Alcohol Content Analysis MEMBER SSSS - hkasme.org20College.pdf · samples of solutions and gas mixtures based on the principle of redox reaction since ... We employ back titration

MEMBERMEMBERMEMBERMEMBERSSSS黃希雯 WONG 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 CollegeRaimondi CollegeRaimondi CollegeRaimondi CollegeChemistry Olympic Hong Kong

把酒問青天把酒問青天把酒問青天把酒問青天Alcohol Content Analysis

SSSS ONG HEI MAN IVY

Leung Ho Pan Alvin

Su Yu Shuang Gordon

Fong Hoi Wai Katie

Mack Chun To Adrian

Raimondi CollegeRaimondi CollegeRaimondi CollegeRaimondi College

把酒問青天把酒問青天把酒問青天把酒問青天 Alcohol Content Analysis

1

Alcohol Content Analysis

2

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.

Alcohol drinking is very common in different social gatherings and celebrations

worldwide. An alcoholic beverage

and it is divided into three classes: beers, wines, and spirits

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

alcohol. Its molecular formula is C

Ethanol is a volatile, colorless liquid that has a slight odor

miscible with water and with m

hydroxyl group and the shortness of its carbon chain

ethanol to be hygroscopic

a reducing property which can be oxidiz

acid CH3COOH.

CH3CH2OH

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 of ethanol in different

aqueous solution of ethanol samples, and the

absorbance method for gas ethanol samples.

Introduction

is very common in different social gatherings and celebrations

lcoholic beverage is a drink that typically contains 3%

divided into three classes: beers, wines, and spirits. Typical alcoholic


alcoholic beverages all contain ethanol which is a

molecular formula is C2H6O and has the following structure:

is a volatile, colorless liquid that has a slight odor. It is a versatile solvent,

miscible with water and with many organic solvents due to the presence of its

hydroxyl group and the shortness of its carbon 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 to ethanal and further oxidized to

CH3 C

O

H CH3 C

O

O[O] [O]



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

is very common in different social gatherings and celebrations

is a drink that typically contains 3%–60% ethanol

Typical alcoholic


alcoholic beverages all contain ethanol which is a 2-carbon

O and has the following structure:

is a versatile solvent,

the presence of its

Hydrogen bonding causes pure

that readily absorb water from air. Chemically, ethanol has

and further oxidized to ethanoic

OH



back titration for

electrochemical method and the

4

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) + 5C2O4

2- (aq) → 2Mn

2+ (aq) + 10CO2 (g) + 8H2O (l)

Procedure:

1. Weigh, accurately, about 6.0 g of hydrated oxalic acid, H2C2O4.2H2O.

2. Dissolve the solid in water and make up to 250.0 cm3 in a volumetric flask.

3. Pipette 25.0 cm3 of the solution into a conical flask.

4. 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.

5

� 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 Trial 1 2 3

Final burette reading / cm3 20.15 38.85 19.35 37.85

Initial burette reading / cm3

1.60 20.15 0.75 19.35

Vol. of KMnO4 delivered / cm3

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

6

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) + 4Mn

2+(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) + 5C2O4

2-(aq) + 16H

+(aq) → 2Mn

2+(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. Dilute the solution to the graduation mark with distilled water.

5. Pipette 10.0 cm3

of diluted ethanol 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. Heat the mixture to about 70°C before titration.

7

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.


� Potential hazard: Irritating


Oxalic acid

� Potential hazard: Corrosive and irritating

8


Sulphuric acid



Result:

Runs Trial 1 2 3 4

Final burette reading / cm3

20.30 37.35 18.40 35.25 37.50

Initial burette reading / cm3 1.90 20.30 1.60 18.40 20.60

Vol. of oxalic acid delivered / cm3 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


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

Volume of ethanol in 10.0 cm3

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%)

9

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)

10.0 cm3 of beer →

water 100.0 cm

3 diluted beer

10.0 cm3 of diluted beer was mixed with 25.0 cm

3 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%.

10

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:

11

1. Pipette 10.0 cm3

of beer into a pear-shaped flask.

2. Carry out steam distillation until about 50.0 cm3

of distillate is obtained.

3. Transfer the distillate and all washings into a 100.0 cm3 volumetric flask.

4. Dilute the mixture to the graduation mark with distilled water.

5. Pipette 10.0 cm3

of diluted solution into a conical flask.

6. Add 40.0 cm3

of 1M H2SO4 (aq) and 20.0 cm

3 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:




Oxalic acid

� Potential hazard: Corrosive and irritating


Sulphuric acid



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

12


of dil. distillate = 6.250 × 10-4

× 5 ÷ 4

= 7.813 × 10-4


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

Density of ethanol = 0.789 g cm-3

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.

For gas samples containing ethanol, the excess amount of MnO

reaction mixture can be found by electrochemical method and colorimetric meth

According to Lange’s handbook of Chemistry

Vapour pressure of ethanol

p = 108.04494- 1554.3

= 108.04494- 1554.3

= 49.2 mmHg

= 6.53 kPa

Apply P V = n R T

If V = gas volume = 10.0 cm

then the expected number of mole of ethanol present in

by

(6.53 ×

Hence, the concentration o

2.66 × 10-3

M.

Electrochemical method

The set-up of the chemical cell:

The half-reactions taking place at the electrodes:

At the cathode, MnO4-(aq) + 8H

At the anode, Zn(s) →

The overall reaction taking place in the

5Zn(s) + 2MnO4-(aq) + 16H

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 meth

According to Lange’s handbook of Chemistry (10th

ed)

Vapour pressure of ethanol (mm Hg) at 22 °C is given by 1554.3÷(222.65+T)

1554.3÷(222.65+22)

10.0 cm3

er of mole of ethanol present in 10 cm3 of gas sample is given

× 103)(10.0 × 10

6) = n × 8.31 × (273 + 22)

n = 2.66 × 10-5

mol

of KMnO4(aq) used in the analysis should be

up of the chemical cell:

reactions taking place at the electrodes:

(aq) + 8H+(aq) + 5e

- → Mn

2+(aq) + 4H2O(l)

→ Zn2+

(aq) + 2e-

ng place in the chemical cell:

(aq) + 16H+(aq) → 5Zn

2+(aq) + 2Mn

2+(aq) + 8H

13

(aq) left in the

reaction mixture can be found by electrochemical method and colorimetric method.

of gas sample is given

greater than

(aq) + 8H2O(l)

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

Since Zn(s) | Zn2+

(aq) half cell is used as a reference half cell, E

a constant. H+(aq) ions are used in large ex

unchanged during the reaction. Thus, the e.m.f. of the chemical cell varies with the

concentration of acidified MnO

The e.m.f. of the chemical cell is given by

E

Therefore, a graph of the e.m.f. of the chemical cell vs. log [MnO

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 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 Law, the absorbance of a coloured solution is

proportional to the concentration of the coloured solution.

Absorbance

Absorbance =



50.059 log

8-

4

2

(aq)][H (aq)][MnO

(aq)][Mn

+

+

20.059 log

(aq)][Zn

1

2+

(aq) half cell is used as a reference half cell, EZn can be regarded as

(aq) ions are used in large excess, and its concentration remains almost


concentration of acidified MnO4-(aq) ions.

The e.m.f. of the chemical cell is given by

Ecell = EMn – EZn ∝ log [MnO4-(aq)]

fore, a graph of the e.m.f. of the chemical cell vs. log [MnO4-(aq)] should give a



the concentration of permanganate ions in the reaction mixture.



ected by the photoelectric cell. Thus, the change in the colour intensity of the

substances in the reaction system can be determined.

(aq) is purple, a green filter (610 nm) is chosen in the measurement of

absorbance of the reaction mixture.

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



can be regarded as

cess, and its concentration remains almost


(aq)] should give a





ected by the photoelectric cell. Thus, the change in the colour intensity of the

(aq) is purple, a green filter (610 nm) is chosen in the measurement of

Lambert Law, the absorbance of a coloured solution is

15

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 cm

3 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.

16

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






Zinc sulphate


� Safety precaution: Wear dust mask, goggles and protective gloves

Calibration Curves

(1) Electrochemical Method

Conc. of MnO

0.04966

0.01242

3.104 × 10

7.759 × 10

1.940 × 10

4.850 × 10

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

0 × 10-4

- 3.715 1.93

4.850 × 10-4

- 4.314 1.86

17

18

(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

19

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. Use gas syringe to get 10 cm3 of ethanol gas sample.

3. Get 5 cm3 of distilled water into the same syringe.

4. Dissolve the ethanol gas in water by shaking the syringe thoroughly.

5. Pipette 10.0 cm3

of 3.104 ×10-3

M KMnO4 into a 100.0 cm3

volumetric flask

6. 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.

20

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.




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 cm

3 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 P V = n R T

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

21

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 P V = n R T

101 × 103 × 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.

22

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%.

23

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 P V = n R T

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.

24

Reference

1. http://en.wikipedia.org/wiki/Alcoholic_beverage

2. http://en.wikipedia.org/wiki/Ethanol

3. Chemistry Practical Redox Titration VA5 / Page 1 of 1

4. 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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Alcohol Content Analysis MEMBER SSSS - hkasme.org20College.pdf · samples of solutions and gas mixtures based on the principle of redox reaction since ... We employ back titration