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-