Lesson 2
Redox Titration
Redox Titration
A redox titration is based on an oxidation-
reduction reaction between analyte and titrant.
q In 1787, Claude Berthollet introduced a method for the
quantitative analysis of chlorine water (a mixture of Cl2, HCl,
and HOCl) based on its ability to oxidize indigo, a dye that is
colorless in its oxidized state.
q In 1814, Joseph Gay-Lussac developed a similar method for
determining chlorine in bleaching powder.
Redox Titration Curve
Region
before the equivalence point
at the equivalence point
after the equivalence point
Treatment
Use the half reaction of the analyte
Use equivalence point equation
Use the half reaction of the titrant
Symmetrical equivalence point: if the
stoichiometry of a redox titration is 1:1.
Asymmetrical equivalence point: if the
titration reaction’s stoichiometry is not 1:1,
then the equivalence point is closer to the top
or to bottom of the titration curve’s sharp rise
Redox
Titration
Titration of Mixture
A titration of a mixture of analytes is possible if their standard
state potentials or formal potentials differ by at least 200 mV.
Example:
titration curve
for a mixture
of Fe2+ and
Sn2+ using
Ce4+ titrant
First to be titrated: stronger reducing agent
Redox
Titration
Indicators for Redox Titration
Redox
Titration
Self indicator: used when the oxidized and reduced form of a titrant
has a different color. Example: permanganate
Specific indicator: reagent that forms a colored compound with a
specific oxidized or reduced form of the titrant or the analyte .
Example: starch produces a blue color with Iodine, SCN- forms a
soluble red-colored complex with Fe3+
Redox indicator: substances that do not participate in the redox
titration, but whose oxidized and reduced forms differ in color.
Specific Indicator
Example:
Starch-Iodine Complex
• Starch is the indicator of choice for those procedures involving
iodine because it forms an intense blue complex with iodine.
The active fraction of starch is amylose, a polymer of the sugar
α-d-glucose.
• In the presence of starch, iodine forms I6 chains inside the
amylose helix and the color turns dark blue
Redox
Titration
Redox
Titration
Redox Indicator
The transition potential of indicator & equivalence point potential should
be nearly equal to give color change change at the equivalence point.
Example: Ferroin. At E° = 1.147 V, a color change from red to
blue occurs in the approximate range 1.088 V to 1.206 V
or
Redox Indicators
Redox
Titration
If the difference in the formal potential between analyte and titrant is > 0.4 V,
a redox indicator usually gives a satisfactory end point.
Auxiliary Reducing Agent
Redox
Titration
To ensure that the analyte is initially present in a single oxidation state,
pre-reduction/pre-oxidation is necessary.
Auxiliary Oxidizing Agents
Sodium Bismuthate
Ammonium Peroxydisulfate
Sodium Peroxide and Hydrogen Peroxide
Redox
Titration
Auxiliary Reducing Agent
Redox
Titration
From: Skood et al. 9E
Some Redox Titration Methods
Dichrometry
Permanganimetry
Cerimetry
Iodimetry
Iodometry
Redox
Titration
Permanganimetry
Redox
Titration
Oxidation with potassium permanganate
Permanganimetry or Permanganometry
KMnO4 is a strong oxidant with an intense violet color and
not available as a primary standard
Primary standard
ü As2O3
ü Mohr’s salt
(NH4)2Fe(SO4)2(H2O)6
ü Na2C2O4 (carried at >60oC to
speed up the reaction)
Indicator: KMnO4 (self-indicator)
Color change - colorless to light pink
Potassium Permanganate
Redox
Titration
In strongly acidic solutions (pH < 1), it is reduced to Mn2+.
In neutral or alkaline solution, it is reduced to brown solid MnO2.
I
n strongly alkaline solution (2 M NaOH), green manganate ion (MnO42-) is produced.
Potassium Permanganate
Redox
Titration
Ordinary distilled water is likely to contain reducing substances (traces of organic
matter, etc) which will react with the KMnO4 to form MnO2. Light, heat, acids, bases,
manganese(II), and manganese dioxide further catalyze the auto-decomposition of
the KMnO4 solution on standing.
Heating the solution speeds up the reaction while filtration with
glass wool removes the MnO2.
The pink permanganate end point tends to fade with time because excess permanganate ions react
slowly with the relatively large concentration of manganese(II) ions present at the end point
Cerimetry
Redox
Titration
Oxidation with Ce4+
Reduction of Ce4+ (yellow) to Ce3+ (colorless) proceeds cleanly in acidic solutions.
Indicator: Ferroin and other substituted phenanthroline redox indicators
Color change: red to blue
Redox
Titration
From: Skood et al. 9E
Dichrometry
Oxidation with potassium dichromate
K2Cr2O7 is available in high purity (excellent primary standard) and
highly stable up to its melting point.
dichromate ion is reduced to green chromium(III) ion:
Indicator: redox indicator like diphenylamine and diphenylamine sulfonate
Color change: green to violet
Redox
Titration
Redox
Titration
Titration involving Iodine
From: https://xn--80aaeoaaddt0bob0e0ezah.xn--p1ai/finance/iodometric-and-iodimetric-titration.php
From: Skood et al. 9E
Redox
Titration
Iodimetry
Oxidation with IODINE
Performed in neutral or mildly alkaline (pH=8) to weakly acid solutions
v If the pH is too alkaline, iodine will undergo disproportionation.
v In acidic medium, starch is hydrolyzed and the oxidizing power of
iodine is decreased
Indicator: Starch - can be added at the beginning of the titration
Color change - colorless to dark blue
From: Skood et al. 9E
Iodometry
Redox
Titration
Titration of IODINE
Reaction between an oxidizing agent (analyte) with KI yields iodine (I2)
The iodine formed is titrated with standard Na2S2O3
Na2S2O3 is not available as a primary standard, must be standardized using KIO3.
Indicator: Starch
- should not be added until immediately before the equivalence point . Otherwise some iodine
tends to remain bound to starch particles after the equivalence point is reached.
-should be carried out below pH 9. In basic solution, Iodine disproportionates to iodide and HOI,
which can oxidize thiosulfate to sulfate
Color change – disappearance of the blue color
From: Skood et al. 9E
Iodimetry
Redox
Titration
From: Skood et al. 9E
Iodometry
Redox
Titration
From: Skood et al. 9E