Oxford University

2nd/3rd Year Undergraduate Experiments in Physical Chemistry

4.04 Redox Reactions

Introduction

An electrode potential is the potential at which a species will be reduced by electrons, relative to that at which protons will be reduced. Redox potentials refer to reductions in which both reactants and products are charged species in solution.

M^y+ + ze^- = M^(y-z)+

The potential of an inert electrode in a redox solution will be determined by the relative concentrations, (strictly activities) of the charged reactants and products. The greater the relative concentration of oxidized species, the more positive will be the redox potential. The relative concentrations of the ions may be altered by titration with an oxidizing or reducing agent. The end-point of such a titration may be found from the resulting changes in emf.

In this experiment we investigate the equilibrium:

Fe (III) + e^- = Fe (II)

which is reproducible and quickly established, unlike many redox systems. The potential is measured with a Pt electrode; a standard calomel electrode completes the cell. Fe(II) ions are titrated with Ce(IV) using potentiometric measurement. The experiment is then repeated using Irn Bru®, a lemonade-type drink, as the solvent in order to investigate the behaviour of Fe(II) and Fe(III) ions in the presence of citrate ion.

Location

Upper Teaching Laboratory, bench 8.

Instructions

Instructions for this experiment are available for reference in a folder in the Lower Teaching Laboratory. You may obtain a personal copy from the technician in that laboratory, or download instructions (pdf) (7 pages, 116k, updated April 6, 2005).

Recommended references for this experiment are

• Electrode potentials by Compton & Sanders, copies of which can be found in the following libraries: BAL, BNC, CCC, EXT, HTF, Hooke, LMH, LIN, MAG, MER, NEW, PBK, ORL, QNS, RSL, SOM, STA, STC, SEH, TRI, WAD, WORC; and
• Electron transfer reactions in organic chemistry by Eberson, in HTF and RSL.

  Safety

  MSDS data for cerium (IV) sulfate
  MSDS data for ferrous ammonium sulphate, hexahydrate
  MSDS data for nitric acid
  MSDS data for sulfuric acid, dilute
  

Updated April 6, 2005.