1. Ch. 15: Redox Titrations
• Determination of Dissolved Oxygen in Waste Water
(Winkler Method)
• Determine what type of bacteria are present (aerobic
or anaerobic)
Applications:
Redox Titration Curves (15-1): not covered

2. Ch. 15: Redox Titrations
• Determine what type of bacteria are present (aerobic
or anaerobic)
A. Oxygen oxidizes Mn2+
to MnO2
B. MnO2 oxidizes I-
to I2
C. Titrate I2 with Thiosulfate (starch as indicator)
I2 + 2S2O3
2-
 2I-
+ S4O6
2-

3. Ch. 15: Redox Titrations
Determination of Water: Karl Fisher Method
• Karl Fisher Reagent:
• Iodine: pyridine: sulfur dioxide
• 1:3:10 dissolved in anhydrous methanol

4. Ch. 15: Redox Titrations
Determination of Water: Karl Fisher Method
I2 + SO2 + 3C5H5N + H2O 
2C5H4NH+
I-
+ C5H5N+
SO4CH3
-
Karl Fisher Reagent
Iodine reduced to I-
Sulfur oxidized to sulfate ion
• Standardize the KF reagent with known amt of water
• Titrate known volume of unknown with KF reagent

5. Ch. 15: Redox Titrations
• Sample Preparation(15-3)
– Analyte may reside in more than one oxidation state
– Convert to a single oxidation state
Treat with reductant or oxidant
– Auxiliary Reducing Agents (donates e-): pre-reduction
• Zn, Al, Cu
Auxiliary Oxidizing Agents (accepts e-): pre-oxidation
•NaBiO3
•(NH4)2S2O8

6. Ch. 15: Redox Titrations
• Prerequisites
– Reagent must react quantitatively with
analyte
– Eliminate excess of reagent
Different ways
1- column (15-6)
2- heat solution (ex. 2H2O2 = 2H2O + O2)

7. Ch. 15: Redox Titrations

8. Ch. 15: Redox Titrations
Standard Oxidants
– Potassium permanganate
– Potassium bromate
– Cerium (IV)
– Potassium dichromate
– Iodine

9. Ch. 15 Redox Titrations
• Choice:
– Strength of analyte as a reducing agent
– Rate of rxn
– Stability
– Cost
– Availability of indicator

10. Ch. 15: Redox Titrations
• Potassium permanganate
– MnO4
-
+ 8H+
+ 5e-  Mn2+
+ 4H2O (acidic)
• Very strong; intense color

11. Ch. 15: Applications of Redox Titrations
• Potassium permanganate
– MnO4
-
+ 8H+
+ 5e-  Mn2+
+ 4H2O
Very strong
Not pure enough to be a primary standard
– Sodium oxalate used as a primary standard
2MnO4
-
+ 5H2C2O4 + 6H+
 2Mn2+
+ 10CO2 + 8 H2O
Adv.: color Disadv: not stable

13. Ch. 15: Applications of Redox Titrations
• Potassium dichromate
Cr2O7
-2
+ 14H+
+ 6e-
 2Cr3+
+ 7H2O
Not as strong as permanganate
Advantages
Stable
Primary std available
Used to titrate Iron
Cr2O7
-2
+ 6Fe2+
+ 14H+
 2Cr3+
+ 6Fe3+
+ 7H2O

14. Ch. 15: Applications of Redox Titrations
• Standard Reducing agents
– Not as popular as standard oxidants
– Solution oxidize in air
– Indirect methods are commonly used for the titration of
oxidizing agents
•Sodium Thiosulfate (S2O3
2-
)
Moderately strong reducing agent
Indirect procedure used w/ I2 (I3
-
) as intermediate

15. Ch. 15: Applications of Redox Titrations
Add excess KI to slightly acidic soln of analyte
Reduction of analyte produces I2
Liberate I2 titrated with standard thiosulfate
Starch  indicator
ex. hypochlorite in bleach
OCl-
+ 2I-
+ 2H+
 Cl-
+ I2 + H2O
I2 + 2S2O3
2-
 2I-
+ S4O6
2-
1mmol OCl-
= 1 mmol I2 = 2 mmol S2O3
-2
Procedure (Iodimetric Titrations)

16. Ch. 15: Applications of Redox Titrations
• Sodium Thiosulfate Standardized via KIO3 or K2Cr2O7
IO3
-
+ 5I-
+ 6H+
 3I2 + 2H2O
I2 + 2S2O3
2-
 2I-
+ S4O6
2-
1mmol IO3
-
= 3 mmol I2 = 6 mmol S2O3
-2
via Potassium dichromate
Cr2O7
-2
+ 6I-
+ 14H+
 2Cr3+
+ 3I2 + 7 H2O
I2 + 2S2O3
2-
 2I-
+ S4O6
2-
1mmol Cr2O7
2-
= 3 mmol I2 = 6 mmol S2O3
-2

17. A solution of sodium thiosulfate was standardized by
dissolving 0.1210 g of potassium iodate (FW 214.0) in
water, adding a large xs of potassium iodide, and
acidifing with HCl. The liberated iodine required
41.64 mL of the thiosulfate solution to decolorize the blue
starch/iodine complex. Calculate the Molarity
IO3
-
+ 5I-
+ 6H+
 3I2 + 2H2O
I2 + 2S2O3
2-
 2I-
+ S4O6
2-