ELECTROCHEMISTRY

• Electrochemistry


  Measurement of the current or voltage generated by the activity of specific ions
• Electrochemical cells


  • Consist of two half-cells and a salt bridge, which can be a piece of filter paper saturated with electrolytes
• Two types of electrochemical cells

  • Galvanic (or voltaic) cells
  • Electrolytic cells
• Galvanic cells


  Store electrical energy and operate spontaneously
• Electrolytic cells


  Require an external source of electrical energy for operation
• Galvanic cell operation

  1. Spontaneous flow of electrons from the electrode with the lower electron affinity (oxidation) to the cathode (reduction)

  2. Reaction continues until one of the chemical components is depleted

  3. Can be forced to operate in reverse direction by applying an external electromotive force
• Oxidation


  Loss of electrons by a species
• Reduction


  Gain of electrons by a species
• Oxidation-reduction (redox) reactions


  Reactions that transfer electrons from one reactant to another
• Reducing agent


  Electron donor
• Oxidizing agent


  Electron acceptor
• Oxidation-reduction titrations


  • Involve changes in oxidation number or transfer of electrons among the reacting substances
  • Use standard solutions that are either oxidizing or reducing agents
• Redox titration curves


  • Usually independent of analyte and reagent concentrations
  • Change in the equivalence-point region becomes larger as the reaction becomes more complete
• General redox indicators


  Substances that change color upon being oxidized or reduced
• General redox indicators

  • Starch/iodine solutions
• Preparing samples for redox titrations

  1. Treat the sample solution with an auxiliary reducing agent to convert all the analyte to a single oxidation state

  2. Auxiliary reagent must react quantitatively with the analyte and any excess must be readily removable
• Auxiliary reducing reagents

  • Zinc
  • Aluminum
  • Cadmium
  • Lead
  • Nickel
  • Copper
  • Silver
• Auxiliary oxidizing reagents

  • Sodium bismuthate
  • Ammonium peroxydisulfate
  • Sodium peroxide
  • Hydrogen peroxide
• Oxidizing agents


  • KMnO4
  • K2Cr2O7
  • Ce(SO2)2
  • I2
  • KIO3
  • KBrO3
• Reducing agents


  • Fe2+
  • Sn2+
  • Na2S2O3
  • As2O3
  • HgNO3
  • CrCl2
  • TiCl3 or Ti2(SO4)3
• Cell potential


  Measure of the tendency of the cell reaction to proceed toward equilibrium
• Cathode


  Electrode at which a reduction reaction occurs
• Anode


  Electrode at which an oxidation reaction occurs
• Standard hydrogen electrode (SHE)


  Universal reference electrode, a typical gas electrode
• Electrochemical techniques

  • Potentiometry
  • Polarography
  • Coulometry
  • Amperometry
  • Conductometry
• Potentiometry


  Measurement of differences in voltage at a constant current
• Polarography


  Measurement of differences in current at a constant voltage
• Coulometry


  Measurement of the amount of electricity (in terms of coulombs) at a fixed potential
• Amperometry


  Measurement of the amount of current that flows when a constant voltage is applied to the measuring electrode
• Conductometry


  Measurement of the current flow between two non-polarizable electrodes between which is known electrical potential is established