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
iodine.
Lab coats, safety glasses and enclosed footwear must
be worn at all times in the laboratory.
Determination of Ethanol Concentration
Cr2O72−in+ 14 H+ + 6 I− → 2 Cr3+ + 3 I2 + 7 H2O
The acid dichromate solution needs to be prepared
Aqueous
Solutions
with care.
Any concentrated
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.
a flask and the alcoholic beverage sample is suspended
adding the
acid, and add the acid slowly with constant
Introduction
2−
in a small container above it (see diagram).The
2 S2O3water
+ I → S4O62− + 2 I−
swirling.This
The
flaskuses
will
gettitration
quitetohot.
method
a redox
find the
and ethanol slowly evaporate and as the ethanol
comes2
concentration of ethanol in an aqueous solution. The
in contact with the
dichromatealcoholic
it first dissolves,
and is
Because
beverages
such as wine or beer
ethanol is oxidised to ethanoic acid by reacting it with
then oxidised. More ethanol evaporates until eventually
an excess of potassium dichromate in acid.
contain
other
oxidisable
all the ethanol from
the beverage
has left
the sample substances that could interfere
Introduction 2− +
and reacted with with
the dichromate.
Since this transfer
2 Cr2O7 + 16 H + 3 C2H5OH →
the titration,
the dichromate solution is placed in
time, it is necessary to leave the flask with the
+ 11 H2Otitration
+ 3 CH3COOH
4 Cr
This method uses
a3+redox
to find the takes
a inflask
and
alcoholic beverage sample is suspended
suspended sample
a warm
placethe
overnight.
The amount
unreacted dichromate
is then solution. The
concentration
ofofethanol
in an aqueous
in
a
small
container
above it (see diagram). The water
determined by adding potassium iodide solution which
Equipment
Needed
ethanol isisalso
oxidised
to
ethanoic
acid
by
reacting
it
with
and ethanol slowly evaporate and as the ethanol comes
oxidised by the potassium dichromate forming
iodine.
250 mL conical flasks
with rubberwith
stoppers
an excess
of potassium dichromate in acid.
in contact
the dichromate it first dissolves, and is
burette
Cr2O72− + 14 H+ + 6 I− → 2 Cr3+ + 3 I2 + 7 H2O
then oxidised. More ethanol evaporates until eventually
2−
Cr2is O
+ 16 H+ + 3 C2Hsolution
OHof→ 5 mL beakers or small
glass vials
The 2
iodine
then
7 titrated with a standard
5
all the
ethanol from the beverage has left the sample and
sodium thiosulfate
3+ and the titration results are used to
beer
or
wine
sample
4
Cr
+
11
H
O
+
3
CH
COOH
reacted with the dichromate. Since this transfer takes
calculate the ethanol content
2 of the original
3 solution.
10 mL and 1 mL pipettes
−
2−
2−
time, it is necessary to leave the flask with the suspended
2 S2O3 + I2 → S4O6 + 2 I
incubator (optional)
sample in a warm place overnight.
Because alcoholic beverages such as wine or beer
contain other oxidisable substances that could interfere
with the titration, the dichromate solution is placed in
Rubber stopper
250 mL Conical flask
(wide mouth)
Solutions Needed
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
250 mL conical flasks with rubber stoppers
concentrated sulfuric acid with constant swirling. Cool
flask under cold water tap and add 0.75 g of potassium
burette
dichromate. Dilute to 250 mL with distilled water.
5 mL beakers
or small
Starch indicator solution:
(1.0% solution)
Dissolve glass
1.0 g vials
of soluble starch in 100 mL of recently boiled water. Stir
until dissolved. beer or wine sample
Glass hook
-1
). Add 7.44 g of
Sodium thiosulfate
(0.03molL
10solution:
mL and
1 mL pipettes
Na2S2O3.5H2O to a 1L volumetric flask, dissolve in distilled
water and dilute up
to the mark. (optional)
incubator
Potassium iodide solution: (1.2molL-1) Dissolve 5 g of KI
in 25 mL of water.
Sample holder
Acid dichromate
solution
1
Method Needed
Solutions
Method
Acid
dichromate
solution: (0.01 molL-1 in 5.0 molL-1
Sample
Preparation
Method
sulfuric
acid)
(see samples
safety notes).
AddmL125inmL
water
Sample
Preparation
1.Method
Dilute
beer
1:20 (10
200ofmL)
with
toSample
a 500
mL
conical
flask. Carefully
add
ofwith
Preparation
1.distilled
Dilute
beer samples
1:20 (10 mL
in 70
200mL
mL)
water.
concentrated
sulfuric
acid 1:20
with(10
constant
swirling.
Cool
Preparation
distilled
water.
1.Sample
Dilute
beer
samples
mL in 200
mL) with
2. under
Dilutecold
winewater
samples
1:50 add
(20 mL
ing1000
mL) with
flask
tap
and
0.75
of
potassium
distilled
water.
1.2.
Dilute
beer samples
1:20
(10
inin200
mL)
Dilute
wine
samples
1:50with
(20mL
mL
1000
mL)with
with
distilled
water.
dichromate.
Dilute
to 250 mL
distilled
water.
distilled
water.
distilled
water.
2.
Dilute
wine samples 1:50 (20 mL in 1000 mL) with
Starch
indicator
solution:
solution) Dissolve 1.0 g
Titration
(described
for (1.0%
one
beverage)
distilled
water.
2.
Dilute
wine
samples
1:50
(20
mL in
1000 water.
mL) with
ofTitration
soluble starch
in 100for
mLone
of recently
boiled
Stir
(described
beverage)
1. Transfer
distilled
water.10 mL of the acid dichromate solution (see
until
dissolved.
Titration
(described
for
one
1.safetyTransfer
theconical
acidbeverage)
dichromate
solution (see
notes)
to10amL
250ofmL
flask with matching
-1
Sodium
thiosulfate
solution:
(0.03molL
).
Add
7.44g(see
Titration
(described
for
one
beverage)
safety
notes)
to
a
250
mL
conical
flask
with
matching
1.rubber
Transfer
10 mL of the acid dichromate solution
stopper.
ofsafety
Na
S
O
.5H
O
to
a
1L
volumetric
flask,
dissolve
in (see
stopper.
amL
250ofmL
conical
flask with matching
2 Transfer
2notes)
3
2to
1.rubber
10
the
acid dichromate
solution
2.
Pipette
1
mL
of
the
diluted
beverage
sample
into
distilled
watertoand
dilute
up
to the
mark.
rubber
stopper.
safety
notes)
a 250
mLcan
conical
flask
withsample
matching
2. sample
Pipette
1 mL
ofThis
the
diluted
into
the
holder.
be abeverage
5mL
beaker
or
glass
-1
Potassium
iodide
solution:
(1.2molL
)
Dissolve
5
g
of KI
rubber
stopper.
the
sample
holder.
This
can
be
a
5mL
beaker
or
glass
2.
Pipette
1
mL
of
the
diluted
beverage
sample
vial. Prepare three samples of the beverage as theinto
entire
inthe
25 sample
mL
ofofwater.
vial.
Prepare
three
samples
of
beverage
asorthe
entire
holder.
This
bethe
5mL
beaker
glass
contents
the
arecan
used
inabeverage
the
titration.
2.
Pipette
1 mLflask
of
the
diluted
sample
into
contents
of
the
flask
are
used
in
the
titration.
vial.
Prepare
three
samples
of
the
beverage
as
the
entire
the
holder.
This can
be a 5mL
or glass
3. sample
Suspend
the sample
holder
overbeaker
the dichromate
Method
contents
of thethe
flask
are used
in the
titration.
vial.
samples
of the
beverage
as the entire
3. Prepare
Suspend
over
the dichromate
solution
andthree
hold sample
in
placeholder
with
the
rubber
stopper
(see
contents
of
the
flask
are
used
in
the
titration.
solution
and hold
in placeholder
with the
rubber
stopper (see
3.
the sample
over
the dichromate
figureSuspend
1).Preparation
Sample
figure
1).
solution
and
hold
in
place
with
the
rubber
stopper (see
3. Suspend
the
sample
holder
over the (an
dichromate
Store
thesamples
flask
overnight
at 25–30°C
incubator
1.4.
Dilute
beer
1:20
(10
mL
in
200
mL)
with
figure
1). and
solution
hold
inovernight
place withatthe
rubber(an
stopper
(see
4.distilled
Store
the
flask
25–30°C
incubator
is
ideal).
water.
figure
1). the flask overnight at 25–30°C (an incubator
is ideal).
4.
Store
Next
morning
allow
to 1000
comemL)
to room
2.5.Dilute
wine
samples
1:50the
(20flask
mL in
with
is
ideal).
4.
Store
the
flask
overnight
at
25–30°C
(an
incubator
5.
Next water.
morning
allow the
to come
to room
temperature,
then loosen
theflask
stopper
carefully
and
distilled
is
ideal).
temperature,
then loosen
theflask
stopper
carefully
and
5.
Next
morning
allow
the
to come
to room
remove
and
discard
the sample
holder.
Titration
(described
for
one
beverage)
remove
and
discard
the sample
holder.
temperature,
then loosen
theflask
stopper
carefully
and
5.
Next
morning
to come
to water,
room
6. Rinse
the wallsallow
of thethe
flask with
distilled
1.remove
Transfer
10
mL
of
the
acid
dichromate
solution
(see
and
discard
the
sample
holder.
temperature,
stopper
carefully
and
6.
thethen
walls
of the
flask
with
distilled
then Rinse
add about
100loosen
mL
of the
distilled
water
and 1water,
mL
of
safety
notes)
to
a
250
mL
conical
flask
with
matching
remove
discard
the
sample
then Rinse
addand
about
100
mL
of
distilled
and 1water,
mL of
6.
the
walls
of the
flask
with
distilled
potassium
iodide
solution.
Swirlholder.
towater
mix.
rubber stopper.
potassium
iodide
solution.
Swirlwith
towater
mix.
then
add about
100
mL
of distilled
and 1water,
mL of
6.
Rinse
the
walls
of
the
flask
distilled
7.
Prepare
3ofblank
titrations
by adding
10 mL
of acid
2.potassium
Pipette
1mL
the
diluted
beverage
sample
into
the
iodide
solution.
Swirl
to
mix.
then
add
about
100
mL
of
distilled
water
and
1
mL
of
7.
Prepare
3
blank
titrations
by
adding
10
mL
of acid
dichromate
solution
to
a
conical
flask,
adding
100
sample holder.
This
can beSwirl
a 5 mL
beaker or glassmL
potassium
iodide
solution.
to
mix.
dichromate
solution
to a conical
flask,
adding
100
mL
7.
Prepare
3
blank
titrations
by
adding
10 mL
acid
of
water
and
1
mL
of
potassium
iodide
solution
and
vial. Prepare three samples of the beverage
asof
the
of
water
and
1
mL
of
potassium
iodide
solution
and
dichromate
solution
to aflask
conical
adding
100
mL
swirling
to mix.
7.
Prepare
3 blank
titrations
byflask,
adding
10 mL
of acid
entire
contents
of the
are
used
in
the
titration.
swirling
mix.
of
water to
and
1 mL of potassium
iodide
solution
andmL
dichromate
solution
to
a
conical
flask,
adding
100
Fill a burette
withholder
sodiumover
thiosulfate
solution
3.8.
Suspend
the
sample
the dichromate
swirling
mix.
of
water
1 mLflask
of
potassium
iodide
solution
and
8.solution
Fill to
aand
burette
with
sodium
thiosulfate
solution
and
titrate
each
with
sodium
thiosulfate.
When
and hold in place with the rubber
stopper
swirling
to
mix.
and
titrate
each
sodium
thiosulfate.
When
8.
Fill
a burette
withwith
sodium
solution
the
brown
iodine
colour
fades
tothiosulfate
yellow
(figure
2), add
(see
figure
1). flask
the
iodine
colour
fades
tothiosulfate
yellow
(figure
2), add
and
titrate
each
flask
sodium
thiosulfate.
When
1 mLbrown
of
solution
and
keep
titrating
until
the
blue
8.
Fillstarch
a burette
withwith
sodium
solution
4.Store
the
flask
overnight
at
25–30°C
1and
mL
of
starch
solution
and
keep
titrating
until
the
blue
the
brown
iodine
colour
fades
to
yellow
(figure
2),
add
colour
disappears
(figures
Titrate
the blankWhen
flasks
titrate
each flask
with 3-5).
sodium
thiosulfate.
(an
incubator
is
ideal).
colour
disappears
(figures
3-5).
Titrate
the(figure
blank
flasks
1the
mLbrown
of
starch
solution
and
keep
titrating
until
the
blue
first,
and
repeat
until
concordant
results
are
obtained
iodine
colour
fades
to
yellow
2),
add
5.colour
Next
morning
allow
the
flask
to
come
to
room
first,
and
repeat
until
concordant
results
are
obtained
disappears
(figures
3-5).
Titrate
the
blank
flasks
to within
0.1keep
mL).titrating
Then titrate
of the
1(titres
mL ofagreeing
starch solution
and
untileach
the blue
temperature,
loosen
the
stopper
carefully
and
(titres
agreeing
to
within
0.1
mL).
Then
titrate
each
of the
first,
and
repeatthen
until
concordant
results
are
obtained
alcohol
samples.
If(figures
the
three
samples
of
the
beverage
do
colour
disappears
3-5).
Titrate
the
blank
flasks
remove
and
discard
the
sample
holder.
alcohol
samples.
If
the
three
samples
of
the
beverage
(titres
agreeing
to
within
0.1
mL).
Then
titrate
each
of
the
not give
further
samples
will needdo
to
first,
andconcordant
repeat untilresults,
concordant
results
are obtained
not
give
concordant
results,
further
samples
will
need
alcohol
samples.
If
the
three
samples
of
the
beverage
do
be
prepared.
(titres
agreeing
to
within
0.1
mL).
Then
titrate
each
of
the
6.Rinse the walls of the flask with distilled water, thento
be
prepared.
not
give
concordant
results,
samples
will needdo
to
alcohol
samples.
If the
three further
samples
of
the beverage
add
about
100 mL
of
distilled
water
and
be
prepared.
not
give
results,solution.
further samples
1 mL
of concordant
potassium iodide
Swirl to will
mix.need to
prepared.
7.bePrepare
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
1 Experimental
setupsetup
for
Figure
1 Experimental
oxidation
of ethanol.
Conical
flask
Figure
1 Experimental
setup for
for oxidation
of ethanol.
contains
yellow
acid
dichromate
oxidation
of
ethanol.
Conical
flask
Conical
flask contains
Figure
1 Experimental
setupyellow
for
solution
and
is
sealed
with
rubber
contains
yellow
acid
dichromate
oxidation
of ethanol. solution
Conical
flask
acid1dichromate
and
Figure
Experimental
setup
for
stopper.
Small
containing
solution
and
isbeaker
sealed
with rubber
contains
yellow
acid
dichromate
is
sealed
with
rubber
stopper.
oxidation
of ethanol.
Conical
flask
beverage
sample
is suspended
above
stopper. Small
containing
solution
and
isbeaker
sealed
with rubber
Small
beaker
containing
contains
yellow
acid
dichromate
from
hook
in
rubber
stopper.
beverage
sample
is
suspended
above
stopper.
Small
beaker
containing
beverage
sample
is
suspended
solution
and
isrubber
sealedstopper.
with
rubber
from
hook
in
beverage
sample
is suspended
stopper.
beaker
containing
aboveSmall
from
hook
in
rubberabove
from
hook
in
rubber
stopper.
beverage
sample is suspended above
stopper.
from
hook
in rubber
stopper.
Figure 2 Titration
of the
iodine
Figure
2 Titration
ofiodine
thethe
formed.
left flask
shows
Figure
2 The
Titration
of the
brown-coloured
solution
resulting
iodine
formed.
The
left
flask
formed.
The
left
flask
shows
the
Figure 2 Titration of the iodine
from
the
formation
of
iodine.
The
shows
the
brown-coloured
brown-coloured
solution
resulting
formed.
left flask
shows
the
Figure
2 The
Titration
of of
the
iodine
right
flask
shows
how
the
brown
from
the
formation
iodine.
The
solution
resulting
from
the
brown-coloured
solution
resulting
formed.
The
left
flask
shows
colour
fades
toofpale
yellow
asthe
the
right
flask
shows
how
the
brown
formation
iodine.
The
right
from
the
formation
of
iodine.
The
brown-coloured
solution
resulting
iodine
is
titrated
with
thiosulfate
colour
fades
to pale
yellow
as the
flask
shows
how
the
brown
right
flask
shows
how
the
brown
from
formation
ofthiosulfate
iodine.
The
(this
isthe
the
stage
at
which
starch
iodine
is
titrated
with
colour
fades
to pale
yellow
as the
colour
fades
to
pale
yellow
right
flask
shows
how
the
brown
solution
should
added).
(this
the
stagebe
at
starch
iodine
is
titrated
with
thiosulfate
as isthe
iodine
is which
titrated
colour
fades
to pale
yellow
aswith
the
solution
should
be
added).
(this
is
the
stage
at
which
starch
thiosulfate
(this
the stage at
iodine
is titrated
withisthiosulfate
Figure
3 Upon
addition
of starch the
solution
should
added).
which
starch
solution
should
(this
is the
stagebe
at
which starch
solution
takes
on
a
blue-black
colour
Figure
3 Upon
of starch
the
solution
shouldaddition
be added).
be added).
due
to
the
formation
of
a
starchsolution
takes addition
on a blue-black
colour
Figure
3 Upon
of starch
the
iodine
due
to complex.
the
formation
of a starchsolution
takes
on
a
blue-black
colour
Figure
3 Upon
addition
of the
Figure
3 Upon
addition
of starch
iodine
due
to complex.
the
formation
of atakes
starchsolution
takes
on a blue-black
colour
starch
the
solution
on a
iodine
due
to complex.
the formation
a starchblue-black
colourofdue
to the
iodine
complex.
formation
of a starch-iodine
complex.
Figure 4 As more thiosulfate is
added
near
the titration
Figure and
4 Aswe
more
thiosulfate
is
endpoint,
the
blue-black
colour
added and
near
the titration
Figure
4 Aswe
more
thiosulfate
is from
the
starch-iodine
complex
fades.
endpoint,
the
blue-black
colour
from
added
and
thethiosulfate
titration
Figure
Asnear
more
Figure
4 As4we
more
thiosulfate
is
the
starch-iodine
complex
fades.
endpoint,
blue-black
colour
is added
and
wethe
near
the from
added
andthe
we
near
titration
the
starch-iodine
complex
fades.
titrationtheendpoint,
blueendpoint,
blue-blackthe
colour
from
theblack
starch-iodine
complex
colour from
the fades.
starch-
iodine complex fades.
Figure 5 The endpoint of the
titration
is reached
when
just
Figure 5 The
endpoint
of the
enough
thiosulfate
is
added
titration
is
reached
when
justto react
Figure 5 The endpoint of the
with
all
the
iodine
present
and
enough
thiosulfate
is
added
to the
react
titration
is reached
when
just
Figure
5the
The
endpoint
of the
Figure
5 iodine
The
endpoint
of the
solution
becomes
colourless.
with
all
present
and
the
enough
thiosulfate
is added
react
titration
is
reached
when
justto just
titration
is reached
when
solution
becomes
colourless.
with
all
the
iodine
present
and
the
enough
thiosulfate
is added
to react
enough
thiosulfate
is added
solution
becomes
colourless.
with
all
the
iodine
present
and
to react with all the iodinethe
solution becomes colourless.
present and the solution
becomes colourless.
brown 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.
– as 6 mol of S2O32- is equivalent to 1 mol of Cr2O72 – and 2 mol of Cr2O72- is equivalent to 3 mol of
C2H5OH
– then 1 mol of S2O32- is equivalent to 0.25 mol of
C2H5OH
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
If you have any questions or comments relating to this
experiment, please contact us. Please note that this
service is for senior school chemistry students in New
Zealand only. We regret we are unable to respond to
queries from overseas.
Outreach
College of Science
University of Canterbury
Private Bag 4800
Christchurch
New Zealand
Phone: +64 3 364 2178
Fax: +64 3 364 2490
Email: [email protected]
www.outreach.canterbury.ac.nz