1. 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) – 4
Titration
Ce4+
Preoxidation : Peroxydisulfate ( (NH4)2S2O8 ) 2 – )
Sodium bismuthate ( NaBiO 3
Hydrogen peroxide (H O
Prereduction : Stannous chloride ( SnCl ) 2
Chromous chloride
Jones reductor (zinc coated with zinc amalgam)
Walden reductor ( solid Ag and 1M HCl)
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2. 2Zn (s) + Hg2+  Zn2+ + Zn(Hg) (s)
Jones reductor :
2Zn (s) + Hg2+  Zn2+ + Zn(Hg) (s)
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3. 901013

4. Reagents used in redox titration
Reducing agents
Ferrous salts :
ammonium iron(II) sulfate hexahydrate (Mohr’s salt) FeSO4(NH4)2SO4· 6H2O
iron(II) ethylene diamine sulfate (Oesper’s salt) FeC2H4(NH3)2(SO4)2· 4H2O
Sodium thiosulfate pentahydrate Na2S2O3·5H2O
Arsenic trioxide: arsenious oxide As2O3
Sodium oxalate and oxalic acid dihydarte Na2(COO)2 , (COOH)2·2H2O
Titanium trichloride TiCl3
Potassium ferrocyanide K4Fe(CN)6 · 3H2O
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5. 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)
I S2O3 2–  2I– + S4O6 2–
Indicator: soluble starch (-amylose)
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6. Standardization of thiosulfate solution:
Primary standard : potassium iodate (KIO3), K2Cr2O7, KBrO3
Titration reactions:
KIO3 + 5KI + 6HCl  3I KCl H2O
I Na2S2O3  2NaI + Na2S4O6
KIO  3I  6Na2S2O3·5H2O  6 Equivalent mw
g  6 × g
g /  1 N × 1000 ml
35.67 g  1 N × 1000 ml
a g  x N × V ml
x N = ( a g × 1 N × 1000 ml) / (35.67 g × V ml)
Stabilizer for sodium thiosulfate solution : Na2CO3
Na2S2O3 + H2O + CO2  Na2CO3 + H2S2O3
H2S2O3  H2SO3 + S
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7. Calculations  Equivalent weight = ( formula weight) / ( e– change)
Equivalents = g / eq. wt meq = mg / eq. Wt.
Normality (N) = eq / L = meq / ml
Reaction eq. wt of reactant
Fe2+  Fe3+ + e FW Fe ÷ 1
KMnO4 + 5e  Mn FW KMnO4 ÷ 5
Na2S2O35H2O  ½ S4O6– + e FW Na2S2O35H2O ÷ 1
Cr2O72 – + 6e  2 Cr FW Cr2O72 – ÷ 6
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8. Molecular model of thiosulfate ion.
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9. 16-2 Finding the end point or
A redox indicator is a compound that changes color when it goes from its oxidized to its reduced state. or
For ferroin, with E° = V
we expect the color change to
occur in the approximate range
1.088 V to V with respect SHE
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10. If the difference in the formal potential is > 0.4 V, then a redox
A redox titration is feasible if the difference between analyte and titrant
is > 0.2 V.
If the difference in the formal potential is > 0.4 V, then a redox
indicator usually gives a satisfactory end point.
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11. Starch-Iodine Complex
Starch is the indicator of choice for those procedures involving iodine because it forms an intense blue complex with iodine. Starch is not a redox indicator; it responds specifically to the presence of I2, not to a change in redox potential.
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
Structure of the repeating unit of the sugar amylose.
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12. H3AsO3 + I3– + H2O = H3AsO4 + 3I– + 2H+
Arsenious oxide, As4O6
As4O H2O = 4H3AsO3
H3AsO3 + I3– + H2O = H3AsO4 + 3I– + 2H+
The As4O6 molecule consists of an As4 tetrahedron with a bridging oxygen atom on each edge
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13. Reagents used in redox titration
Oxidizing agents
Potassium permanganate KMnO4 : Permanganometry
Ceric sulfate / Ceric ammonium sulfate Ce(SO4)2·2(NH4)2SO4· 4H2O : Cerimetry
Potassium dichromate K2Cr2O7 : Dichrometry
Iodine I2 : Iodimetry, Iodometry
Potassium iodate KIO3 : Iodatimetry
Potassium bromate KBrO3 : Bromatimetry
Sodium nitrite NaNO2 :
Calcium hypochlorite Ca(ClO)2 :
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15. Permanganate titration  
Oxidation with permanganate : Reduction of permanaganate
KMnO4 Powerful oxidant that the most widely used.
In strongly acidic solutions (1M H2SO4 or HCl, pH  1)
MnO4– + 8H+ + 5e = Mn H2 O Eo = 1.51 V
violet color colorless manganous
KMnO4 is a self-indicator.
In feebly acidic, neutral, or alkaline solutions
MnO4– + 4H+ + 3e = MnO2 (s) + 2H2 O Eo = V
brown manganese dioxide solid
In very strongly alkaline solution (2M NaOH)
MnO4– + e = MnO42 – Eo = V
green manganate  
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16. Permanganate titration  
Duration of colour in end point (30 seconds)
MnO4– + 3Mn2+ + 2H2O  5MnO2 + 4H K=1*1047
Stability of aqoues solution of MnO4-
MnO4– + 2H2O  4MnO2 (s) + 3O2 (g) +4OH-  
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17. 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) :
2KMnO Na2(COO)2 + 8H2SO4 = 2MnSO4 + K2SO4 + 5Na2SO CO2 + 8H2O
2KMnO  Na2(COO)  10 Equivalent
mw mw
g /  g /  1 Eq.
g  g
1N × 1000 ml  g
x N × V ml a g
x N = ( a g × 1N × 1000 ml) / ( g × V ml)
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18. 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.
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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19. 901013

20. Applications of permanganometry H2O2
2KMnO H2O2 + 3H2SO4 = 2MnSO4 + K2SO4 + 5O2 + 8H2O
(2) NaNO2
2NaNO2 + H2SO4 = Na2SO HNO2
2KMnO HNO2 + 3H2SO4 = 2MnSO4 + K2SO4 + 5HNO3 + 3H2O
(3) FeSO4
2KMnO FeSO4 + 8H2SO4 = 2MnSO4 + K2SO4 + 5Fe2(SO4)3 + 8H2O
(4) CaO
CaO HCl = CaCl2 + H2O
CaCl H2C2O4 = CaC2O HCl (excess oxalic acid)
2KMnO H2C2O4 + 3H2SO4 = 2MnSO4 + K2SO4 + 10CO2 + 8H2O (back tit)
(5) Calcium gluconate
[CH2OH(CHOH)4COO]2Ca HCl = CaCl + 2CH2OH9CHOH)4COOH
(NH4)2C2O CaCl2 = CaC2O NH4Cl
CaCl H2SO4 = H2C2O4 + CaSO4
2KMnO H2C2O4 + 3H2SO4 = 2MnSO4 + K2SO4 + 10CO2 + 8H2O
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21. Preparation and standardization:
Oxidation with Ce4+
Ce4+ + e = Ce V in 1 N HClO4
yellow colorless V in 1N HNO3
1.47 V in 1N HCl
1.44 V in 1M HSO4
Indicator : ferroin, diphenylamine
Preparation and standardization:
Ammonium hexanitratocerate, (NH4)2Ce(NO3)6, (primary standard grade)
Ce(HSO4)4, (NH4)4Ce(SO4)4·2H2O
Standardized with Sodium oxalate.
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23. Applications of cerimetry
(1) Menadione (2-methylnaphthoquinon: vitamin K3) O
CH3
HCl, Zn
Reduction OH
CH3
2 Ce(SO4)2
Iron
2FeSO (NH4)4Ce(SO4)4 = Fe2(SO4)3 + Ce2(SO4)3 + 4 (NH4)2SO4
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24. 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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25. Very large : quantitative : complete reaction
Ex. Redox titration ( hydroquinone vs dichromate standard solution )
Cr2O72– H e  2 Cr H2O Eo= 1.33 HO OH  O O
+ 2H e Eo= 0.700 3
3 HO
OH + Cr2O72– + 8H+  3 O
O Cr H2O
Eo= Eocathode – Eoanode = – = V
K = 10 nEo/ = 10 6(0.63) / =
redox indicator : diphenylamine
colorless to violet
Very large : quantitative : complete reaction
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26. 16-7 Methods Involving IodineIodimetry 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.
Iodine only dissolves slightly in water. Its solubility is enhanced by interacting with I-
A typical 0.05 M solution of I2 for titrations is prepared by dissolving 0.12 mol of KI plus 0.05 mol of I2 in 1 L of water. When we speak of using iodine as a titrant, we almost always mean that we are using a solution of I2 plus excess I−.
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27. Preparation and Standardization of Solutions
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.
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 I2 by quantitative reverse disproportionation:
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28. Cu2++4I- 2CUI + I2
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32. 901013

33. KBrO3 BrO3– + 5Br– + 6H+  3Br2 + H2O 2I– + Br2  I2 + 2Br–
Bromatimetry
KBrO BrO3– + 5Br– + 6H+  3Br2 + H2O
2I– + Br2  I2 + 2Br–
I S2O32–  2I– + 2S4O62–
Substitution reactions BrO3– + 5Br– + 6H+  3Br2 + H2O
2I– + Br2  I2 + 2Br– I S2O32–  2I– + S4O62–
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34. Al3+ + 3HOC9H6N  Al(OC9H6N)3 (s) + 3H+
pH 4-9
Al HOC9H6N  Al(OC9H6N)3 (s) + 3H+
hot 4M HCl
Al(OC9H6N)3 (s)  3HOC9H6N + Al3+
3HOC9H6N Br2  3HOC9H4NBr2 + 6HBr
1 mol Al3+  3 mol HOC9H6N  6 mol Br2  2 mol KBrO3
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35. Addition reactions
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36. Determining water with the Karl Fisher Reagent
The Karl Fisher reaction :
I SO H2O  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 Fischer reagent is popular.
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37. Pyridine free Karl Fisher reagent
In recent years, pyridine, and its objectionable odor, have been replaced in the Karl Fisher reagent by other amines, particularly imidazole.
(1) Solvolysis ROH + SO2  RSO3– ROH2+
Buffering B + RSO3– ROH2+  BH+SO3R– ROH
Redox B•I BH+SO3R– + B + H2O  BH+SO4R– BH+I–
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