Applications of Redox Titrations

1

• Applications of Redox Titrations
• Auxiliary reagents are often used to place the
  analyte in an appropriate form prior to the
  analytical titration step
• Oxidizing agents
• NaBiO3 is a slightly soluble solid that is a very
  strong oxidizing agent
• NaBiO3(s) 4H 2e- BiO Na 2H2O
• Suspend the solid in the solution to be oxidized,
  heat and filter the unused solid
• IO4-
• IO4- 2H 2e- IO3- H2O
• S2O82-
• S2O82- 2e- 2SO42-
• Unused S2O82- can be decomposed by heating
• 2S2O82- 2H2O 4SO42- O2 4H
• Each of the above oxidizing agents can oxidize
  Mn2 to MnO4- and S2O42- can oxidize Cr3 to
  Cr2O72-

2

• Applications of Redox Titrations
• Auxiliary reagents ...
• Reducing agents
• Walden reductor consists of a gravity-chromatograp
  hic-column like tube filled with Ag and a
  solution of HCl
• Ag Cl- AgCl 1e-
• Jones reductor uses similar glassware filled with
  amalgamated zinc
• 2Zn Hg2 Zn(Hg)(s) Zn2 preparation
  of amalgam
• Zn(Hg) Zn2 Hg 2e- oxidation of
  zinc amalgam
• The Hg prevents the oxidation of Zn by H and
  solutions containing moderate concentrations of
  H can pass through the reductor without undue
  contamination of the analytical solution by Zn2

3
Applications of Redox Titrations Uses of Walden
and Jones reductors

• Other reducing agents can be used including Sn,
  Sn(II), Al, Cd, Ni, Pb, Cu, Ag(Cl-)
• Most methods use oxidizing agents as titrants
• MnO4-, Ce(IV), Cr2O72-, I2, I3-

4

• Applications of Redox Titrations
• MnO4- is the most used oxidizing agent titrant
• MnO4- 8H 5e- Mn2 4H2O Eo 1.51
  V in acid
• Self-indicating but endpoint fades due to
• 2MnO4- 3Mn2 5MnO2 4H K 1046 but
  slow
• MnO4- 4H 3e- MnO2 2H2O if pH gt 4,
  neutral or basic
• MnO4- is unstable as it oxidizes water
• 4MnO4- 2H2O 4MnO2 3O2 4OH- K gtgt 1
  but slow
• Reaction catalyzed by light, heat, acids,
  Mn(II), and MnO2
• Solutions containing excess MnO4- should not be
  heated because of this decomposition reaction
  which cannot be corrected for by using a blank
• Warm solutions can be titrated with MnO4-,
  however
• Preparation of KMnO4 solutions
• Cannot prepare standard solutions by direct
  weighing as KMnO4 is contaminated with MnO2
• Prepare solutions hot and allow to stand
  overnight, then filter using Gooch crucible to
  remove the MnO2
• Store in a dark place except when in use

5

• Applications of Redox Titrations
• MnO4- is the most used oxidizing agent titrant
• Standardization of MnO4- solutions
• Primary standard Na2C2O4 in acid is used
• H2C2O4 2CO2 2H 2e-
• reaction is rapid in hot solution but catalyzed
  by Mn2, so the initial MnO4- takes several
  seconds to react and discharge the color
• Method of McBride - slowly titrate a 60 oC
  solution
• Low results are due to O2 H2C2O4 H2O2
  CO2
• Method of Fowler and Bright - add 95 of
  required MnO4- to a cold H2C2O4 solution, heat
  to 60 oC and finish the titration
• Other standards include As2O3, Fe, KI

6

• Applications of Redox Titrations
• MnO4- is the most used oxidizing agent titrant
• Analysis of iron alloys and iron ores with MnO4-
• Dissolve in HCl - Fe(II) forms a stable complex
  with Cl- which enhances the dissolution process
• Often the resulting solution consists as a
  mixture of Fe(II) and Fe(III)
• Use a Jones reductor to reduce Fe(III) to Fe(II),
  but other metal impurities may be reduced that
  would react with MnO4-
• Use a Walden reductor which does not reduce
  Cr(III) or Ti(IV) but V species could interfere
• Use SnCl2 Sn2 2Fe3 Fe2 Sn4
• remove excess Sn2 Sn2 Hg2 2Cl-
  Sn4 Hg2Cl2(s)
• avoid too much HgCl2 Sn2 Hg2 Hg
  Sn4
• Analysis of Ca2
• Precipitate as CaC2O4 and dissolve in acid
• Titrate the H2C2O4 with MnO4-

7

• Applications of Redox Titrations
• Cerium(IV) solutions
• Ce4 1e- Ce3 Eo 1.44 V in 1 M
  H2SO4
• Primary standard grade Ce(NO3)42NH4NO3 can be
  used to prepare standard solutions in solutions
  at least 0.1 M in H2SO4
• Other reagents in reagent-grade quality can be
  used and the resulting solutions standardized by
  titration of primary standard grade sodium
  oxalate in 1M H2SO4 at 50 oC
• Ce(IV) solutions are indefinitely stable even at
  100 oC
• Solutions of Ce(IV) with the same oxidizing
  capacity as MnO4- solutions cost 50 times as
  much as the MnO4- solutions
• Ce(IV) unlike MnO4- will not oxidize Cl- in HCl
  solutions, so solutions prepared with HCl can be
  used

8
Applications of Redox Titrations Some
applications of KMnO4 and Cerium(IV) in acid
solutions
9

• Applications of Redox Titrations
• Iodimetry and Iodometry
• Iodimetry is the direct use of I2 or I3-
  solutions
• Saturated I2(aq) is 0.001 M, so I2(s) is added to
  a small volume of concentrated KI and diluted
  to form I3-
• I3- solutions are not very stable and must be
  standardized every few days
• 4I- O2 4H 2I2 2H2O solution
  increases I2 concentration
• I3- is a weak oxidizing agent so steps are taken
  to enhance its potential
• Adjust pH
• pH does not affect the potential of the I3-/I-
  couple but
• H3AsO4 2H 2e- H3AsO4 H2O
• Form complexes
• Fe(III) can be made to form EDTA complexes under
  conditions which Fe(II) does not form complexes
  thus enhancing the reaction of Fe(III) with I3-
• 2Fe2 EDTA I3- 2FeY- 3I-
• In alkaline solutions I2 OH- HOI
  I-
• 3HOI 3OH- IO3-
  2I- 3H2O

10

• Applications of Redox Titrations
• Iodimetry
• End points use I3- since the eye can detect 5 x
  10-6 M I3-
• Use an immiscible organic liquid to extract the
  I2
• Use starch
• Standardization of I3- solutions
• Dissolve As2O3 in 1M NaOH and neutralize excess
  base with HCl to prevent oxidation of As(III) to
  As(IV)
• H3AsO3 I3- H2O H3AsO4 3I- 2H K
  0.16
• Keep pH 7 - 8 with CO2/HCO3- buffer to draw
  reaction forward
• Iodometry is the indirect use of I2 of I3-
  produced by the reaction of I- added to a
  solution containing an oxidizing analyte.
• The I(0) is titrated with standardized S2O32-
• 2S2O32- I3- S4O62- 3I- note 1e-
  per S2O32- 2 S2O32- S4O62- 2e-
• Other oxidizers form SO42- 4HOI S2O32-
  H2O 2SO42- 4I- 6H
• HOI produced when I3- solution becomes basic

11

• Applications of Redox Titrations
• Iodometry
• S2O32- solutions
• pH must be controlled S2O32- H
  HS2O3-
• HS2O3- S(s) HSO3-
• If pH low, reaction takes seconds
• Stabilize with Na2CO3 or borax pH7-9
• Must neutralize OH- by acidfying I3- solution to
  prevent formation of HOI
• Certain bacteria should be excuded - sterilize
  the solution by boiling the water
• Sunlight can cause decomposition of S2O32-
  solutions
• Atospheric O2 can oxidize the S2O32-
• Standardization of S2O32- solutions
• Use a reagent that produces a known amount of I3-
  and titrate the I3-
• IO3- 0.5I- 6H 5e- 0.5I3-
  3H2O
• 7.5I-
  2.5I3- 5e-
• IO3- 8I- 6H 3I3-
  3H2O Obtain 6 mol I(0)/mol IO3-

12

• Applications of Redox Titrations
• Iodometry
• Errors
• Air oxidation of I- 6I- O2 4H 2I3-
  2H2O
• Reaction is slow catalyzed by light, high H
  concentration and Cu(II)
• Volatalization of I2 maintain large excess of
  I-, titrate rapidly, keep solutions cool
• Decomposition of S2O32-
• Avoid basic solutions so HOI not produced and
  S2O32- will not go to SO42-
• Do not add starch too early as high
  concentrations of I3- will decompose it