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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
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- + S4O62-
Ch. 15: Redox Titrations
Determination of Water: Karl Fisher Method
• Karl Fisher Reagent: • Iodine: pyridine: sulfur dioxide• 1:3:10 dissolved in anhydrous methanol
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
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
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)
Ch. 15: Redox Titrations
Ch. 15: Redox Titrations
Standard Oxidants
– Potassium permanganate– Potassium bromate– Cerium (IV)– Potassium dichromate– Iodine
Ch. 15 Redox Titrations
• Choice:– Strength of analyte as a reducing agent– Rate of rxn– Stability– Cost– Availability of indicator
Ch. 15: Redox Titrations
• Potassium permanganate
– MnO4- + 8H+ + 5e- Mn2+ + 4H2O (acidic)
• Very strong; intense color
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
Ch. 15: Applications of Redox Titrations
• Potassium dichromate
Cr2O7-2 + 14H+ + 6e- 2Cr3+ + 7H2O
Not as strong as permanganate
AdvantagesStablePrimary std available
Used to titrate Iron
Cr2O7-2 + 6Fe2+ + 14H+ 2Cr3+ + 6Fe3+ + 7H2O
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 (S2O32-)
Moderately strong reducing agentIndirect procedure used w/ I2 (I3
-) as intermediate
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 thiosulfateStarch indicator
ex. hypochlorite in bleach
OCl- + 2I- + 2H+ Cl- + I2 + H2O
I2 + 2S2O32- 2I- + S4O6
2-
1mmol OCl- = 1 mmol I2 = 2 mmol S2O3-2
Procedure (Iodimetric Titrations)
Ch. 15: Applications of Redox Titrations
• Sodium Thiosulfate Standardized via KIO3 or K2Cr2O7
IO3- + 5I- + 6H+ 3I2 + 2H2O
I2 + 2S2O32- 2I- + S4O6
2-
1mmol IO3- = 3 mmol I2 = 6 mmol S2O3
-2
via Potassium dichromate
Cr2O7-2 + 6I- + 14H+ 2Cr3+ + 3I2 + 7 H2O
I2 + 2S2O32- 2I- + S4O6
2-
1mmol Cr2O72- = 3 mmol I2 = 6 mmol S2O3
-2
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 required41.64 mL of the thiosulfate solution to decolorize the bluestarch/iodine complex. Calculate the Molarity
IO3- + 5I- + 6H+ 3I2 + 2H2O
I2 + 2S2O32- 2I- + S4O6
2-