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)
–
4
Fe(II)
Titration
Ce4+
Preoxidation :
2–
Peroxydisulfate ( (NH4)2S)2O8 )
Sodium bismuthate ( NaBiO3)
Hydrogen peroxide (H2O2)
Prereduction : Stannous chloride ( SnCl2)
Chromous chloride
Jones reductor (zinc coated with zinc amalgam)
Walden reductor ( solid Ag and 1M HCl)
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Reagents used in redox titration
Reducing agents
1) ammonium iron(II) sulfate hexahydrate (Mohr’s salt)
FeSO4(NH4)2SO4· 6H2O
2) iron(II) ethylene diamine sulfate (Oesper’s salt) FeC2H4(NH3)2(SO4)2· 4H2O
3) Sodium thiosulfate pentahydrate
4) Arsenic trioxide: arsenious oxide
Na2S2O3·5H2O
As2O3
5) Sodium oxalate and oxalic acid dihydarte Na2(COO)2 ,
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(COOH)2·2H2O
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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

6Na2S2O3·5H2O
 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.
or
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.
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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)2·2(NH4)2SO4· 4H2O : Cerimetry
3) Potassium dichromate K2Cr2O7
4) Iodine I2 :
:
Dichrometry
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 –
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Eo = 0.558 V
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Permanganate titration

Duration of colour in end point (30 seconds)
MnO4– + 3Mn2+ + 2H2O  5MnO2 + 4H+ K=1*1047
Stability of aqoues solution of MnO4MnO4– + 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.
Standardization by titration of sodium oxalate (primary standard) :
2KMnO4 + 5 Na2(COO)2 + 8H2SO4 = 2MnSO4 + K2SO4 + 5Na2SO4 + 10 CO2 + 8H2O
2KMnO4
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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 (mw=158.04) 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.
2)
Filter the liquid through a sintered-glass filter crucible to remove solid MnO2.
3)
Transfer the filtrate to a clean stoppered bottle freed from grease with cleaning
mixture.
4)
Protect the solution from evaporation, dust, and reducing vapors, and keep it in the
dark or in diffuse light.
5)
If in time managanese dioxide settles out, refilter the solution and restandardize it.
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Applications of permanganometry
(1) 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)4COO]2Ca + 2HCl = CaCl2 + 2CH2OH9CHOH)4COOH
(NH4)2C2O4 + CaCl2 = CaC2O4 + 2 NH4Cl
CaCl2
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2KMnO4 + 5 H2C2O4 + 3H2SO4 = 2MnSO
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4 + K2SO4 + 10CO2 + 8H2O
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Oxidation with Ce4+
Ce4+ + e = Ce3+
1.7 V in 1 N HClO4
yellow
1.61 V in 1N HNO3
colorless
1.47 V in 1N HCl
1.44 V in 1M H2SO4
Indicator : ferroin, diphenylamine
Preparation and standardization:
Ammonium 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
O
CH3
O
OH
CH3
Reduction
2 Ce(SO4)2
OH
(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
3
HO
3 HO
 2 Cr3+ + 7 H2O

OH
Eo= 1.33
O
OH + Cr2O72– + 8H+ 
O
+ 2H+ + 2e
Eo= 0.700
O + 2 Cr3+ + 7 H2O
3O
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
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Very large : quantitative : complete reaction
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colorless to
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Iodimetry and iodometry
• Iodimetry:
• a reducing analyte is titrated directly with iodine (to pr
oduce I−).
I2 + V.C→ 2I- + ……
•
• iodometry :
• an oxidizing analyte is added to excess I− to produce i
odine, which is then titrated with standard thiosulfate s
olution.
I- + Cu2+→ I2 + Cu+
I2 + S2O32- → 2I- + S4O62920310
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standard I31) Iodine only dissolves slightly in water. Its solubility is en
hanced by interacting with I-
2) An excellent way to prepare standard I3- is to add a wei
ghed quantity of potassium iodate to a small excess of K
I. Then add excess strong acid (giving pH ≈ 1) to produ
ce I3- by quantitative reverse disproportionation:
3) Cu2++4I- 2CUI + I2
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Stability of I2 Solutions
• In acidic solutions of I3- are unstable because the exc
ess I− is slowly oxidized by air:
• In neutral solutions, oxidation is insignificant in the abs
ence of heat, light, and metal ions.
• At pH ≳ 11, triiodide disproportionates to hypoiodo
us acid (HOI), iodate, and iodide.
I2 + OH- ⇌ IO- + I- + H+
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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 1:3:10. The reaction with water
involves the following reactions :
C5H5N•I2 + C5H5N•SO2 + C5H5N + H2O  2 C5H5N•HI + C5H5N•SO3
C5H5N+•SO3– + CH3OH  C5H5N(H)SO4CH3
Pyridinium sulfite can also consume water.
C5H5N+•SO3– + H2O  C5H5NH+SO4H–
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
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