Manganate (VII) titrations

• In manganate(VII) titrations, MnO4- (aq) ions are reduced and so the other chemical used must be a reducing agent that is oxidised
• Manganate Titrations Procedure

  1. Standard solution of potassium manganate (VII), KMnO4 added to burette
  2. Using pipette, add measured volume of solution being analysed to conical flask - excess of dilute sulfuric acid is added
  3. Manganate(VII) solution reacts and is decolourised as it is being added. End point of the titration = the first permanent pink colour
  4. Repeat titration until obtain concordant titres
• In the Manganate titration (VII) an excess of dilute sulfuric acid is added to provide the H+ (aq) ions required for the reduction of MnO4-(aq) ions
• In the Manganate titration (VII) you do not need to add an indicator, as the reaction is self-indicating
• Manganate (VII) Titration

  The end point of the titration is judged by the first permanent pink colour, indicating when there is an excess of MnO4- ions present.
• Concordant titres are two titres that agree within + / - 0.10 cm3
• KMnO4 (aq) is a deep purple colour
• In manganate(VII) titrations, burette readings are read from the top, rather than the bottom, of the meniscus.
• Manganate(VII) titrations can be used for the analysis of many different reducing agents. Two examples are:

  • iron(II) ions, Fe2+ (aq)
  • ethanedioic acid, (COOH)2 (aq)
• Reduction half-equations of acidified manganate(VII) ions with iron (Il) ions

  MnO4- (aq) + 8 H+(aq) + 5 e- → Mn2+(aq) + 4 H2O (l)
• Oxidation half equation:

  Fe2+(aq) -> Fe 3+(aq) + e-
• Overall equation:

  MnO4 - (aq) + 8 H+ (aq) + 5 Fe2+ (aq) -> Mn2+ (aq) + 5 Fe3+ (aq) + 4 H2O(l)
• How the 2 half equations combined

  • The common link is 2 × 5 = 10
  • The reduction half-equation has been mutiplied by 2
  • The oxidation half-equation has been mutiplied by 5
  • This gives 10e is each half equation
  • The half-equations are then added and the 10e are cancelled
• Manganate(VII) titrations can be used to analyse reducing agents that reduce MnO4 - to Mn 2+
• KMnO4 can be replaced with other oxidising agents, the most commonly used being acidified dichromate(VI), H+/ Cr2O7 2-