This document discusses redox titration methods. It describes the Winkler method for determining dissolved oxygen in waste water and determining whether bacteria present are aerobic or anaerobic. The Karl Fischer method for determining water content is also outlined, using iodine, sulfur dioxide, and pyridine dissolved in methanol to quantitatively reduce iodine in the presence of water. Common oxidizing agents used in redox titrations include potassium permanganate, potassium bromate, cerium(IV), and potassium dichromate. Sodium thiosulfate is also described as a moderately strong standard reducing agent often used in indirect iodometric titrations to determine oxidizing agents.

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-