mass between distillate components.
8.
Azeotropic mixtures
(a) have a constant boiling point
(b) show ideal behavior
(c) can only be separated with a highly efficient fractionating column
(d) have anomalous boiling points.
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