REDOX

• What is an oxidation-reduction (redox) reaction?

  Reactions where electrons are transferred
• In the reaction Cu²⁺ + Zn(s) <----> Cu(s) + Zn²⁺, what is being transferred?

  Electrons
• How can reagents act as both oxidizing and reducing agents?

  Depending on the ability to transfer electrons
• What is a REDOX titration?

  A volumetric analysis involving oxidation number change
• What is typically used as a titrant in REDOX titrations?

  Usually an oxidizing agent
• What occurs at the equivalence point in a REDOX titration?

  Concentration of oxidized and reduced forms equalizes
• What are nonspecific redox indicators?

  • Respond to electrode potential of solution
  • Different colors for oxidized and reduced forms
  • Must be weaker than analyte to react last
• What are specific redox indicators?

  • React with specific chemical species
  • Depend on concentration of specific analyte or titrant
  • Used with specific titrant or analyte
• What color complex does starch form with iodine?

  Deep blue complex
• How does KMnO₄ serve as its own indicator?

  Permanganate ion is colored, Mn²⁺ is colorless
• What is the process for sample dissolution in titration?

  • Dissolve samples in suitable solvent
  • Ensure analyte exists in a single oxidation state
  • Use pretreatment processes if necessary
• What is the purpose of a Jones reductor?

  To reduce analyte to a single oxidation state
• What is amalgamation in the context of a Jones reductor?

  Formation of amalgamated zinc
• What is the reaction for amalgamation in a Jones reductor?

  2Zn(s) + Hg²⁺ → Zn²⁺ + Zn(Hg)(s)
• What does the Walden reductor use as a reducing agent?

  Metallic silver
• How does HCl affect the oxidation of silver in a Walden reductor?

  Encourages oxidation of silver
• What are some auxiliary oxidizing agents used for oxidation in sample preparation?

  Sodium bismuthate, ammonium peroxydisulfate
• How is sodium bismuthate used in oxidation?

  Suspended in analyte solution and boiled
• What is the half-reaction for the reduction of bismuthate?

  NaBiO₃(s) + 4H⁺ + 2e^- <----> BiO⁺ + 2H₂O
• What does ammonium peroxydisulfate do in acidic solution?

  Converts Cr(III) to dichromate
• What is the half-reaction for ammonium peroxydisulfate?

  S₂O₈^2- + 4H⁺ + 2e^- <--- > SO₄^2-
• What is the half-reaction for hydrogen peroxide in acidic solution?

  H₂O₂ + 2H⁺ + 2e^- <--- > 2H₂O
• What are the requirements for REDOX titrants?

  Strong, fast, stable, selective, with indicator
• What is potassium permanganate used for in titrations?

  As a powerful oxidant in acidic solutions
• What is the half-reaction for potassium permanganate in acidic solution?

  2MnO₄^- + 5H₂C₂O₄ + 6H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O
• What is cerium (IV) used for in titrations?

  As a strong oxidizing titrant in perchloric acid
• What is the half-reaction for cerium (IV)?

  Ce⁴⁺ + e^- → Ce³⁺
• Why is iodine used as a redox titrant?

  Reacts rapidly with strong reductants
• What is the solubility of iodine in water?

  Not very soluble
• How is potassium iodide related to iodine solutions?

  Used to dissolve iodine for titration
• What is the half-reaction for potassium dichromate in acidic conditions?

  Cr₂O₇^2- + 14H⁺ + 6e^- <--- > 2Cr³⁺ + 7H₂O
• What is the half-reaction for the reduction of triiodide ion?

  I₃^- + 2e^- <--- > 3I^-
• How can iodine solutions be standardized?

  Against anhydrous sodium thiosulfate solution
• Why are iron (II) solutions not used much in direct titrations?

  They serve as titrants for excess oxidants
• What is sodium thiosulfate used for in titrations?

  To determine oxidizing agents indirectly
• What is the half-reaction for thiosulfate ion?

  S₂O₃^2- <--- > S₄O₆^2- + 2e^-
• How are thiosulfate solutions standardized?

  Against potassium iodate in KI solution
• Why are iodide solutions not suitable for direct titration?

  Due to formation of iodine
• What is the iodometric method?

  Excess iodide added, liberated iodine titrated
• What are the applications of various titrants?

  • Dichromate: Fe(II) in acidic condition
  • Sodium thiosulfate: Various oxidizing agents
  • Iodine solutions: As, Sb, HCN, H2CO
  • Permanganate or Cerium(IV): Various metals and compounds