Redox Titrations

• The transfer of ELECTRONS (rather than protons in acid-base titrations) • An OXIDISING agent is titrated against as REDUCING agent • The COLOUR CHANGE of the reaction often provides the END POINT (without the need of an indicator) Redox titrations = self-indicating

Fe2+(aq) —– Fe3+(aq) + e- MnO4-(aq) + 8H+(aq) + 5e- — Mn2+(aq) + 4H2O(l) Overall equation: MnO4- + 8H+ + 5Fe2+ —— Mn2+ + 5Fe3+ + 4H2O

• Write balanced equation • Work out MOLES of known substance • Use MOLAR RATIO to deduce moles of unknown • Calculate REQUIRED QUANTITY

Thiosulfate solution ~ REDUCING agent • a known concentration of the solution is typically added from the burette OXIDISING agents which can be analysed with thiosulfate solution are: • Iodine, I2(aq) • Copper (II), Cu2+(aq) • Dichromate (VI), Cr2O72-(aq) • Chlorate (I), CLO-(aq)

Reduction (I2 is oxidising agent): I2(aq) (brown) + 2e- ——- 2I-(aq) (colourless) Oxidation (S2O32- is reducing agent): 2S2O32-(aq) —– S4O62-(aq) + 2e- Overall redox equation: 2S2O32-(aq) + I2(aq) —— 2I-(aq) + S4O62-(aq)

• solution DECOLOURISES as reaction proceeds STARCH: • added when colour of solution is PALE STRAW • PURPLE colour appears due to presence of I2 END POINT ~ when the purple/black/blue colour disappears as as the iodine has reacted