Oxidizing Agents in Titrations

Titrations with Oxidizing Agents

• Potassium permanganate (KMnO4) is a widely used oxidizing agent with a strong oxidizing power (E° = 1.51V) and can act as a self-indicator.
• In acidic medium, KMnO4 is reduced to Mn2+.
• In basic medium, it is precipitated as brown MnO2.
• Potassium permanganate is standardized by titration with primary standard sodium oxalate.

Properties of Potassium Permanganate

• It is a strong oxidant with an intense violet color.
• It is not a primary standard.
• Aqueous solutions of KMnO4 are not entirely stable because they tend to oxidize water, leading to decomposition.
• The decomposition reaction is catalyzed by light, heat, acids, and manganese dioxide.

Standardization of Potassium Permanganate

• It is standardized using sodium oxalate (H2C2O4).
• The reaction is: 2MnO4- + 5H2C2O4 + 16H+ → 2Mn2+ + 10CO2 + 8H2O.

Conditions for Titrations with Potassium Permanganate

• The titration is carried out in acidic solution (1 M H2SO4) and not in HCl, because permanganate titration is not possible in the presence of chloride.
• The titration is carried out at 70°C to expel CO2 and to catalyze the reaction, and to avoid the precipitation of MnO2.
• The reaction occurs slowly even at elevated temperatures, but once Mn is formed, it acts as a catalyst for the reaction.

End Point Detection in Potassium Permanganate Titrations

• The intense purple color of MnO4- serves as an indicator.
• The reaction occurs slowly, but once Mn2+ is formed, it acts as a catalyst for the reaction.

Oxidation with Potassium Permanganate

• KMnO4 is a strong oxidant with an intense violet color.
• In strongly acidic solutions (pH < 1), it is reduced to Mn2+.
• In neutral or alkaline solutions, it is reduced to brown solid MnO2.
• In strongly alkaline solutions (2 M NaOH), it is reduced to green manganate ion (MnO42-).

Applications of Potassium Permanganate

• It is used for the determination of reducing species in acidic solutions.
• The acid used for acidification is H2SO4, and not HCl, because MnO4- oxidizes Cl- in acid medium.

Iodometry

• Iodide ion is a weak reducing agent and will reduce strong oxidizing agents.
• It is not used as a titrant mainly because of the lack of a convenient visual indicator system and the low speed of the reaction.
• When an excess of iodide (I-) is added to a solution of an oxidizing agent, I2 is produced in an amount equivalent to the oxidizing agent present.
• The I2 produced can be titrated with reducing agents, and the result will be the same as if the oxidizing agent was titrated directly.
• The titrating agent is sodium thiosulphate.
• This method is called the Iodometric method.

Example Calculations

• Examples are provided for calculating the percentage of copper in a sample using iodometric titration, standardizing a solution of Na2S2O3 using iodometric titration, and calculating the molar concentration of Ce4+ in a solution using titration with Fe2+.

Other Questions

• Accuracy: The degree of agreement between the measured value and true value.
• Equivalent weight of KCl: The equivalent weight of KCl is 74.55 g.
• pH of a solution with a concentration of OH- ions of 2.5 x 10-4: The pOH is lower than 7.
• pH of ammonium chloride (0.05 M): The pH is equal to 5.33.
• Number of moles for 200 mL of 2M H2SO4: The number of moles is 4.
• Suitable indicator for some titrations: Methyl orange is a suitable indicator while phenolphthalein is unsuitable.
• Percentage by volume for a solution prepared by adding 80 mL methanol to 300 mL of water: The percentage by volume is 21.05%.
• Volume of 0.10 M NaOH required to exactly neutralize 20.0 mL of 0.05 N H2SO4: The volume required is 10.0 mL.
• Word equation for a neutralization reaction: Base + acid → salt + water.