1.a. Construction of ethanol-water phase diagram
A pure liquid substance boils at a temperature at which its vapour pressure 𝑝𝑝𝑖𝑖
0
is equal to the pressure of the gas above it 𝑝𝑝𝑖𝑖
0
= 𝑝𝑝0 (where 𝑝𝑝0 is usually
laboratory pressure). Experimental values of vapour pressures above pure liquid are
usually given in tables (see TABLE I) or described by mathematical functions with
experimental parameters. For ethanol is can be used:
( )5,226
37,3674
57999,23ln 0
+
−=
t
pet
(4.1.)
where t is temperature in CO
.
A liquid mixture boils at pressure condition when its total vapour pressure 𝑝𝑝𝐶𝐶 is equal
to the pressure of the gas above it 𝑝𝑝𝐶𝐶 = 𝑝𝑝0. The total vapour pressure of the
multicomponent liquid mixture is given by
pC = ∑ pi
G
(4.2.)
where 𝑝𝑝𝑖𝑖
𝐺𝐺
are the equilibrium partial pressures of the constituents of the liquid
mixture.
The equilibrium partial pressure 𝑝𝑝𝑖𝑖
𝐺𝐺
of the majority component i can be calculated
using the Raoult's law formula (RL):
𝑝𝑝𝑖𝑖
𝐺𝐺
= 𝑥𝑥𝑖𝑖
𝐿𝐿
𝑝𝑝𝑖𝑖
0
(4.3.)
where molar fraction of majority component 𝑥𝑥𝑖𝑖
𝐿𝐿
is near 1.
On the contrary, if the component i in the liquid mixture is a minor component (ie
0→L
ix ), its equilibrium partial pressure is governed by Henry's law (HL):
𝑝𝑝𝑖𝑖
𝐺𝐺
= 𝑥𝑥𝑖𝑖
𝐿𝐿
𝐻𝐻𝑖𝑖
px
(4.4.)
where px
iH is Henry's constant that is dependent on temperature, pressure, and
composition of the liquid mixture.
Assuming the gaseous phase exhibits ideal behaviour (that is 𝑝𝑝𝑖𝑖
𝐺𝐺
= 𝑥𝑥𝑖𝑖
𝐺𝐺
𝑝𝑝𝐶𝐶) and that
the RZ is valid, the molar fraction of the component i in the gaseous phase 𝑥𝑥𝑖𝑖
𝐺𝐺
can
be calculated using the known molar fraction of the component i in the liquid phase
𝑥𝑥𝑖𝑖
𝐿𝐿
as:
𝑥𝑥𝑖𝑖
𝐺𝐺
= 𝑥𝑥𝑖𝑖
𝐿𝐿 𝑝𝑝0
𝑖𝑖
𝑝𝑝𝐶𝐶
(4.5.)
The calculated gaseous phase composition 𝑥𝑥𝑖𝑖
𝑔𝑔
deviates from the experiment for nonideal
mixture.
The Raoult's (Henry's) law cannot be generally used if the component is not a major
(minor) component. We refer to the so-called positive deviation from the RL for
component i if the calculated (use eqn. (4.3.)) partial pressure 𝑝𝑝𝑖𝑖
𝐺𝐺
is lower than the
experimental partial pressure. Otherwise, the negative deviation from the RL is
referred. We are talking about the predominance of the positive deviations from the
RL, if the sum (see eqn (4.2.)) of the component partial pressures obtained using
Raoult's law is less than the total experimental pressure (ie. if pC < 𝑝𝑝0 when liquid
mixture boils in the distillation apparatus).

Liquid binary ethanol-water mixture
can be distilled using a simple
distillation apparatus in FIGURE 1.
During the distillation of the liquid
ethanol-water mixture, the liquid
mixture boils at temperature 𝑇𝑇𝐵𝐵 in the
distillation flask and vapour rises to
upper parts. The vapour is rich in more
volatile component (here ethanol). The
vapour liquefies at temperature 𝑇𝑇𝐴𝐴 in
the condenser and fall into reflux
reservoir. The excess of the
condensate (refluxing liquid) goes back
to the distilling flask. After a certain
reflux time, the temperatures 𝑇𝑇𝐵𝐵, 𝑇𝑇𝐴𝐴 are
nearly equal (if the insulation is good)
and compositions in both distilling flask
and reflux reservoir are different but
stabilised (the chemical composition of
steam and condensate in the reservoir
is the same).
The composition of boiling mixture and
condensed vapour can be determined
from the phase diagram. The mixture
of water and ethanol binary is an
example of a non-ideal binary system
(see phase diagram in FIGURE 2). If the
mixture boils at temperature 𝑡𝑡1 , the
composition of the liquid mixture in the
distillation flask (see point A in FIGURE
2) is 𝑥𝑥𝐸𝐸𝐸𝐸
𝐿𝐿
. The composition of the
vapour (see point B) is 𝑥𝑥𝐸𝐸𝐸𝐸
𝐺𝐺
, and the
overall composition of the system (see
point O, ie. the composition of the
mixture before heating) is 𝑥𝑥𝐸𝐸𝐸𝐸
0
. Any
change in the overall composition (for
example condensate withdrawal)
causes a boiling temperature change.
The compositions of the liquid phase
and the gas phase will change but
always lie on phase boundary L/(L+G)
and (L+G)/G respectively.
The mass of vapour relative to the
mass of condensate is negligible thus
the mass fractions of gaseous 𝑤𝑤 𝐺𝐺
and
liquid 𝑤𝑤 𝐿𝐿
phases is governed by the
lever phase rule that can be written in
the form:
FIGURE 1: Apparatus for reflux distillation.
1…distilling flask (𝑇𝑇𝐵𝐵), 2… distillation adapter
with reflux reservoir (𝑇𝑇𝐴𝐴 ), 3… condenser, 4…
thermometer, 5… syringe, 6… double oblique
stopcock, 7…insulation.
FIGURE 2: Phase diagram of the water-ethanol
system. Azeotropic point 78.18 °C, boiling: water
100.0 °C, ethanol 78.4 °C.
𝑤𝑤 𝐺𝐺
=
𝑚𝑚 𝐺𝐺
𝑚𝑚 𝐺𝐺
+𝑚𝑚𝐿𝐿 and 𝑤𝑤 𝐿𝐿
=
𝑚𝑚𝐿𝐿
𝑚𝑚 𝐺𝐺
+𝑚𝑚𝐿𝐿 (4.6.)
where 𝑚𝑚𝐺𝐺
and 𝑚𝑚𝐿𝐿
are the mass of the mixture in the reservoir and in the distillation
flask respectively.
TASK: Determine the compositions of the co-existing gaseous and liquid
phases during distillation of ethanol-water mixture at reflux conditions.
Compare the experimental results with the phase diagram. Find out which deviation
from the Raoul’s law reveals the distilled mixture.
LABORATORY AIDS AND CHEMICALS: Apparatus for reflux distillation (see FIGURE
2, for main parts) including: thermometer (75-100 °C), spherical glass beads 3-
5mm, laboratory heating mantle 1000ml, double oblique stopcock. 12 laboratory
tubes, laboratory test tube rack, 2 syringes (20ml), 2 scale pipettes (5 and 10 ml).
Distilled water, laboratory ethanol (95.6% by weight, preferably without denaturation).
Oscillation densitometer. Ethanol-water standard mixtures with a molar ratio of
ethanol: 0.05 (prepared by mixing 848 volumes of water + 152 volumes of laboratory
ethanol); 0.10 (volume ratio: 723/276); 0.2 (534/466); 0.3 (396/605); 0.50 (207/793),
and 0.70 (84/916).
INSTRUCTIONS: Assemble the distillation apparatus (see FIGURE 1). Insert
several glass beads into the distillation flask and add ethanol-water mixture
(250-300 ml). A suitable composition is about 50 vol% ethanol. You can also recycle
the samples of the previous measurement.
1. Turn the inlet of cooling water to the condenser on. Switch on laboratory heating
mantle. Bring the mixture to boil.
2. Let the mixture to reflux. Watch the temperature and wait for stabilization, then
record the temperature TB in the distillation flask and TA in the distillation adapter
(if is it at disposal).
3. Take the couple of the mixture samples. One from the distillation flask, the second
from the reservoir to the separate test tubes.
4. Pour out so much distillate from the reservoir to let the mixture boil at temperature
about 3 ºC above the foregoing reflux temperature.
5. Continue according to point 2-4. Take out 6-7 couples of samples covering the
80-98ºC temperature range.
6. Within free time during distillation, use the densitometer for measurements
needed for a construction of calibration curve. You can also analyse the cold
samples. Collect the samples and the calibration solutions for recycling. Measure
density of both pure water and laboratory ethanol.
7. When the distillation has finished, switch off the heating mantle and turn the
cooling water inlet off. Measure the air pressure in the laboratory.
Determination of calibration curve. Use pre prepared ethanol-water standards.
Take a standard sample using syringe, rinse capillary of oscillation densimeter by first
portion of the liquid standard, and leave the rest of the standard in oscillating
capillary. Measure the density of all standard solutions, all samples, pure water and
laboratory ethanol.
SAFETY: Do not leave the distillation apparatus without your supervising.
Distillation is the most common cause of fires in laboratories.
REPORT: Calibration table 1: for each standard sample, water and laboratory
ethanol: ethanol concentration in molar fraction 𝑥𝑥𝐸𝐸𝐸𝐸, density (g cm3
).
?




Calibration graph 1: density vs. ethanol concentration 𝑥𝑥𝐸𝐸𝐸𝐸. Table 2: for each sample
taken from reservoir (G) and distillation flask (L): sample label (couple number
completed with G or L symbol), sample density, ethanol concentration according to
the calibration graph, temperature above distilling flask (TA) or temperature near to
condenser (TB), the average of these temperatures for each couple. Graph 2:
experimental phase diagram of binary system ethanol-water (like FIGURE 2, use
average temperature for temperature on y-axis). IN ADDITION: laboratory pressure 𝑝𝑝0.
Table 3: for pure ethanol, each sample and pure water: sample label, density,
ethanol and water contents, boiling point (Find tabulated values for pure water and
ethanol. Take average of TB and TA for refluxing samples.), ethanol 𝑝𝑝𝐸𝐸𝐸𝐸
0
and water
𝑝𝑝𝑊𝑊
0
vapour pressures for samples in distilling flask at temperature equal to boiling
point given in previous column (use TABLE I for pure ethanol and eqn (4.1.) for pure
water), ideal partial pressures of constituents 𝑝𝑝𝐸𝐸𝐸𝐸
𝐺𝐺
and 𝑝𝑝𝑊𝑊
𝐺𝐺
above liquid mixture
calculated according Raoults law (4.3.), sum pC = 𝑝𝑝𝐸𝐸𝐸𝐸
𝐺𝐺
+ 𝑝𝑝𝑊𝑊
𝐺𝐺
, difference pC − 𝑝𝑝0 , ideal
ethanol content in gaseous phase 𝑥𝑥𝐸𝐸𝐸𝐸
𝐺𝐺 (𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖) (use eqn (4.5.)), difference 𝑥𝑥𝐸𝐸𝐸𝐸
𝐺𝐺 (𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖) −
𝑥𝑥𝐸𝐸𝐸𝐸
𝐺𝐺 (𝑒𝑒𝑒𝑒𝑒𝑒. ).
TABLE I: Saturated vapour pressures (in kPa) of water at different temperatures.
T
o
C 0 1 2 3 4 5 6 7 8 9
0 0.609 0.656 0.704 0.755 0.811 0.870 0.923 0.999 1.070 1.145
10 1.225 1.309 1.399 1.494 1.595 1.701 1.813 1.932 2.059 2.192
20 2.331 2.480 2.637 2.802 2.977 3.160 3.353 3.556 3.771 4.043
30 4.232 4.481 4.743 5.018 5.307 5.610 5.926 6.260 6.609 6.975
40 7.358 7.759 8.180 8.618 9.079 9.560 10.061 10.587 11.133 11.707
50 12.304 12.928 13.579 14.258 14.963 15.694 16.466 17.263 18.101 18.966
60 19.870 20.801 21.786 22.796 23.847 24.938 26.081 27.265 28.489 29.766
70 31.082 32.452 33.875 35.351 36.868 38.450 40.087 41.789 43.544 45.366
80 47.242 49.183 51.205 53.293 55.434 57.656 59.956 62.337 64.798 67.325
90 70.11 72.82 75.61 78.50 81.46 84.53 87.70 90.96 94.30 97.77
100 101.33 105.00 108.77 112.67 116.67 120.80 125.05 129.40 133.91 138.51