College of Science
Determination of Vitamin C Concentration by Titration
(Redox Titration Using Iodine Solution)
Safety
Lab coats, safety glasses and enclosed footwear must
be worn at all times in the laboratory.
Introduction
Solutions Needed
Iodine solution: (0.005 mol L−1). Weigh 2 g of potassium
iodide into a 100 mL beaker. Weigh 1.3 g of iodine and
add it into the same beaker. Add a few mL of distilled
water and swirl for a few minutes until iodine is
dissolved. Transfer iodine solution to a 1 L volumetric
flask, making sure to rinse all traces of solution into the
volumetric flask using distilled water. Make the solution
up to the 1 L mark with distilled water.
This method determines the vitamin C concentration
in a solution by a redox titration using iodine. Vitamin
C, more properly called ascorbic acid, is an essential
antioxidant needed by the human body (see additional
Starch indicator solution: (0.5%). Weigh 0.25 g of
notes). As the iodine is added during the titration, the
soluble starch and add it to 50 mL of near boiling water
ascorbic acid is oxidised to dehydroascorbic acid, while
(Redox
Titration
the iodine is reduced to
iodide
ions. Using Iodine Solution)in a 100 mL conical flask. Stir to dissolve and cool before
using.
−
ascorbic acid + I2 →
2
I
+
dehydroascorbic
acid
Introduction
Thisiodine
method determines
vitamin C concentration
Due to this reaction, the
formed istheimmediately
in a solution by a redox titration using iodine. Vitamin
reduced to iodide as long
as there is any ascorbic acid
C, more properly called ascorbic acid, is an essential
Burette containing
present. Once all the ascorbic
has
been
oxidised,
iodine solution
antioxidantacid
needed
by the
human
body (see additional
notes).
As the iodine
added
during the titration, the
the excess iodine is free
to react
with isthe
starch
acid is oxidised to dehydroascorbic acid, while
indicator, forming theascorbic
blue-black
starch-iodine complex.
the iodine is reduced to iodide ions.
This is the endpoint of the titration.
−
Determination of Vitamin C Concentration
by Titration
ascorbic acid + I2 → 2 I + dehydroascorbic acid
The method is suitableDue
fortouse
with vitamin
tablets,
this reaction,
the iodineCformed
is immediately
fresh or packaged fruitreduced
juicestoand
fruits
and
iodidesolid
as long
as there
is any ascorbic acid
present. Once all the ascorbic acid has been oxidised,
vegetables.
the excess iodine is free to react with the starch
indicator,
forming
the blue-black
complex.
NB: This method is more
straight
forward
thanstarch-iodine
the
This
is
the
endpoint
of
the
titration.
alternative method using potassium iodate, but as
method is
is suitable
use with
vitamin
the potassium iodate The
solution
more for
stable
than
theC tablets,
fresh or packaged fruit juices and solid fruits and
iodine as a primary standard,
vegetables.the alternative method is
more reliable.
NB: This method is more straight forward than the
alternative method using potassium iodate, but as
the potassium iodate solution is more stable than the
iodine as a primary standard, the alternative method is
more reliable.
Equipment Needed
burette and stand
Equipment
100 mL or 200 mL volumetric
flaskNeeded
20 mL pipette
burette and stand
100 mL or 200 mL volumetric flask
10 mL and 100 mL measuring cylinders
20 mL pipette
250 mL conical flasks 10 mL and 100 mL measuring cylinders
250 mL conical flasks
Solutions Needed
Iodine solution: (0.005 mol L−1). Weigh 2 g of potassium
Conical flask
Vitamin C sample solution
Method
4.Calculate the concentration in mol L−1 of ascorbic
acid in the solution obtained from fruit/vegetable/
Sample
3.juice.Using
equation
the titration (below)
StarchPreparation
indicator solution: (0.5%). Weigh 0.25 g of
Also, the
calculate
the of
concentration,
in mg/100mL
determine
the
number
of
moles
of
ascorbic
soluble
starch
and
add
it
to
50
mL
of
near
boiling
water
or mg/100g of ascorbic acid, in the sampleacid
of fruit/
For vitamin C tablets: Dissolve a single tablet in 200 mL
reacting.
in
a
100
mL
conical
flask.
Stir
to
dissolve
and
cool
before
vegetable/juice.
of distilled water (in a volumetric flask if possible).
using.
ascorbic acid + I2 → 2 I− + dehydroascorbic acid
For fresh fruit juice: Strain the juice through
Notes
cheesecloth to remove seeds and pulp which may block Additional
4. Calculate the concentration in mol L−1 of ascorbic
Method
pipettes.
1. acid
Iodine
stains
both skin
and clothing
so proper care is
in the
solution
obtained
from fruit/vegetable/juice.
advised.
If staining
does occur, alcohol
may remove
ForSample
packaged
fruit juice: This may also need to be
Also,
calculate
the concentration,
in mg/100mL
or mg/
Preparation
skin
stains
and
cleaners
are
available
for
fabric
stains.
strained through cheesecloth if it contains a lot of pulp
100g of ascorbic acid in the sample of fruit/vegetable/
For
vitamin
C
tablets:
Dissolve
a
single
tablet
in
200
mL
or seeds.
2.juice.
Vitamin C, or ascorbic acid, is a water soluble
of distilled water (in a volumetric flask if possible).
antioxidant that plays a vital role in protecting the
For fruits and vegetables: Cut a 100 g sample into small
For fresh
fruit in
juice:
Strainand
the juice
through
body from infection
pieces
and grind
a mortar
pestle.
Add 10cheese
mL
Additional
Notesand disease. It is not synthesised
cloth
to
remove
seeds
and
pulp
which
may
block
by the human body and therefore must be
portions of distilled water several times while grinding
1.acquired
Iodine
stains
bothsources
skin and– clothing
proper
from
dietary
primarilysofruits
andcare
thepipettes.
sample, each time decanting off the liquid extract
is
advised.
If
staining
does
occur,
alcohol
may
remove
vegetables. The chemical structure and antioxidant
intoFor
a 100
mL volumetric
flask.
strain
the
packaged
fruit juice:
ThisFinally,
may also
need
toground
be
skin
stains and
cleaners
are available
fabric stains.
(reducing)
action
of ascorbic
acid arefor
illustrated
in
fruit/vegetable
pulp
through
cheesecloth,
rinsing
the
strained through cheese cloth if it contains a lot of pulp
the redox
halfC,equation
below:
2.
Vitamin
or ascorbic
acid, is a water soluble
pulp
a few 10 mL portions of water and collecting
or with
seeds.
antioxidant
that
plays
a
vital
role in protecting the body
all filtrate and washings in the volumetric flask. Make
For fruits and vegetables: Cut a 100 g sample into small 3. The concentration of the prepared iodine solution
from
infection
and disease.
It is not synthesised
the extracted solution up to 100 mL with distilled water.
can be
more accurately
determined
by titrationby
with
pieces and grind in a mortar and pestle. Add 10 mL
the
human
body
and
therefore
must
be
acquired
from
a
standard
solution
of
ascorbic
acid
or
a
standard
Alternatively
the
100
g
sample
of
fruit
or
vegetable
portions of distilled water several times while grinding
dietary
sources
– primarily
fruits andusing
vegetables.
solution
of potassium
thiosulfate
a starchThe
may
blendedeach
in atime
fooddecanting
processoroff
together
withextract
thebesample,
the liquid
chemical
structure
and
antioxidant
(reducing)
action of
about
mL mL
of distilled
water.
After
blending,
into50
a 100
volumetric
flask.
Finally,
strainstrain
the ground indicator. This should be done if possible as iodine
ascorbic
solutionsacid
canare
beillustrated
unstable. in the redox half equation
thefruit/vegetable
pulp through cheesecloth,
it with
a fewthe
pulp throughwashing
cheese cloth,
rinsing
below:
10mL
portions
of distilled
water, and
makeand
thecollecting
extracted
pulp
with a few
10 mL portions
of water
HO
HO
solution
up to
100washings
mL in a volumetric
flask. flask. Make
all filtrate
and
in the volumetric
oxidation
O
O
O
O
HO
+ 2H+ + 2ethe extracted solution up to 100 mL with distilled water. HO
Titration
Alternatively the 100 g sample of fruit or vegetable may
be blended
a food
processor
togethersolution
with about
1. Pipette
a 20inmL
aliquot
of the sample
into
50
mL
of
distilled
water.
After
blending,
strain
the
a 250 mL conical flask and add about 150 mL of
pulp through
cloth,
washing
it with asolution.
few 10
distilled
watercheese
and 1 mL
of starch
indicator
mL portions of distilled water, and make the extracted
2. Titrate the sample with 0.005 mol L−1 iodine solution.
solution up to 100 mL in a volumetric flask.
The endpoint of the titration is identified as the first
permanent trace of a dark blue-black colour due to
Titration
the
starch-iodine complex.
HO
OH
ascorbic acid
reduction
O
O
dehydroascorbic acid
3. The concentration of the prepared iodine solution
4.The
average
titre volume
should ideally
be in the
can be
more accurately
determined
by titration
with a
range
of
10
–
30
mL.
If
the
titre
required
for
a
20
mL
standard solution of ascorbic acid or a standard solution
aliquot
of sample
solution
is well
outside
this range
of
potassium
thiosulfate
using
a starch
indicator.
This
then
a
larger
or
smaller
aliquot
volume
should
be
should be done if possible as iodine solutions can be
chosen. If the volume of the titre is too low, dilute
unstable.
the standard. If the titre volume is too high, dilute
3. Repeat
the titration
further
aliquots
sampleinto 4.
1. Pipette
a 20 mLwith
aliquot
of the
sampleofsolution
The average titre volume should ideally be in
the sample.
solution
youflask
obtain
results
(titres
a 250 mLuntil
conical
andconcordant
add about 150
mL of
distilled
the range of 10 – 30 mL. If the titre required for a 20
5.mL
Ascorbic
is susceptible
by this
agreeing
mL). indicator solution.
water andwithin
1 mL of0.1starch
aliquotacid
of sample
solutiontoisoxidation
well outside
atmospheric
oxygen
over
time.
For
this
reason,
the
range then a larger or smaller aliquot volume should
2. Titrate the sample with 0.005 mol L−1 iodine
samples
be prepared
immediately
before
be
chosen.should
If the volume
of the
titre is too low,
dilute
Calculations
solution. The endpoint of the titration is identified as
the
titrations.
However,
if
the
samples
have
to bethe
the
standard.
If
the
titre
volume
is
too
high,
dilute
the first permanent
trace
of a dark
blue-black
colour
due prepared several hours earlier, oxidation can be
1. Calculate
the average
volume
of iodine
solution
used
sample.
to
the
starch-iodine
complex.
minimised by the addition of a small amount of
from your concordant titres.
5.
is susceptible
oxidation
by
acid (egacid
1 g oxalic
acid perto
100
mL of sample
3. Repeat
titration
with reacting.
further aliquots of sample oxalicAscorbic
2. Calculate
thethe
moles
of iodine
atmospheric
oxygen
over
time.
For
this
reason,
the
solution).
solution until you obtain concordant results (titres
3. Using
thewithin
equation
of the titration (below) determine
samples should be prepared immediately before the
agreeing
0.1 mL).
6.Identification of the endpoint in this titration is
the number of moles of ascorbic acid reacting.
titrations. However, if the samples have to be prepared
significantly affected by the colouration of the
several hours earlier, oxidation can be minimised by the
−
sample solution used. If the solutions are colourless
ascorbic
acid + I2 → 2 I + dehydroascorbic acid
Calculations
addition of a small amount of oxalic acid (eg 1 g oxalic
or are pale in colour, there is no problem identifying
acid per 100 mL of sample solution).
1. Calculate the average volume of iodine solution
used from your concordant titres.
6. Identification of the endpoint in this titration is
significantly affected by the colouration of the sample
2. Calculate the moles of iodine reacting.
solution used. If the solutions are colourless or are pale
in colour, there is no problem identifying the endpoint.
For strongly coloured juices there can be a problem with
theendpoint
endpoint.
Foritstrongly
juices
there can
the
and
is advisedcoloured
to carry out
a “rough”
be
a
problem
with
the
endpoint
and
it
is
advised
titration in order to become familiar with any distinct
in colour, there
is nochange
problem
identifying
the
endpoint.
to carry
out awhich
“rough”
titration
order to(itbecome
colour
occurs
at theinendpoint
may
For stronglyjust
coloured
a problem
with
be a juices
darkening
ofbethe
colour)
This
will also
help by
familiar
withthere
anycan
distinct
colour
change
which
the endpoint and it is advised to carry out a “rough”
establishing
anendpoint,
approximate
volume
solution
1 Vitamin C tablet. Left photo: before endpoint, added iodine reacts
occurs
at the
(itdistinct
may
justofbeiodate
a darkening
FigureFigure
1 Vitamin
C tablet. Left photo: before endpoint, added iodinetitration in order
to become
familiar with any
with ascorbic acid leaving the solution colourless. Centre photo: At the
required.
of
the
colour)
This
will
also
help
by
establishing
an
reacts with ascorbic acid leaving the solution colourless.
colour change which occurs at the endpoint (it may
titration endpoint all the ascorbic acid has reacted and the excess iodine
injust
colour,
is no problem
identifying
thealso
endpoint.
Centrereacts
photo:
Atthe
thestarch
titration
endpoint
the ascorbic
acid
hasphoto:
darkening
of the
colour)
This will
help
bysolution
with
indicator
to givealla pale
blue colour.
Right
If be athere
approximate
volume
of
iodine
required.
7.
The
above
method
may
bewith
used
to carry
out a
For
strongly
coloured
juices
there
can
be
a
problem
addition
of1iodine
continued
after
the the
endpoint,
further
establishing
an
approximate
volume
of
iodate
solution
reacted
andFigure
the
excess
reacts
with
starch
indicator
to give
VitaminisCiodine
tablet. Left
photo:
before
endpoint,
added
iodineiodine-starch
reacts
in
colour,
there
is
no
problem
identifying
the
endpoint.
number
of
interesting
investigations
regarding
the
endpoint
and itabove
is advised
to carry out
a “rough”
is
formed.
in each
of these
images
a flask
showing
pale
ascorbic
acidNB:
leaving
theIf
solution
colourless.
Centre
photo:
At the the
7. The
method
may
be
used to
carry out the
a
required.
a palecomplex
bluewith
colour.
Right
photo:
addition
of iodine
is continued
after
For
strongly
coloured
juices
there
can
be
a
problem
with
titrationof
endpoint
all the ascorbic
acid has
and the excess iodine
blue colour
the endpoint
is shown
forreacted
comparison.
concentration
of
vitamin
C
in
various
foods
and drinks:
titration
in order to become familiar with any distinct
the endpoint,
further
iodine-starch
complex
is
formed.
NB:
in
each
of
reacts with the starch indicator to give a pale blue colour. Right photo: If
number
interesting
investigations
regarding the
7.the endpoint
The
above
method
may
to carry
out
a
and
it
isof
advised
to
carry
out
“rough”
change
which
occurs
at be
theused
endpoint
(it amay
addition
of iodine
is continued
theblue
endpoint,
further
these images
a flask
showing
theafter
pale
colour
ofiodine-starch
the endpointcolour
isnumber
•
Vitamin
C
content
of
different
types
of
fruits/
of
interesting
investigations
regarding
the
titration
in
order
to
become
familiar
with
any
distinct
concentration
of
vitamin
C
in
various
foods and
complex is formed. NB: in each of these images a flask showing the pale
just be a darkening of the colour) This will also help by
shown for blue
comparison.
colour of the endpoint is shown for comparison.
concentration
of
vitamin
C
in
various
foods
and
drinks:
vegetables/juices.
colour
change
which
occurs
at
the
endpoint
(it
may
establishing an
approximate volume of iodate solution
Figure 1 Vitamin C tablet. Left photo: before endpoint, added iodine reacts
drinks:
with ascorbic acid leaving the solution colourless. Centre photo: At the
just
be aCdarkening
colour)
This
will also help by
required.
•
Vitamin
content
ofofdifferent
typesof
ofdifferent
fruits/
•
Vitamin
Cthe
content
types/brands
of vitamin
titration endpoint all the ascorbic acid has reacted and the excess iodine
•
Vitamin
C
content
of
different
types of fruits
establishing
an
approximate
volume
of
iodate
solution
Figure
Vitamin
C tablet.
Left photo:
endpoint,
iodine Ifreacts vegetables/juices.
reacts1with
the starch
indicator
to givebefore
a pale blue
colour.added
Right photo:
7.
The
above
method
may
be
used
to
carry
out
a
tablets.
with
ascorbic
acid leaving
the solution
Centreiodine-starch
photo: At the
addition
of iodine
is continued
after thecolourless.
endpoint, further
required.
vegetables/juices.
number
of interesting
regarding of
thevitamin
• Vitamin
C content
ofinvestigations
different types/brands
titration
theinascorbic
acid has
reacted
and
the excess
iodine
complexendpoint
is formed.allNB:
each of these
images
a flask
showing
the pale
•
Vitamin
CC content
of
tablets
or
in the
reacts
with the
starch
indicator
to give
pale blue colour. Right photo: If
blue colour
of the
endpoint
is shown
foracomparison.
concentration
of
vitamin
inmay
various
foods
and
drinks:
tablets.
7. The
above
method
be
used
to
carry
outfood/drink
a types/brands
• Vitamin C content of different
of addition of iodine is continued after the endpoint, further iodine-starch
presence
and
absence
of
added
oxalic
acid
or
metal ions
interesting
investigations
regarding
• Vitamin
ofofdifferent
types
of fruits/
complex is formed. NB: in each of these images a flask showing the pale
•number
VitaminCof
Ccontent
content
tablets
or
food/drink
in the the
vitamin
tablets.
blue colour of the endpoint is shown for comparison.
over
various
periods
ofacid
time.
concentration
of
vitamin
C inoxalic
various
foods
and drinks:
vegetables/juices.
presence
and
absence
of added
or metal
ions
•
Vitamin
C
content
of
tablets
food/drink
in the over
various
periods
of
time.
•
Vitamin
C
content
of
different
types
of
fruits/
• Vitamin C content
of different
types/brands
of vitaminor
• Vitamin
C content
of food/drink
before
and after
Figure 2 Commercial fruit juice. Left flask: before the endpoint, the colour
presence
and
absence
of
added
oxalic
acid
or vegetables/juices.
tablets.
content of food/drink
before
and after
to cooking
conditions.
of theFigure
solution
reflectsfruit
thejuice.
paleLeft
yellow
ofendpoint,
the fruitthejuice.
Centre• Vitamin
flask: Csubjection
2 Commercial
flask: colour
before the
colour
Figure
At theoftitration
endpoint
allpale
theyellow
ascorbic
andflask:
any excess
subjection
to
cooking
conditions.
the solution
reflects the
colouracid
of thehas
fruitreacted
juice. Centre
content
metal
various
periods
of time.
C content
of tablets
orover
food/drink
in the
• Vitamin
ofCions
different
types/brands
of vitamin
2 Commercial
fruitwith
juice.
Leftallflask:
beforetothe
endpoint,
the colour• Vitamin
of
the titration
endpoint
the
ascorbic acid
has reacted
andblue-black
any excess complex.
•CVitamin
content
of fruits/vegetables
at different
iodineAtreacts
the starch
indicator
form
a dark
presence
and
absence
of
added
oxalic
acid
or
metal
ions
•
Vitamin
C
content
of
fruits/vegetables
at
different
tablets.
the solution
reflects
the
pale
yellow
colour
of
the
fruit
juice.
iodine
reacts
with
the
starch
indicator
to
form
a
dark
blue-black
complex.
In this case the result is a darkening of the solution’s colour from yellow
•
Vitamin
C
content
of
food/drink
before
and after stages
of
ripeness.
In this
result is aendpoint
of all
the solution’s
colour from
yellow
periods of time.
stages
Centretoflask:
At case
thethe
titration
thethe
ascorbic
hasjust aover
brown-grey.
Right
flask:darkening
This illustrates
effect
ofacid
adding
mL various
or of ripeness.
to brown-grey. Right flask: This illustrates the effect of adding just a mL or
•
Vitamin
C
content
of
tablets
or
food/drink
in
the
subjection
tobefore
cooking
conditions.
two
more
ofexcess
iodine
afterthethe
endpoint
is reached,
resulting
the
reacted
and
any
iodine
with
theis reached,
starch
indicator
two
more
of iodinesolution
solutionreacts
after
endpoint
resulting
in theto • in
Vitamin
C content
of food/drink
after
presence
and absence
of added
oxalicand
acid
or metal ions
Figure
2 of
Commercial
fruit
juice. Left flask:
before the endpoint, the colour
further
iodine-starch
complex.
formation
of further
iodine-starch
complex.
form aformation
dark
blue-black
complex.
In
this
case
the
result
is
a
darkening
subjection
toContact
cooking
conditions.
of the solution reflects the pale yellow colour of the fruit juice. Centre flask:
Us
Contact
Us
•
Vitamin
C
content
of
fruits/vegetables
at different over
various
periods
of
time.
At the titration
endpoint
all the
ascorbic
has reacted and
any excess
of the solution’s
colour
from
yellow
toacid
brown-grey.
Right
flask:
C content
of fruits/vegetables
atrelating
different
iodine reacts with the starch indicator to form a dark blue-black complex.
you
stages
If• you
have
questions
orripeness.
comments
to
this
IfCany
have
any
questions
or
relating to this
Vitamin
content
of of
food/drink
before
andcomments
after
This illustrates
thetheeffect
of adding
just
abefore
mL the
orcolour
two
more
of
iodine• Vitamin
Figure
Commercial
juice. Leftof
flask:
endpoint,
colour
In this2case
resultfruit
is a darkening
the solution’s
from the
yellow
stages
of ripeness.
experiment,
please
contact
us:
subjection
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Figure 4 Red capsicum. Left flask: Before the endpoint, the solution
iodine reacts with the starch indicator to form a dark blue-black complex.
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two more of iodine solution after the endpoint is reached, resulting in the
formation of further iodine-starch complex.
retains its original colour. Centre flask: Once all the ascorbic acid has been
oxidised, a slight excess of added iodine forms a dark complex with starch
indicator giving a purple colour. This is the titration endpoint. Right flask:
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