Analytical Chemistry lecture note: Application of Redox titration

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Preadjustment of analyte oxidation state
It is necessary to adjust the oxidation state of
the analyte to one that can be titrated
with an auxiliary oxidizing or reducing agent.
Ex. Preadjustment by auxiliary
reagent
Fe(II), Fe(III) Fe(II)

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Titration
Ce4
Prereduction Stannous chloride ( SnCl2)
Chromous chloride
Jones reductor (zinc coated with zinc
amalgam) Walden
reductor ( solid Ag and 1M HCl)
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Jones reductor 2Zn (s) Hg2 ? Zn2 Zn(Hg)
(s)
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Reagents used in redox titration Reducing
agents 1) ammonium iron(II) sulfate
hexahydrate (Mohrs salt) FeSO4(NH4)2SO4
6H2O 2) iron(II) ethylene diamine sulfate
(Oespers salt) FeC2H4(NH3)2(SO4)2 4H2O
3) Sodium thiosulfate pentahydrate
Na2S2O35H2O 4) Arsenic trioxide arsenious
oxide As2O3 5) Sodium oxalate and
oxalic acid dihydarte Na2(COO)2 ,
(COOH)22H2O
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Sodium thiosulfate, Na2S2O3 Thiosulfate ion is a
moderately strong reducing agent that has been
widely used to determine oxidizing agents by an
indirect procedure that involves iodine as an
intermediate. With iodine, thiosulfate ion is
oxidized quantitatively to tetrathionate ion
according to the half-reaction 2S2O3 2
? S4O6 2 2e Eo 0.08 Ex.
Determination of hypochlorite in bleaches
CaCl(OCl)H2O OCl 2I 2H
? Cl I2 H2O (unmeasured excess KI)
I2 2 S2O3 2 ? 2I S4O6 2
Indicator soluble starch
(?-amylose)
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Standardization of thiosulfate solution Primary
standard potassium iodate (KIO3), K2Cr2O7,
KBrO3 Titration reactions KIO3 5KI
6HCl ? 3I2 6KCl 3 H2O I2
2Na2S2O3 ? 2NaI Na2S4O6 KIO3 ?
3I2 ? 6Na2S2O35H2O ? 6
Equivalent S2O32- H ? HSO3- S(s) pH,
Microorganisms, Concentration, Cu2,
Sunlight Stabilizer for sodium thiosulfate
solution Na2CO3 Na2S2O3 H2O CO2
? Na2CO3 H2S2O3 H2S2O3 ?
H2SO3 S
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16-2 Finding the end point
A redox indicator is a compound that changes
color when it goes from its oxidized to its
reduced state.
For ferroin, with E 1.147 V we expect the
color change to occur in the approximate range
1.088 V to 1.206 V with respect SHE
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Starch-Iodine Complex

• Starch is the indicator of choice for those
  procedures
• involving iodine because it forms an intense
  blue colour
• with iodine.
• Starch is not a redox indicator
• it responds specifically to the presence of I2,
  not to a change in redox
  potential.

Structure of the repeating unit of the sugar
amylose.
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Arsenious oxide, As4O6 As4O6 6H2O
4H3AsO3 H3AsO3 I3 H2O H3AsO4
3I 2H
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Reagents used in redox titration Oxidizing
agents 1) Potassium permanganate KMnO4
Permanganometry 2) Ceric sulfate /
Ceric ammonium sulfate Ce(SO4)22(NH4)2SO4
4H2O Cerimetry 3) Potassium dichromate
K2Cr2O7 Dichrometry 4) Iodine I2
Iodimetry, Iodometry 5) Potassium
iodate KIO3 Iodatimetry 6)
Potassium bromate KBrO3 Bromatimetry
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Permanganate titration Oxidation with
permanganate Reduction of permanaganate KMnO4
Powerful oxidant that the most widely used.
1) In strongly acidic solutions (1M H2SO4 or
HCl, pH ? 1) MnO4 8H 5e Mn2
4H2 O Eo 1.51 V
KMnO4 is a self-indicator. 2) In feebly acidic,
neutral, or alkaline solutions MnO4
4H 3e MnO2 (s) 2H2 O Eo
1.695 V
3) In very strongly alkaline solution (2M
NaOH) MnO4 e MnO42

Eo 0.558 V

?
?
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Permanganate titration Duration of
colour in end point (30 seconds)
MnO4 3Mn2 2H2O ? 5MnO2 4H K11047
Stability of aqoues
solution of MnO4- MnO4 2H2O ?
4MnO2 (s) 3O2 (g) 4OH-

?
?
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Standardization of KMnO4 solution Potassium
permanganate is not primary standard, because
traces of MnO2 are invariably present. Standardi
zation by titration of sodium oxalate (primary
standard) 2KMnO4 5 Na2(COO)2 8H2SO4
2MnSO4 K2SO4 5Na2SO4 10 CO2 8H2O
2KMnO4 ? 5 Na2(COO)2
? 10 Equivalent
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• Preparation of 0.1 N potassium permanganate
  solution
• KMnO4 is not pure.
• Distilled water contains traces of organic
  reducing substances
• which react slowly with permanganate to form
  hydrous managnese dioxide.
• Manganesse dioxide promotes the
  autodecomposition of permanganate.
• 1) Dissolve about 3.2 g of KMnO4 (mw158.04)
  in 1000ml of water,
• heat the solution to boiling, and keep
  slightly below the boiling point for 1 hr.
• Alternatively , allow the solution to
  stand at room temperature for 2 or 3 days.
• Filter the liquid through a sintered-glass filter
  crucible to remove solid MnO2.
• Transfer the filtrate to a clean stoppered bottle
  freed from grease with cleaning mixture.
• Protect the solution from evaporation, dust, and
  reducing vapors, and keep it in the dark or in
  diffuse light.
• If in time managanese dioxide settles out,
  refilter the solution and restandardize it.

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• Applications of permanganometry
• H2O2
• 2KMnO4 5 H2O2 3H2SO4 2MnSO4 K2SO4
  5O2 8H2O
• (2) NaNO2
• 2NaNO2 H2SO4 Na2SO4
  HNO2
• 2KMnO4 5 HNO2 3H2SO4 2MnSO4 K2SO4
  5HNO3 3H2O
• (3) FeSO4
• 2KMnO4 510 FeSO4 8H2SO4 2MnSO4
  K2SO4 5Fe2(SO4)3 8H2O
• (4) CaO
• CaO 2HCl CaCl2 H2O
• CaCl2 H2C2O4 CaC2O4 2HCl
  (excess oxalic acid)
• 2KMnO4 5 H2C2O4 3H2SO4 2MnSO4
  K2SO4 10CO2 8H2O (back tit)
• (5) Calcium gluconate
• CH2OH(CHOH)4COO2Ca 2HCl CaCl2
  2CH2OH9CHOH)4COOH
• (NH4)2C2O4 CaCl2 CaC2O4 2
  NH4Cl
• CaCl2 H2SO4 H2C2O4 CaSO4
• 2KMnO4 5 H2C2O4 3H2SO4 2MnSO4
  K2SO4 10CO2 8H2O

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Oxidation with Ce4 Ce4 e Ce3
1.7 V in 1 N HClO4 yellow
colorless 1.61 V
in 1N HNO3
1.47 V in 1N HCl

1.44 V in 1M H2SO4 Indicator ferroin,
diphenylamine Preparation and standardization Am
monium hexanitratocerate, (NH4)2Ce(NO3)6,
(primary standard grade) Sodium oxalate.
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Applications of cerimetry (1) Menadione
(2-methylnaphthoquinon vitamin K3)
HCl, Zn Reduction
2 Ce(SO4)2

• Iron
• 2FeSO4 2 (NH4)4Ce(SO4)4
  Fe2(SO4)3 Ce2(SO4)3 4 (NH4)2SO4

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Oxidation with potassium dichromate Cr2O72
14H 6e 2Cr3 7H2O Eo
1.36 V K2Cr2O7 is a primary standard. Indicator
diphenylamine sulphonic acid
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Ex. Redox titration ( hydroquinone vs
dichromate standard solution )
Cr2O72 14H 6e ? 2 Cr3 7 H2O
Eo 1.33
HO
OH
?
O
O
2H 2e Eo 0.700
3
3 HO
OH Cr2O72 8H ? 3 O
O 2 Cr3 7 H2O
Eo Eocathode Eoanode 1.33 0.700
0.63 V K 10 nEo/0.05916 10 6(0.63) /
0.05916 10 64 redox indicator
diphenylamine
colorless to violet
Very large quantitative complete reaction
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Iodimetry and iodometry

• Iodimetry
• a reducing analyte is titrated directly with
  iodine (to produce I-).
• iodometry
• an oxidizing analyte is added to excess I- to
  produce iodine, which is then titrated with
  standard thiosulfate solution.

I2 V.C? 2I-
I- Cu2? I2 Cu
I2 S2O32- ? 2I- S4O62-
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standard I3-

• 1) Iodine only dissolves slightly in water. Its
  solubility is enhanced by interacting with I-
• 2) An excellent way to prepare standard I3- is to
  add a weighed quantity of potassium iodate to a
  small excess of KI. Then add excess strong acid
  (giving pH 1) to produce I3- by quantitative
  reverse disproportionation
• 3) Cu24I-? 2CUI I2

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Stability of I2 Solutions

• In acidic solutions of I3- are unstable because
  the excess I- is slowly oxidized by air
• In neutral solutions, oxidation is insignificant
  in the absence of heat, light, and metal ions.
• At pH ? 11, triiodide disproportionates to
  hypoiodous acid (HOI), iodate, and iodide.

I2 OH- ? IO- I- H
3IO- ? IO3- 2I-
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Iodimetry
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iodometry
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Bromatimetry
BrO3 5Br 6H ? 3Br2 H2O
2I Br2 ? I2 2Br

I2 2 S2O32 ? 2I S4O62
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Addition reactions
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Determining water with the Karl Fisher
Reagent The Karl Fisher reaction I2 SO2
2H2O ? 2HI H2SO4 For the determination
of small amount of water, Karl Fischer(1935)
proposed a reagent prepared as an anhydrous
methanolic solution containing iodine, sulfur
dioxide and anhydrous pyridine in the mole ratio
1310. The reaction with water involves the
following reactions C5H5NI2 C5H5NSO2
C5H5N H2O ? 2 C5H5NHI C5H5NSO3 C5H5NSO3
CH3OH ? C5H5N(H)SO4CH3 Pyridinium sulfite
can also consume water. C5H5NSO3 H2O ?
C5H5NHSO4H It is always advisable to use fresh
reagent because of the presence of various side
reactions involving iodine. The reagent is stored
in a desiccant-protected container. The end point
can be detected either by visual( at the end
point, the color changes from dark brown to
yellow) or electrometric, or photometric
(absorbance at 700nm) titration methods. The
detection of water by the coulometric technique
with Karl Fischer reagent is popular.