把酒問青天
Alcohol Content Analysis
MEMBERS
MEMBERS
黃希雯 WONG
ONG HEI MAN IVY
梁皓彬 Leung Ho Pan Alvin
蘇鈺翔 Su Yu Shuang Gordon
方凱惠 Fong Hoi Wai Katie
麥晉陶 Mack Chun To Adrian
Raimondi College
Chemistry Olympic Hong Kong
1
Abstract
In this investigation, we aimed to estimate the concentration of ethanol present in
samples of solutions and gas mixtures based on the principle of redox reaction since
ethanol has a reducing property.
Ethanol reacts with acidified potassium permanganate quite slowly. We employ
back titration method to determine the concentration of ethanol present in an aqueous
solution containing alcohol such as beer.
For gas samples containing ethanol, its concentration is not high enough to be
determined by titration method. Absorbance method and e.m.f. measurement are used
instead to determine the concentration of potassium permanganate left in the reaction
mixture of ethanol and excess acidified potassium permanganate solution. These
indirect methods can be used to find the concentration of ethanol present in a gas
sample. We expect that these simple methods can be applied to find the blood alcohol
content (BAC), which function as a breathalyzer.
To sum up, we used three different methods to determine the concentration of
ethanol present in different kinds of samples including volumetric analysis, e.m.f.
measurement method and absorbance method.
2
Introduction
Alcohol drinking is very common in different social gatherings and celebrations
worldwide. An alcoholic
lcoholic beverage is a drink that typically contains 3%–60%
3%
ethanol
and it is divided into three classes: beers, wines, and spirits.
spirits Typical alcoholic
beverage such as rice wine, red wine and beer are commonly drunk by people
nowadays.
As mentioned, these alcoholic beverages all contain ethanol which is a 2-carbon
alcohol. Its molecular formula is C2H6O and has the following structure:
Ethanol is a volatile, colorless liquid that has a slight odor.
odor It is a versatile solvent,
miscible with water and with many
m
organic solvents due to the presence of its
hydroxyl group and the shortness of its carbon chain.
chain Hydrogen bonding causes pure
ethanol to be hygroscopic that readily absorb water from air. Chemically, ethanol has
a reducing property which can be oxidized
oxidiz to ethanal and further oxidized to ethanoic
acid CH3COOH.
O
O
CH3CH2OH
[O]
CH3
[O]
C
H
CH3
C
O
OH
Based on the above property, we are going to use acidified potassium
permanganate as oxidizing agent. Then, we employ three different methods to
determine the concentration
entration of ethanol in different samples, i.e. the back titration for
aqueous solution of ethanol samples, and the electrochemical method and the
absorbance method for gas ethanol samples.[1], [2]
3
Standardization
Principle:
Before reacting ethanol with acidified potassium permanganate, we should find
the exact concentration of potassium permanganate by titrating against a standard
solution of oxalic acid in an acidic medium.
2MnO4- (aq) + 16H+ (aq) + 5C2O42- (aq) → 2Mn2+ (aq) + 10CO2 (g) + 8H2O (l)
Procedure:
1.
2.
3.
4.
Weigh, accurately, about 6.0 g of hydrated oxalic acid, H2C2O4.2H2O.
Dissolve the solid in water and make up to 250.0 cm3 in a volumetric flask.
Pipette 25.0 cm3 of the solution into a conical flask.
Acidify the solution with an approximately equal volume of 1 M H2SO4 and heat
the mixture to about 70 °C.
5. At this temperature the flask is almost too hot to hold by the neck. Titrate the hot
solution with potassium permanganate (~0.10M) until at the end point a very faint
pink colour persists.
6. Repeat the titration until three concordant results are obtained.
7. Record the result and calculate the concentration of potassium permanganate.
Precautions:
The titration must be carried out slowly with the constant swirling and, because of
the intense colour of the permanganate, it is found advantageous to read the top of the
liquid rather than the bottom of the meniscus.
Chemical used:
Potassium permanganate
Potential hazard: Contact can severely irritate and burn skin and eyes with possible
eyes damage. Breathing potassium permanganate can irritate the nose, throat and
also lungs which cause coughing or shortness of breath. Higher exposures can
cause a build-up of fluid in the lungs with severe shortness of breath. It may affect
the liver and kidneys.
Safety precaution: Wear goggles, protective gloves and clothing
Oxalic acid
It is corrosive and contact can severely irritate and burn skin and eyes with
possible eyes damage. Inhaling oxalic acid can irritate the nose, throat and lungs
causing coughing, wheezing and shortness of breath. It may damage the kidneys.
Exposure to oxalic acid can affect nervous system and can cause dizziness,
headache, nausea and vomiting, convulsion, coma and even death.
4
Safety precaution: Wear protective gloves and clothing, goggles
Sulphuric acid
Potential hazard: Highly corrosive, irritating and dehydrating(if concentrated)
Safety precaution: Wear protective gloves and clothing
Results:
Mass of the hydrated oxalic acid: 6.00g
Volume of the oxalic acid solution used in the titration: 25.0cm3
Colour change at the end-point: from colourless to very faint pink
Runs
Final burette reading / cm
3
Initial burette reading / cm3
Vol. of KMnO4 delivered / cm
3
Trial
1
2
3
20.15
38.85
19.35
37.85
1.60
20.15
0.75
19.35
18.55
18.70
18.60
18.50
Calculation:
Average volume of KMnO4 delivered = (18.55 + 18.70 + 18.60 + 18.50) ÷ 4
=18.59 cm3
Number of mol of H2C2O4.2H2O used in titration
= 6.00 ÷ (1 × 6 + 12 × 2 + 16 × 6) × 25.0 / 250.0
= 4.762 × 10-3 mol
Number of mol of KMnO4 used in titration = 4.762 × 10-3 × 2/5
= 1.905 × 10-3 mol
Concentration of MnO4- = 1.905 × 10-3 ÷ (18.59 ÷ 1000) = 0.1025 M
5
Part I
Determination of the concentration of ethanol present in a sample:
Theory:
Ethanol is an organic reducing agent, and can be oxidised to ethanoic acid by an
acidified potassium permanganate solution. An exactly known amount of excess
acidified potassium permanganate solution is mixed with an ethanol sample in order
to ensure all ethanol has been reacted. The oxidation of ethanol is shown as follows:
5C2H5OH(aq) + 4MnO4-(aq) + 12H+(aq) → 5CH3COOH (aq) + 4Mn2+(aq) +11H2O(l)
The amount of the permanganate ions left after the reaction can be found by
titrating the reaction mixture with standard oxalic acid. Then the number of mole and
concentration of ethanol in the sample can be determined.
2MnO4-(aq) + 5C2O42-(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l)
A. Testing the validity of the back titration method using a known ethanol
solution
1. Prepare 5% volume by volume of ethanol by pipetting 5.0 cm3 of pure ethanol into
a 100.0 cm3 volumetric flask.
2. Dilute ethanol to the graduation mark with distilled water.
3. Pipette 10.0 cm3 of the 5% (v/v) ethanol solution into a 100.0 cm3 volumetric flask.
4.
5.
6.
7.
Dilute the solution to the graduation mark with distilled water.
Pipette 10.0 cm3 of diluted ethanol solution into a conical flask.
Add 40.0 cm3 of 1M H2SO4 (aq) and 20.0 cm3 of KMnO4(aq) into the conical flask.
Stopper the conical flask and let it stand for 1 hour.
8. Heat the mixture to about 70°C before titration.
6
9. Titrate the mixture with standard oxalic acid solution until the mixture turns from
pink to colourless.
Chemical used:
Ethanol
Potential hazard: highly flammable and irritating.
Safety precaution: Shut off ignition sources when using ethanol and wear eye
protection, a lab coat and gloves.
Potassium permanganate
Potential hazard: Irritating
Safety precaution: Wear goggles, protective gloves and clothing
Oxalic acid
Potential hazard: Corrosive and irritating
7
Safety precaution: Wear protective gloves and clothing, goggles
Sulphuric acid
Potential hazard: Highly corrosive, irritating and dehydrating(if concentrated)
Safety precaution: Wear protective gloves and clothing
Result:
Runs
Final burette reading / cm3
Initial burette reading / cm
3
Vol. of oxalic acid delivered / cm
3
Trial
1
2
3
4
20.30
37.35
18.40
35.25
37.50
1.90
20.30
1.60
18.40
20.60
18.40
17.05
16.80
16.85
16.90
Calculation:
Concentration of oxalic acid = 0.1905 M
Concentration of KMnO4 = 0.09910 M
Volume of KMnO4 mixed with ethanol = 20.0 cm3
Average volume of oxalic acid delivered = (16.80 + 16.85 + 16.90) ÷ 3 = 16.85 cm3
Number of mole of oxalic acid = 0.1905 × (16.85 ÷ 1000)
= 3.210 × 10-3
Number of mole of MnO4- = 3.210 × 10-3 × 2 ÷ 5
= 1.284 × 10-4
Number of mole of KMnO4 reacting with 10.0 cm3 of dil. ethanol solution
= 0.09910 × 20.0 ÷ 1000 – 0.001284
= 6.98 × 10-4
Number of mole of ethanol in 10.0 cm3 of dil. ethanol solution = 6.98 × 10-4 × 5 ÷ 4
= 8.725 × 10-4
Number of mole of ethanol in 100.0 cm3 of dil. ethanol solution
= 8.725 × 10-4 × 100.0 ÷ 10.0
= 8.725 × 10-3
Concentration of ethanol in 10.0 cm3 of original ethanol solution
= 8.725 × 10-3 ÷ 10.0 ×10-3
= 0.8725 M
Density of ethanol = 0.789 g cm-3
Mass of ethanol in 10.0 cm3 of original ethanol solution= 8.725 ×10-3 × 46
= 0.4014 g
3
Volume of ethanol in 10.0 cm of original ethanol solution= 0.4014 ÷ 0.789
= 0.509 cm3
% (v/v) of ethanol in original ethanol solution= 0.509 ÷ 10.0 × 100%
= 5.09% (which is close to 5.0%)
8
Experimental error:
The reaction between ethanol and acidified potassium permanganate may be
incomplete.
There may be a loss of solution during transferring.
Some ethanol may evaporate away from the reaction mixture.
Improvement:
Be careful in the chemical transferring process.
Control experiments with varying the time of reaction i.e. let the ethanol, KMnO4
and H2SO4 mixture stay overnight, standing for 3 hours and 2 hours can be held.
Stopper the conical flask immediately when ethanol is mixed with acidified
potassium permanganate.
Discussion
The common chemical used to determine the concentration of ethanol in many
experiments is usually dichromate ion solution. In the investigation, we did not use
dichromate ion as it requires a special indicator which is not common in school
laboratory and the dichromate ion is not the only ion has colour, when it is oxidized,
chromium(III) ion has colour also. Therefore it is hard to apply the same method used
in our investigation using dichromate ion instead of permanganate ion.
To sum up, it is possible to determine the concentration of ethanol using its
oxidizing properties with the back titration method. In the investigation, we have
proved the validity of the back titration method using a known ethanol solution with
the experimental concentration 5.09% which was close to the theoretical value 5.00%.
Furthermore, we applied the back titration method on determination of ethanol
concentration in a beer sample and we found that there are some other ingredients in
beer which may affect the result.
B. Determination of concentration of ethanol in beer (without steam distillation)
water
10.0 cm3 of beer  
→ 100.0 cm3 diluted beer
10.0 cm3 of diluted beer was mixed with 25.0 cm3 of KMnO4.
H2SO4 (aq) was added to the mixture and titrated it with oxalic acid again.
Concentration of oxalic acid = 0.1905M
Concentration of KMnO4 = 0.1025M
Volume of oxalic acid used in titration = (17.05 + 17.35) ÷ 2 = 17.20 cm3
% of ethanol in beer = 9.13%
According to the label on the beer, the % should be around 5.0%.
9
Experimental error:
As there are other ingredients besides ethanol in beer, typical beer addictives such as
Betaglucanase, Ammonia caramel, Rhoiso-alpha acids, Sulphur dioxide, Protease,
Amyloglucosidase, Propylene glycol alginate and Silicone may also react with
potassium permanganate.
As the error is large and it may be due to other ingredients in beer, which can react with
acidified potassium permanganate, steam distillation can be carried out in order to separate
ethanol from the ingredients.
Therefore we employed steam distillation method in order to minimize the error.
Improvement:
As the error is large and it may be due to other ingredients in beer, which can react
with acidified potassium permanganate, steam distillation can be carried out in order
to separate ethanol from the ingredients.
C. Determination of concentration of ethanol in beer (obtained from steam
distillation)
Since other ingredients present in beer may react with acidified potassium
permanganate, the ethanol is removed by steam distillation. About 50 cm3 of a
distillate containing ethanol and water is collected for further investigation.
Procedure:
10
1. Pipette 10.0 cm3 of beer into a pear-shaped flask.
2.
3.
4.
5.
Carry out steam distillation until about 50.0 cm3 of distillate is obtained.
Transfer the distillate and all washings into a 100.0 cm3 volumetric flask.
Dilute the mixture to the graduation mark with distilled water.
Pipette 10.0 cm3 of diluted solution into a conical flask.
6. Add 40.0 cm3 of 1M H2SO4 (aq) and 20.0 cm3 of KMnO4(aq) into the conical flask.
7. Stopper the conical flask and let it stand for 1 hour.
8. Titrate the mixture with a standard solution of oxalic acid.
Chemical used:
Potassium permanganate
Potential hazard: Irritating
Safety precaution: Wear goggles, protective gloves and clothing
Oxalic acid
Potential hazard: Corrosive and irritating
Safety precaution: Wear protective gloves and clothing, goggles
Sulphuric acid
Potential hazard: Highly corrosive, irritating and dehydrating(if concentrated)
Safety precaution: Wear protective gloves and clothing
Result:
Runs
Trial
1
2
3
4
Final burette
reading / cm3
23.95
41.70
18.90
36.70
30.75
Initial burette
reading / cm3
5.90
23.95
0.95
18.95
13.00
Vol. of oxalic acid delivered / cm3
18.05
17.75
17.95
17.75
17.75
Concentration of oxalic acid = 0.1905 M
Concentration of KMnO4 = 0.09910 M
Average volume of oxalic acid delivered = (17.75 + 17.95 + 17.75 + 17.75) ÷ 4
= 17.81 cm3
Number of mole of MnO4- in excess= 0.1905 × 17.81 ÷ 1000 × 2 ÷ 5
= 1.357 × 10-3
Number of mole of KMnO4 reacting with 10.0 cm3 of dil. distillate
= 0.09910 × 20.0 ÷ 1000 – 0.001357
= 6.250 × 10-4
11
Number of mole of ethanol in 10.0 cm3 of dil. distillate = 6.250 × 10-4 × 5 ÷ 4
= 7.813 × 10-4
Number of mole of ethanol in 100.0 cm3 of dil. distillate = 7.813 × 10-4 × 100.0 ÷ 10.0
= 7.813 × 10-3
Volume of beer used in steam distillation = 10.0 cm3
Concentration of ethanol in beer = 7.813 × 10-3 ÷ 10.0 × 10-3
= 0.7813M
-3
Density of ethanol = 0.789 g cm
Mass of ethanol in 10.0 cm3 of original ethanol solution= 7.813 × 10-3 × 46 = 0.3594 g
Volume of ethanol in 10.0 cm3 of beer= 0.3594 ÷ 0.789
=0.456 cm3
% (v/v) of ethanol in original ethanol solution= 0.456 ÷ 10.0 × 100%
= 4.56% (which is close to 5.0%)
Experimental error:
The reaction between ethanol and potassium permanganate may be incomplete.
The impurities may not be totally separated.
Not all ethanol is separated by steam distillation.
There may be a loss of solution during transferring.
Improvement:
Be careful in the chemical transferring process.
Control experiments with varying the time of reaction i.e. let the ethanol, KMnO4
and H2SO4 mixture stay overnight, stand for 3 hours and 2 hours can be held.
At least 50 cm3 of distillate is collected in steam distillation so that almost all
ethanol in beer can be separated.
12
Part II
For gas samples containing ethanol, the excess amount of MnO4-(aq) left in the
reaction mixture can be found by electrochemical method and colorimetric method.
meth
According to Lange’s handbook of Chemistry (10th ed)
Vapour pressure of ethanol (mm Hg) at 22 °C is given by
p = 108.04494- 1554.3÷(222.65+T)
= 108.04494- 1554.3÷(222.65+22)
= 49.2 mmHg
= 6.53 kPa
Apply
PV=nRT
If V = gas volume = 10.0 cm3
then the expected number
er of mole of ethanol present in 10 cm3 of gas sample is given
by
(6.53 × 103)(10.0 × 106) = n × 8.31 × (273 + 22)
n = 2.66 × 10-5 mol
Hence, the concentration of
o KMnO4(aq) used in the analysis should be greater than
2.66 × 10-3 M.
Electrochemical method
The set-up
up of the chemical cell:
The half-reactions
reactions taking place at the electrodes:
At the cathode,
MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l)
At the anode,
Zn(s) → Zn2+(aq) + 2e-
The overall reaction taking
ng place in the chemical cell:
5Zn(s) + 2MnO4-(aq) + 16H+(aq) → 5Zn2+(aq) + 2Mn2+(aq) + 8H2O(l)
13
According to Nernst equation, the empirical relationship between electrode potential
and concentration of ionic species involved can be shown by the Nernst equation:

At 298 K, EMn = EMn -
EZn = EZn -
0.059
5
0.059
2
log
[Mn
2+
(aq)]
+
8
[MnO4 (aq)][H (aq)]
1
log
[Zn
2+
(aq)]
Since Zn(s) | Zn2+(aq) half cell is used as a reference half cell, EZn can be regarded as
a constant. H+(aq) ions are used in large excess,
excess, and its concentration remains almost
unchanged during the reaction. Thus, the e.m.f. of the chemical cell varies with the
concentration of acidified MnO4-(aq) ions.
The e.m.f. of the chemical cell is given by
Ecell = EMn – EZn ∝ log [MnO4-(aq)]
Therefore,
fore, a graph of the e.m.f. of the chemical cell vs. log [MnO4-(aq)] should give a
straight line. Measure the e.m.f. of the chemical cell produced by acidified potassium
permanganate solutions of known concentration. A calibration curve is plotted to find
the concentration of permanganate ions in the reaction mixture.
Colorimetric Method
Use a colorimeter to measure the absorbance of a solution at a fixed wavelength.
Monochromatic light is passed through the reaction mixture and the transmitted light
is detected
ected by the photoelectric cell. Thus, the change in the colour intensity of the
substances in the reaction system can be determined.
Since MnO4-(aq) is purple, a green filter (610 nm) is chosen in the measurement of
absorbance of the reaction mixture.
According to Beer-Lambert
Lambert Law, the absorbance of a coloured solution is
proportional to the concentration of the coloured solution.
Absorbance ∝ conc. of coloured solution
Absorbance = - log (Transmittance)
14
A graph of absorbance vs. concentration of MnO4-(aq) ions should give a straight line.
Measure the absorbance at 610 nm produced by acidified potassium permanganate
solutions of known concentration. A calibration curve is plotted to find the
concentration of permanganate ions in the reaction mixture.
Preparing different known concentrations of KMnO4 solutions for calibration
curves:
Procedure:
1. Pipette 25.0 cm3 of 0.0990M KMnO4 solution into a beaker.
2. Add 25.0 cm3 of H2SO4 (aq) to the beaker.
3. Pipette 25.0 cm3 of solution to a 100.0 cm3 of volumetric flask and dilute the
solution to the graduation mark with distilled water.
4. Repeat step 3 in order to prepare acidified potassium permanganate of different
concentrations.
Calibration Curve for Electrochemical Method
Procedure:
1. Setting up chemical cell by pour the acidified KMnO4 solution and 0.1 M zinc
sulphate solution into two plastic vials separately.
2. Connect the zinc electrode and carbon electrode to the multimeter by wires.
3. Put the zinc plate into the zinc sulphate solution and the carbon electrode to the
acidified KMnO4 solution
4. Connect the half cells with a salt bridge soaked with a saturated solution of sodium
sulphate.
5. Record the e.m.f. reading of multimeter.
6. Clean the carbon electrode after each set of KMnO4 solution, and repeat the e.m.f.
measurement.
15
Calibration Curve for Colorimetric Method
Procedure:
1. Set the frequency used for absorbance measurement to 610 nm.
2. Calibrate the colorimeter with distilled water, i.e. adjust the reading to absorbance
equal to zero.
3. Pour an acidified KMnO4 solution of a known concentration into a test tube.
4. Put the test tube into the colorimeter.
5. Record the reading of transmittance.
6. Repeat the measurement with acidified potassium permanganate solutions with
different exact concentrations.
Chemical used
Sulphuric acid
Potential hazard: Highly corrosive, irritating and dehydrating(if concentrated)
Safety precaution: Wear protective gloves and clothing
Potassium permanganate
Potential hazard: Irritating
Safety precaution: Wear goggles, protective gloves and clothing
Zinc sulphate
Potential hazard: Irritating
Safety precaution: Wear dust mask, goggles and protective gloves
16
Calibration Curves
(1) Electrochemical Method
Conc. of MnO4-(aq) / M
log [MnO4-(aq)]
e.m.f. / V
0.04966
- 1.304
2.14
0.01242
- 1.906
2.11
3.104 × 10-3
- 2.508
2.06
7.759 × 10-4
- 3.110
1.99
1.9400 × 10-4
- 3.715
1.93
4.850 × 10-4
- 4.314
1.86
17
(2) Colorimetric Method
Conc. of MnO4-(aq) / M
Transmittance (T)
Absorbance (- log T)
0.04966
0.008
2.10
0.01242
0.010
2.00
3.104 × 10-3
0.340
0.469
7.759 × 10-4
0.728
0.138
1.940 × 10-4
0.880
0.0555
4.850 × 10-4
0.925
0.0339
18
Experimental error:
The multimeter readings are not steady during the e.m.f. measurement.
There may be some chemical left on the electrode which may affect the
concentration and thus the reading.
The concentration of zinc sulphate solution may change after several
measurements.
The zinc electrode may be coated with some oxide on the metal surface.
Improvement:
Same electrode should be used and always be kept in the same position in the half
cell.
Clean the electrode clearly after each measurement.
Zinc sulphate solution should be replaced for each measurement.
The calibration and the determination of the concentration of acidified potassium
permanganate left in the reaction mixture should be carried out at once.
Determination of concentration of ethanol in gas sample (obtained from
evaporation of pure ethanol in a balloon)
Procedure:
1. Evaporate pure ethanol in balloons for overnight.
2.
3.
4.
5.
6.
Use gas syringe to get 10 cm3 of ethanol gas sample.
Get 5 cm3 of distilled water into the same syringe.
Dissolve the ethanol gas in water by shaking the syringe thoroughly.
Pipette 10.0 cm3 of 3.104 ×10-3 M KMnO4 into a 100.0 cm3 volumetric flask
Transfer the ethanol mixture to the volumetric flask with a filter funnel.
7. Rinse the syringe and filter funnel with distilled water, and transfer all the
washings into the volumetric flask.
8. Add distilled water to volumetric flask and make the solution up to mark.
19
9. Let the mixture stand for 1 hour.
10. Measure the concentration of MnO4- in excess using e.m.f. and absorbance
measurement.
Chemical used:
Ethanol
Potential hazard: highly flammable and irritating.
Safety precaution: Shut off ignition sources when using ethanol and wear eye
protection, a lab coat and gloves.
Potassium permanganate
Potential hazard: Irritating
Safety precaution: Wear goggles, protective gloves and clothing
Result:
Transmittance
e.m.f.
Set 1
0.890
1.96 V
Set 2
0.895
1.96 V
Calculation
Average transmittance = (0.890 + 0.895) ÷ 2
Absorbance of reaction mixture = - log 0.893 = 0.0491
From the calibration graph, y = 142.6 x + 0.0265,
Concentration of MnO4- left in 100.0 cm3 of mixture = (0.0491 - 0.0265) ÷ 142.6
= 1.58 × 10-4 M
Number of mole of MnO4- left = 1.58 × 10-4 × 100 ÷ 1000
= 1.58 × 10-5 mol
Number of mole of MnO4- reacting with ethanol in gas sample
= 3.104 ×10-5 – 1.58 × 10-5
= 1.52 × 10-5 mol
Number of mole of ethanol in 10.0 cm3 of gas sample = 1.52 × 10-5 × 5 ÷ 4
= 1.91 × 10-5 mol
Apply
PV=nRT
101 × 103 × V = 1.91 × 10-5 × 8.31 × (273 + 22)
V = 0.463 cm3
% (v/v) of ethanol in gas sample = 0.463 ÷ 10.0 × 100%
= 4.63%
Average e.m.f. = (1.96 + 1.96) ÷2 = 1.96V
20
From the calibration curve, y = 0.15 x + 2.50
log [MnO4- (left)] = (1.96 - 2.5) ÷ 0.150 = -3.6
[MnO4- (left)] = 2.51 × 10-4 M
Number of mole of MnO4- left = 2.51 × 10-4 ×100 ÷ 1000
= 2.51 × 10-5 mol
Number of mole of MnO4- reacting with ethanol in gas sample
= 3.104 × 10-5 – 2.51 × 10-5
= 5.94 × 10-6 mol
Number of mole of ethanol in 10.0 cm3 of gas sample = 5.94 × 10-6 × 5 ÷ 4
= 7.47 × 10-6 mol
Apply
PV=nRT
3
101 × 10 × V = 7.47 × 10-6 × 8.31 × (273+22)
V = 0.180 cm3
% (v/v) of ethanol in gas sample = 0.180 ÷ 10.0 × 100% = 1.80%
Experimental error:
There may be some air trapped in the gas syringe and the short rubber tubing.
There may be gas leakage when water is got into the same syringe.
The ethanol gas may not be totally dissolved in water.
The reaction between ethanol and permanganate may be incomplete.
Improvement:
Minimize the length of the rubber tubing.
Rinse the syringe with the gas sample several times.
Hold the gas syringe carefully to prevent gas leakage.
Rinse the syringe with water several times in order to transfer all the ethanol.
Let the reaction mixture stand for longer time and carry out control experiment
with different duration.
21
Conclusion
The result of testing validity of the back titration method to determine the
concentration of ethanol using a known ethanol solution was 5.09% which was close
to 5.0%, i.e. the concentration of ethanol used.
The result of determination of ethanol concentration in a beer sample was 4.56%
which was obtained by steam distillation and was close to the concentration 5.0%
provided by the manufacturer.
The result of determination of ethanol concentration in a gas sample using
colorimetric method was 4.63%.
The result of determination of ethanol concentration in a gas sample using e.m.f.
method was 1.80%.
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Discussion
In this experiment, we used potassium permanganate instead of potassium
dichromate as the oxidizing agent. The first reason of not using potassium dichromate
is our school had not purchased the indicator for the titration of ethanol with
potassium dichromate. The second reason is, if we use potassium dichromate, both
potassium dichromate and the product, chromium(III) ions, are orange and green
respectively, the colour change at the end point of the titration will be not sharp
enough to determine the end point without the indicator. However by using potassium
permanganate, the colour change of the end point of the titration is purple to
colourless, which is sharp enough for us to determine the end point. This helps us
improve our result more accuracy.
For part II, although the electrochemical method and the colorimetric method we
used to determine the concentration of ethanol in a gas sample did not give same
results, at least the results were within an acceptable range.
According to Lange’s handbook of Chemistry (10th ed)
Vapour pressure of ethanol at 22 °C is given by pmmHg = 108.04494- 1554.3÷(222.65+T)
At 22 °C, p =108.04494- 1554.3÷(222.65+22) = 49.2 mmHg = 6.53 kPa
Apply
PV=nRT
The vapour pressure of ethanol in the gas sample determined by colorimetric method
was
P × 10 ÷ 106 = 1.91 × 10-5 × 8.31 × 295
P = 4.68 kPa
The vapour pressure of ethanol in the gas sample determined by e.m.f. method
P × 10 ÷ 106 = 7.47 × 10-6 × 8.31 × 295
P = 1.83 kPa
So we think that the concentration of ethanol can still be determined by the
electrochemical method and colorimetric method. If these two methods are valid and
become more sensitive, we hope that they can be employed to determine the breath
alcohol content (BrAC) of the gas sample or alcohol content in a urine sample
collected from the drunk drivers.
Hong Kong's maximum blood alcohol level (BAL) is 50 mg of alcohol per 100 ml of
blood, 0.22 mg alcohol per litre breath alcohol content (BrAC), or 67 mg of alcohol
per 100 ml of urine.
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Reference
1.
2.
3.
4.
http://en.wikipedia.org/wiki/Alcoholic_beverage
http://en.wikipedia.org/wiki/Ethanol
Chemistry Practical Redox Titration VA5 / Page 1 of 1
http://courses.chem.indiana.edu/s117/documents/1.Percentalcohol.pdf
5. Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003, Pages133-140
A Rapid Method for Determination of Ethanol in Alcoholic Beverages Using
Capillary Gas Chromatography
6. BBC NEWS | Business | What exactly is in your beer?
7. Lange’s handbook of Chemistry (10th ed)
8. http://www.td.gov.hk/en/road_safety/drink_driving/drink_driving_is_a_criminal_o
ffence/index.html
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