Determination of Ethanol Concentration in Aqueous Solutions

College of Science
Determination of Ethanol Concentration in Aqueous Solutions
Safety
The amount of unreacted dichromate is then
determined by adding potassium iodide solution which
is also oxidised by the potassium dichromate forming
Lab coats, safety glasses and enclosed footwear must iodine.
be worn at all times in the laboratory.
Determination of Ethanol Concentration 2− in + − 3+
The acid dichromate solution needs to be prepared Cr O + 14 H + 6 I → 2 Cr + 3 I2 + 7 H O
2 7
2
with care. Aqueous Any concentrated Solutions acid spills must be cleaned The iodine is then titrated with a standard solution of
up by very carefully diluting with water before wiping sodium thiosulfate and the titration results are used to
up. Take care to put the water in the flask first before calculate the ethanol content of the original solution.
adding the Introduction acid, and add the acid slowly with constant a flask and the alcoholic beverage sample is suspended
in a small container above it (see diagram).The water 2− 2− −
swirling. This The method flask uses will a redox get titration quite to hot. find the
2 S O + I2 → S O + 2 I
and ethanol slowly evaporate and as the ethanol 2 3 comes 4 6
in contact with the Because dichromate alcoholic it first dissolves, beverages and is such as wine or beer
then oxidised. More ethanol evaporates until eventually
Introduction
all the ethanol from contain the beverage other has left oxidisable the sample substances that could interfere
and reacted with with the dichromate. the titration, Since this transfer the dichromate solution is placed in
takes time, it is necessary to leave the flask with the
This method uses a redox titration to find the suspended sample a in flask a warm and place the overnight. alcoholic beverage sample is suspended
concentration of ethanol in an aqueous solution. The in a small container above it (see diagram). The water
ethanol is oxidised to ethanoic acid by reacting Equipment it with Needed and ethanol slowly evaporate and as the ethanol comes
an excess of potassium dichromate in acid. 250 mL conical flasks in contact with rubber with stoppers the dichromate it first dissolves, and is
burette then oxidised. More ethanol evaporates until eventually
5 mL beakers or small glass vials all the ethanol from the beverage has left the sample and
beer or wine sample
reacted with the dichromate. Since this transfer takes
10 mL and 1 mL pipettes
time, it is necessary to leave the flask with the suspended
incubator (optional)
sample in a warm place overnight.
Solutions Needed
1
concentration of ethanol in an aqueous solution. The
ethanol is oxidised to ethanoic acid by reacting it with
an excess of potassium dichromate in acid.
2− + 2 Cr O + 16 H + 3 C2H OH →
2 7
5
4 Cr3+ + 11 H O + 3 CH COOH
2 3
The amount of unreacted dichromate is then
determined by adding potassium iodide solution which
is also oxidised by the potassium dichromate forming
iodine.
2− + − 3+ Cr O + 14 H + 6 I → 2 Cr + 3 I2 + 7 H O
2 7 2− + 2
The 2 iodine Cris Othen titrated + 16 with H a + standard 3 C2Hsolution OH of →
2 7
5
sodium thiosulfate and the titration results are used to
4 Cr calculate the ethanol content of the original solution.
2− 2− −
2 S O + I2 → S O + 2 I 2 3
4 6
Because alcoholic beverages such as wine or beer
contain other oxidisable substances that could interfere
with the titration, the dichromate solution is placed in
3+ + 11 H O + 3 CH COOH
2 3
250 mL Conical flask
(wide mouth)
Sample holder
Acid dichromate
solution
Rubber stopper
Glass hook
Equipment Needed
Acid dichromate solution: (0.01 molL-1 in 5.0 molL-1 sulfuric acid) (see safety notes). Add 125 mL of water
to a 500 mL conical flask. Carefully add 70 mL of
concentrated sulfuric acid with constant swirling. Cool
flask under cold water tap and add 0.75 g of potassium
dichromate. Dilute to 250 mL with distilled water.
Starch indicator solution: (1.0% solution) Dissolve 1.0 g
of soluble starch in 100 mL of recently boiled water. Stir
until dissolved.
Sodium thiosulfate solution: (0.03molL -1 ). Add 7.44 g of
Na S O .5H O to a 1L volumetric flask, dissolve in distilled
2 2 3 2
water and dilute up to the mark.
Potassium iodide solution: (1.2molL -1 burette
beer or wine sample
10 mL and 1 mL pipettes
incubator (optional)
) Dissolve 5 g of KI
in 25 mL of water.
250 mL conical flasks with rubber stoppers
5 mL beakers or small glass vials

Solutions Method Needed
Method
Sample Method Preparation
Sample Method Preparation
Acid dichromate solution: (0.01 molL
1. Dilute beer samples 1:20 (10 mL in 200 mL) with
distilled 1. Sample Dilute water. Preparation beer samples 1:20 (10 mL in 200 mL) with
distilled Sample 1. Dilute water. Preparation beer samples 1:20 (10 mL in 200 mL) with
2. Dilute wine samples 1:50 (20 mL in 1000 mL) with
distilled 2. 1. distilled Dilute water.
water. wine beer samples 1:20 1:50 (10 (20 mL mL in in 200 1000 mL) mL) with with
distilled 2. Dilute water. wine samples 1:50 (20 mL in 1000 mL) with
Titration 2. distilled Dilute water. (described wine samples for one 1:50 (20 beverage) mL in 1000 mL) with
1. Titration distilled Transfer water. (described 10 mL of for the one acid beverage) dichromate solution (see
1. Titration safety Transfer notes) (described to 10 a mL 250 of mL for the conical one acid beverage) dichromate flask with matching solution (see
Titration 1. rubber safety Transfer notes) stopper. (described to 10 a mL 250 of mL for the conical one acid beverage) dichromate flask with matching solution (see
1. rubber safety
2. Transfer notes) stopper. to
Pipette 1 10 a
mL mL 250
of of mL
the the conical
diluted acid dichromate flask with matching
beverage sample solution into (see
the 2. safety rubber
sample Pipette notes) stopper.
holder. 1 to mL a 250 of This the mL can diluted conical be a beverage flask 5mL beaker with sample matching or glass into
vial. the rubber 2. sample Prepare Pipette stopper. holder. three 1 mL of samples This the can diluted of be the a beverage 5mL beverage beaker sample as or the glass into entire
contents vial. 2. the sample Prepare Pipette of holder. the three 1 mL flask of samples This the are can diluted used of be the in a beverage the 5mL beverage titration. beaker sample as or the glass into entire
contents of the flask are used in the titration.
3. the vial. sample Prepare
Suspend holder. three samples
the sample This can of
holder be the a 5mL beverage
over beaker as
the dichromate or the glass entire
solution 3. vial. contents Prepare Suspend of the
and three hold the flask sample in samples are used
place holder with of the in the
the over beverage titration.
rubber the dichromate stopper as the entire (see
figure solution contents 3. Suspend 1). and of the hold the flask sample in place are used holder with in the the over rubber titration. the dichromate stopper (see
figure 1).
4. 3. solution
Store Suspend and hold
the the flask sample in place
overnight holder with the
at 25–30°C over rubber the (an dichromate stopper (see
incubator
is 4. solution figure
ideal). Store 1). and the hold flask in overnight place with at the 25–30°C rubber (an stopper incubator (see
is figure 4. ideal). Store 1). the flask overnight at 25–30°C (an incubator
5. Next morning allow the flask to come to room
temperature, 5. 4. is ideal). Next Store morning the then flask loosen allow overnight the flask stopper at 25–30°C to come carefully (an to incubator room and
remove temperature, is 5. ideal). Next and morning discard then loosen allow the sample the flask stopper holder. to come carefully to room and
remove and discard the sample holder.
6. 5. temperature,
Rinse Next morning then loosen
the walls allow of the the
flask flask stopper
with to come carefully
distilled to water, room and
then 6. temperature, remove Rinse and
add about the discard then walls 100 loosen the
mL of the sample
of the distilled flask stopper holder. with water distilled carefully and 1 water, mL and of
potassium then remove 6. Rinse add and about the iodide discard walls 100 solution. mL the of the of sample distilled flask Swirl holder. with to water mix. distilled and 1 water, mL of
potassium iodide solution. Swirl to mix.
7. 6. then
Prepare Rinse add about the 3 blank walls 100 mL of titrations the of distilled flask by with water
adding distilled and 1
10 mL water, mL of
of acid
dichromate 7. then potassium Prepare add about iodide
solution 3 blank 100 solution. mL titrations to of a conical distilled Swirl by to
flask, adding water mix.
adding and 10 mL 1 100 mL of acid of mL
of dichromate potassium 7. water Prepare and iodide solution 1 3 mL blank solution. of potassium titrations to a conical Swirl by iodide to flask, adding mix. solution adding 10 mL 100 of and acid mL
swirling of 7. dichromate water Prepare to and mix. solution 1 3 mL blank of potassium titrations to a conical by iodide flask, adding solution adding 10 mL 100 of and acid mL
swirling to mix.
8. dichromate of water and
Fill a burette solution 1 mL of potassium
with to sodium a conical iodide
thiosulfate flask, solution adding solution 100 and mL
and 8. of swirling water titrate Fill to a and burette mix.
each 1 mL flask of with potassium with sodium sodium thiosulfate iodide thiosulfate. solution solution When and
the and swirling 8. brown titrate Fill to a burette iodine each mix. flask colour with with sodium fades sodium to thiosulfate yellow thiosulfate. (figure solution When 2), add
1 the 8. and mL brown titrate of Fill starch a burette iodine each solution flask colour with with and sodium fades sodium keep to thiosulfate titrating yellow thiosulfate. (figure until solution the When 2), add blue
colour 1 and the mL brown titrate of disappears starch iodine each solution flask colour (figures with and fades 3-5). sodium keep to Titrate titrating yellow thiosulfate. the (figure until blank the When 2), flasks add blue
first, colour the 1 mL brown of and disappears starch repeat iodine solution until colour (figures concordant and fades 3-5). keep to Titrate titrating yellow results the (figure are until blank obtained the 2), flasks add blue
(titres first, 1 colour mL of and agreeing disappears starch repeat solution to until (figures within concordant and 0.1 3-5). keep mL). Titrate titrating Then results the titrate are until blank obtained each the flasks blue of the
alcohol (titres colour first, and agreeing disappears samples. repeat to until If (figures within the concordant three 0.1 3-5). mL). samples Titrate Then results of the titrate the are blank beverage obtained each flasks of the do
not alcohol first, (titres give and agreeing samples. concordant repeat to until If within the results, concordant three 0.1 mL). further samples Then results samples of titrate the are beverage obtained will each need of the do to
be not (titres alcohol prepared. give agreeing samples. concordant to If within the results, three 0.1 mL). further samples Then samples of titrate the beverage will each need of the do to
be alcohol not prepared. give samples. concordant If the results, three further samples samples of the beverage will need do to
not be prepared. give concordant results, further samples will need to
be prepared.
-1 in 5.0 molL-1 sulfuric acid) (see safety notes). Add 125 mL of water
to a 500 mL conical flask. Carefully add 70 mL of
concentrated sulfuric acid with constant swirling. Cool
flask under cold water tap and add 0.75 g of potassium
dichromate. Dilute to 250 mL with distilled water.
Starch indicator solution: (1.0% solution) Dissolve 1.0 g
of soluble starch in 100 mL of recently boiled water. Stir
until dissolved.
Sodium thiosulfate solution: (0.03molL-1 ). Add 7.44g
of Na S O .5H O to a 1L volumetric flask, dissolve in
2 2 3 2
distilled water and dilute up to the mark.
Potassium iodide solution: (1.2molL-1 ) Dissolve 5 g of KI
in 25 mL of water.
Method
Sample Preparation
1. Dilute beer samples 1:20 (10 mL in 200 mL) with
distilled water.
2. Dilute wine samples 1:50 (20 mL in 1000 mL) with
distilled water.
Titration (described for one beverage)
1. Transfer 10 mL of the acid dichromate solution (see
safety notes) to a 250 mL conical flask with matching
rubber stopper.
2. Pipette 1mL of the diluted beverage sample into the
sample holder. This can be a 5 mL beaker or glass
vial. Prepare three samples of the beverage as the
entire contents of the flask are used in the titration.
3. Suspend the sample holder over the dichromate
solution and hold in place with the rubber stopper
(see figure 1).
4. Store the flask overnight at 25–30°C
(an incubator is ideal).
5. Next morning allow the flask to come to room
temperature, then loosen the stopper carefully and
remove and discard the sample holder.
6. Rinse the walls of the flask with distilled water, then
add about 100 mL of distilled water and
1 mL of potassium iodide solution. Swirl to mix.
7. Prepare 3 blank titrations by adding 10 mL of acid
dichromate solution to a conical flask, adding 100
mL of water and 1 mL of potassium iodide solution
and swirling to mix.
8 Fill a burette with sodium thiosulfate solution and
titrate each flask with sodium thiosulfate. When the
2
2
2
Figure Figure 1 Experimental 1 Experimental setup setup for
oxidation Figure for oxidation 1 Experimental of ethanol. of ethanol. Conical setup for flask
contains oxidation Figure Conical 1 Experimental
yellow of flask ethanol. acid contains dichromate Conical setup yellow for flask
solution contains and yellow is sealed acid dichromate with rubber
stopper. solution
Figure
oxidation acid 1 dichromate Experimental
of ethanol. solution Conical
Small and is beaker sealed
setup containing with rubber
for
flask and
beverage stopper.
oxidation
contains is sealed yellow
Small sample of with ethanol.
acid rubber dichromate
beaker is suspended Conical stopper.
containing
flask above
from beverage
contains
solution Small and
hook
beaker yellow
is sealed
sample in rubber acid containing
is suspended stopper. dichromate
with rubber
above
from
solution
stopper. beverage Small
hook
and
in
sample is
beaker
rubber
sealed
stopper.
is containing
with suspended rubber
stopper.
beverage
above Small from
sample
beaker hook
is suspended
in containing rubber
above
beverage
from hook
sample
in rubber
is suspended
stopper.
stopper.
above
Figure
from hook
2 Titration
in rubber
of the
stopper.
iodine
formed. Figure Figure 2 The Titration 2 Titration left flask of the shows of iodine the the
brown-coloured formed. Figure
iodine
2 The Titration
formed. left flask solution
of
The
the shows left
iodine
resulting flask the
from brown-coloured formed. shows the formation solution of iodine. resulting The
right from
Figure flask the
2
The the
Titration
left brown-coloured
flask
formation shows how of
of
the
shows
the iodine.
iodine
the
brown The
colour right
formed.
brown-coloured solution resulting
flask fades The
shows
left to pale flask
solution from
how yellow shows
resulting the
the brown as the the
iodine colour
brown-coloured
from formation the formation of iodine.
is fades titrated to pale
solution
of iodine. The
with yellow thiosulfate resulting
The right
as the
(this iodine
from
right flask flask
is the
shows
is the titrated
formation
shows how stage at with which of
the thiosulfate
iodine.
brown
starch The
solution (this
right
colour
is
flask
fades fades
the should shows
to pale to
stage be at
how pale yellow
added). which
the yellow
starch
brown
as the
solution
colour
iodine as the is
fades
titrated iodine
should
to pale
with is titrated
be added).
yellow
thiosulfate
as with the
iodine
(this thiosulfate is
is
the
titrated
stage (this at
with
which is thiosulfate the starch stage at
Figure 3 Upon addition of starch the
(this
solution which is the
should starch stage
be
at solution added).
which starch should
solution Figure 3 Upon takes addition on a blue-black of starch colour
solution should be added).
the
be added).
due solution Figure
to
3
the
Upon takes formation
addition on a blue-black of
of
a starch-
starch colour the
iodine due to complex. the formation of a starchiodine
Figure
solution
Figure
complex.
3 Upon takes 3 Upon addition on a addition blue-black
of starch of colour
the
solution
due
solution starch
to the
takes takes the
formation
solution on on a blue-black blue-black
of a
takes
starch-
colour colour on a
due
iodine
due to complex.
blue-black the formation colour of of due a starch- to the
iodine formation complex.
of a starch-iodine
complex.
Figure 4 As more thiosulfate is
added Figure and 4 As we more near thiosulfate the titration is
endpoint, added Figure and 4 As
the we more
blue-black near thiosulfate the titration colour
is
from
the endpoint, added
starch-iodine the blue-black complex colour fades. from
the
Figure Figure and
starch-iodine
4 As 4 we
more more As near more thiosulfate thiosulfate
the thiosulfate titration
complex fades.
is
endpoint,
added is added and
the
we and blue-black
near near we the near titration titration
colour the from
the
endpoint, titration starch-iodine
the endpoint, blue-black
complex the colour
fades. blue- from
the black starch-iodine starch-iodine colour from complex the fades. starchiodine
complex fades.
Figure 5 The endpoint of the
titration Figure 5 The is reached endpoint when of the just
enough titration Figure 5
thiosulfate
The is reached endpoint when is added
of the just to react
with enough all the thiosulfate iodine present is added and to the react
solution with
Figure
titration
all
5 5
the
The
is reached
becomes iodine
endpoint
when
colourless. present
of of the
just
Figure 5 The endpoint and of the
solution
titration
enough thiosulfate
is
becomes
reached
colourless.
when
is added
just
to react
titration is reached when just
enough
with all the
thiosulfate
iodine present
is added added
and
to
the
react
with
solution
enough
all the
becomes
thiosulfate
iodine
colourless.
is added
to react with all present the iodine and the
solution becomes colourless.
present and the solution
becomes colourless.

own iodine colour fades to yellow (figure 2), add 1mL
of starch solution and keep titrating until the blue
colour disappears
(figures 3–5). Titrate the blank flasks first, and repeat
until concordant results are obtained (titres agreeing
to within 0.1 mL). Then titrate each of the alcohol
samples. If the three samples of the beverage do not
give concordant results, further samples will need to be
prepared.
Result Calculations
The blank titration tells you how much acid dichromate
was present at the start. As no alcohol was added the
full amount of the dichromate is still present. The
blank titrations are carried out so the result can be
compared with those of the sample titrations.
1. Determine the average volume of sodium
thiosulfate used for your sample from your
concordant sample results.
2. Determine the average volume of sodium
thiosulfate used for the blank titration from your
concordant blank results.
3. Subtract the volume of the sodium thiosulfate
solution used for the sample titration from the
volume used for the blank titration. This volume
of the sodium thiosulfate solution is now used to
determine the alcohol concentration.
4. Calculate the number of moles of sodium
thiosulfate in this volume.
5. Using the equations, determine the relationship
between the moles of sodium thiosulfate and the
moles of ethanol.
2- 2-
– as 6 mol of S O is equivalent to 1 mol of Cr2O 2 3
7
2- – and 2 mol of Cr O is equivalent to 3 mol of
2 7
C H OH 2 5
2- – then 1 mol of S O is equivalent to 0.25 mol of
2 3
C H OH 2 5
6. Use this ratio to calculate the moles of alcohol in the
sample solution.
7. Remember to allow for the dilution factor
eg. if the dilution was 1:20 the result needs to be
multiplied by 20.
8. Convert the answer in moles per litre to percentage
(grams per 100mL) to compare with the figure given
on the bottle of the alcoholic beverage tested.
Contact Us
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experiment, please contact us. Please note that this
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