Redox Titrations

• Redox titration

  oxidising agent is titrated against a reducing agent
• potassium manganate (VII) titrations

  MnO₄⁻ is the oxidising agent

  • it is reduced to Mn²⁺
  • -1 —> 2+
• potassium manganate (VII) titrations

  Iron (Fe²⁺) is the reducing agent

  • it is oxidised to Fe³⁺ (aq)
• Potassium Manganate (VII) titration

  the reaction mixture must be acidified so excess acid is added to the iron (II) ions before the reaction starts

  • acid must not react with the manganate (VII) ions
  • acid provides the protons but doesn’t oxidise in these conditions
  • acid is typically dilute sulfuric acid
• Potassium Manganate (VII) Titrations

  the titration is self indicating

  • reacts and decolourises as it is being added
  • end point is when the permanent pink colour forms
• how many times do you repeat a titration?

  until you get concordant results
• KMnO₄ is a deep purple

  • burette readings are taken from the top as it is difficult to see the bottom of the meniscus
• procedure of the Potassium Manganate (VII) Titration

  1.) prepare 250 cm³ solution of impure FeSO₄ • 7H₂O in a volumetric flask

  2.) using a pipettes measure 25 cm³ of this solution into a conical flask

  3.) add 10 cm³ of 1 moldm⁻³ of H₂SO₄ (aq) (this is in excess)

  4.) using a burette titrations the solution

  5.) determine percentage purity
• percentage purity

  mass of compound / mass of impure sample x 100
• most transition metal ions naturally change colour when changing oxidation states
• Procedure of Iodine-Thiosulfate titrations

  1.) add standard solution of Na₂S₂O₃ to a burette

  2.) prepare solution of the oxidising agent and add using a pipettes to a conical flask

  • add excess potassium iodide

  3.) titrate the solution with Na₂S₂O₃ (aq)
• Idodine-Thiosulfate titrations

  add excess potassium iodide as that is the oxidising agent that reacts the iodide ions

  • produces iodine which turns the solution a orange-brown colour
• Iodine-Thiosulfate titrations

  when titrated iodine is reduced back to I ⁻ and the brown colour fades gradually

  • this makes it difficult to see the end point
  • add starch indicator to clarify the end point
• determining the end point in the Iodine-Thiosulfate titrations

  when the end point is being approached the iodine fades to be a straw colour

  • add starch indicator at this point
  • solution turns blue-black and as the iodine reactions ( when more sodium thiosulfate is added) the blue-black colour fades
  • end point is when the blue-black colour disappears
• analysis of copper

  copper 2+ ions are produced by dissolving the compound in water
• analysis of coper

  insoluble copper (II) compounds can be reacted with acids to form copper 2+ ions

  • for alloys like brass, react with concentrated nitric acid
• why would hydrochloric acid not suitable for the redox titration?

  can be oxidised to chlorine gas (by Manganate VII ions)
• why would nitric acid not be suitable for the redox reaction?

  is an oxidising agent and may oxidise the substance being analysed
• why would ethanoic acid not be suitable for the redox titration?

  weak acid and the concentration of hydrogen ions will be insufficient
• why would concentrated sulfuric acid not be suitable for the redox titration?

  may oxidise the substance being analysed