Iodine titration by krishna baokar

1
OXIDATION-REDUCTION
(Iodine titrations)
Mr. SHRIKRISHNA BAOKAR
Delonix Society’s
BARAMATI COLLEGE OF PHARMACY, BARHANPUR, BARAMATI
Email: krishnabaokar@gmail.com
Website: krishnabaokar.wordpress.com
Contact No. 9960225455
delonixsociety.org.in

Strictly for reference 2
The learner will be able to:
• Differentiate Iodometry with Iodimetry.
• Determine solubility of Iodine.
• Learn conditions and precautions in iodine
titration.
• Determine end point by various method.
• Know why iodine is standardized with
sodium thiosulphate.
Learning Outcomes

Why starch is added towards the end point of
the titration and not in the beginning?
Reflection Spot

• Iodine titrations are very important part of
oxidation-reduction titrations,
• Iodine titrations are basically two types….
1) Iodimetry (Direct titrations),
2) Iodometry (Indirect titrations).
IODINE TITRATIONS: Introduction

• Iodimetry (Direct titrations) :
refers to a standard solution of iodine, that
means iodine is directly involved in the
reaction,
• Iodometry (Indirect titrations) :
Iodine standard solution is not directly
involved but iodine that is liberated due to
chemical reaction and this liberated iodine
is estimated.
IODINE TITRATIONS: Introduction

Solubility of Iodine
• Iodine is poorly soluble in water (0.335g/L is
the saturation solubility of iodine in water).
• where as it is readily soluble in aqueous
iodide solutions like potassium iodide or
sodium iodide solution.
• Hence the actual half cell reaction is……
IODINE TITRATIONS: Solubility

I2 (Solid) + 2e ↔ 2I- and Eo = 0.5345 Volts
the above reaction occurs at the end of the
titrations but in Iodometry titrations where
excess iodine is present tri iodide is formed,
I2 (Aqueous) + I- ↔ I3
- and Eo = 0.5355 Volts

• So the reactive species in Iodimetry is tri
iodide and the reaction should be written as
I3
- but for the sake of convenience we write
it as I2 .
• For example :
I3
- + 2 S2O3
2- = 3I- + S4O6
2-
is more accurate than
I2 + 2 S2O3
2- = 2I- + S4O6
2-

• The E0 value of this system is +0.5345V, hence
iodine is a weaker oxidizing agent than
potassium permanganate, dichromate,
ceric etc.
• Thus there are several reducing agents
which can be oxidized by free iodine
(those with E0 value of less than 0.5345V).

• But there are also numbers of oxidizing
agents which can be reduced by iodide
ions (those with E0 value of more than
0.5345V).
• So Iodine is a opportunistic oxidizing agent
i.e. it can act either as an oxidizing agent or
as a reducing agent depending on the
condition.
•

• For example :
• Free iodine like other halogens can accept
electrons and therefore is an oxidizing agent.
• Iodide ions (I-) can readily donate electrons
acting as a reducing agent.
• Hence there is a dual possibility of using redox
properties of I2/I- system in volumetric analysis;
for determination of reducing agents by
oxidation with iodine and oxidizing agents by
reduction with iodides.

• As iodine is volatile, the titration is conducted in
the cold.
• This is also necessary because the sensitivity of
starch as indicator diminishes with rise of
temperature.
• If a starch solution turned blue by a single drop
of iodine, when this solution is heated, the blue
color disappears when it is cooled the color
returns.
IODINE TITRATIONS: Conditions for Titration

• Iodometry titrations cannot be performed in
strongly alkaline solutions, because iodine
reacts with alkaline in accordance with the
equation
I2 + 2NaOH NaIO + NaI + H2O
I2 + 2 OH- IO- + I- + H2O
• The presence of hypoiodide (IO- ions) is
inadmissible, because it is stronger oxidant than
I2 and partially oxidizes thiosulphate to sulphate
S2O3
-- + 4IO- + 2OH 4I- + 2SO4
-- + H2O
The higher the OH- concentration in solution, the
more thiosulphate is converted into sulphate

• This side reaction makes exact calculation
of the analytical result impossible. Care must
therefore be taken that the solution pH does
not exceed 9

• If the reaction result in formation of H+ ions,
they must be removed to ensure that the
reaction proceeds to practical completion in
the required direction; this is done by addition
of Sod. Bicarbonate (NaHCO3)which is react as
follows
HCO3
- + H+ H2CO3 H2O + CO2
the solution becomes slightly alkaline (pH ~ 8),
but this does not interfere with the titration

• As the solubility of iodine in water is low, a
considerable excess of KI must be used in
Iodometry for determinations of oxidizing
agents. The iodine liberated by the reaction
then dissolves by forming the unstable
complex salt K[I3] with KI
KI + I2 K[I3] or I- + I2 [I3]-
formation of this compound does not
interfere with titration of iodine with
thiosulphate, because the solution contains
sufficient iodine owing to the reversible
character of the above reaction.

• In case where H+ ions are consumed in the
reaction increases of solution acidity has a
similar effect
• When the reaction mixture is left to stand
before the start of the titration it is kept in a
dark place, because light accelerates the
side reaction in which I- ions are oxidized to
I2 by atmospheric oxygen
4I- + 4H+ + O2 2I2 + 2H2O

• The titration should not be prolonged unduly.
• Titrations should be performed in cold solution in
an “Erlenmeyer’s Flask”.
• The solutions containing excess iodide and acid
should not be allowed to stand for long period of
time. If prolonged standing is inevitable then the
air inside the flask should be displaced by using
sodium carbonate or dry ice before adding
iodide.
• Whenever the solution needs to be standing it
should be done in dark than under direct sunlight.
• The solution containing iodide should be stored in
amber colored glass.
IODINE TITRATIONS: Precautions

• Two important sources of errors in iodine
titrations are the volatility of iodine and
oxidation of iodide in acid solutions by
atmospheric oxygen.
• In the presence of excess iodide, the volatility
of iodine is decreased markedly due to the
formation of tri-iodide ions.
• The atmospheric oxidation of iodide is
negligible in neutral solutions in the absence of
catalysts.

• Rate of oxidation increases with increase in
pH and in the presence of ions like copper,
nitrites.
• The presence of sunlight will also increase
the rate of oxidation reaction.
• Further the oxidation of iodide ion may be
induced by autocatalysis.
• Hence above precautions need be
observed during iodine titrations.

• Starch is used as indicator,
• Advantage: In expensive,
• Disadvantage: Insoluble in cold water,
instability in suspension, formation of water
insoluble complex with iodine etc.
There is also drift endpoint observed in dilute
solutions.
• Starch contains two major components
Amylose and Amylopectin
IODINE TITRATIONS: End Point Determination

• Amylose which is a straight chain will give blue
color end point where as Amylopectin will give
red purple endpoint.
• With excess of iodine in the beginning of the
titration the indicator gives green color but at
the end it will give an intense blue color. And
the end point is very sharp.

• Carbon tetra chloride: used in certain
reactions instead of starch.
• Intense coloration in presence of iodine is a
favorable factor.
• Reddish brown to colorless is the end point.
• Equally satisfactory results can be obtained
by chloroform.

Preparation of standard iodine solution
• A standard solution containing potassium iodide
and Iodate is quite stable and yields
quantitative iodine when treated with acid.
KIO3 +5KI + 3H2SO4 → 3I2 + 3K2SO4 + 3H2O
• Standard solution can be prepared by weighing
exact quantity of KIO3 and dissolving in a
solution containing slight excess of KI.
• By controlling the amount of Iodate an exact
amount of iodine can be made available in the
solution.
IODINE TITRATIONS: Preparation & Standardization

• The amount of iodine liberated is equivalent
to the acid content of the solution.
• When a solution of acid the normality of
which has to be determined is treated with
slight excess of potassium iodide and
potassium Iodate solution.
• The amount of iodine liberated will be
proportional to the amount of acid present.

• By titrating the liberated iodine with sodium
thio sulphate, the strength of the acid can
be calculated.
• It helps in 2 ways,
a) It forms a source of known quantity of
iodine in titrations,
b) In determination of acid content of
substance iodometrically or in
standardization of solutions of strong acids

Standardization of iodine solution
• Iodine solution can be standardized using
1) arsenic trioxide (or)
2)sodium thiosulphate solution.
• Sodium thiosulphate in turn should be
recently standardized using potassium
Iodate, potassium dichromate and standard
iodine.

Reason for standardization of Na2S2O3.5H2O
• The reason is we are not sure of the water
molecule present in sodium thio sulphate due to
its efflorescent nature (Na2S2O3.5H2O),
• It is unstable if prepared in ordinary distilled
water and decomposes in presence of CO2 to
form sulphur,
• Moreover decomposition is due to bacteria
(Thiobacillus thioparus) on long standing which
can be prevented by adding a drop of
chloroform.

Standardization using arsenic trioxide
• As2O3 + 6NaOH 2Na3AsO3 + 3H2O
Sodium arsenite
• 2Na3AsO3 + 6HCl 2H3AsO3 + 6NaCl
Arsenious acid
• H3AsO3 + I2 + H2O H3AsO4 + 2HI
Arsenic acid
HI + NaHCO3 NaI + H2O + CO2

• Standerdization of sod. thiosulphate is done by 3 methods:
1)Potassium iodate
Io3
- + 5I- +6H+ = 3I2 +3 H2O,
(Titrate the liberated iodine with thiosulphate solution)
2)Potassium dichromate
Cr2O7
2- +6I- +14H+ = 2Cr3+ +3I2 +7H2O,
This method is prone to errors like formation of hydroiodic acid
which is oxidized by air hence current of Co2 has to be
maintained and the reaction is slow. (Titrate the liberated
iodine)
3)With standard solution of iodine
2S2O3
2- +I2 = S4O6
2- +2I- (pH < 5)
an intermediate step is involved which results in the formation of
S2O3I- which reacts with thiosulphate and follow the main
course of over all reaction.

Applications
• Iodimetry titration :
Example : Assay of ascorbic acid
( standardization by arsenic trioxide),
• Iodometry Titration:
Example : Assay of potassium permanganate,
Assay of potassium bromate,
Assay of copper sulphate,
Assay of chlorinated lime
(standardization by sodium thio sulphate)
IODINE TITRATIONS: Applications

32
Thank You……..
Delonix Society’s
BARAMATI COLLEGE OF PHARMACY, BARHANPUR, BARAMATI

Iodine titration by krishna baokar

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