Coulometry

• what are the factors affecting the potential of an electrochemical cell in the presence of current?

  IR Drop and Polarization
• In galvanic cell at equilibrium, there is no ohmic potential because I = 0
• How can ohmic potential be minimized?

  • By adding support electrolyte (high ionic strength)
  • Using 3-electrode setup where current flows between the WE and 3rd AE
• Current causes the potential to be much larger than the thermodynamic potential when operating an electrolytic cell.
• What is the difference between equilibrium potential and the actual potential?

  Overvoltage
• What are the Polarization sources?

  • Concentration polarization - rate of mass transport to electrode is insufficient to maintain current
  • Kinetic polarization - magnitude of current is limited by the rate of the electrode reactions
  • Adsorption/desorption
• 1. it is the site of electron transfer between the electrode and the reactive species
  2. Ions must be transported towards this layer a constant rate to maintain a steady current

  Hemholtz layer
• 1. It is the transfer of ions/molecules from higher [C] to lower [C]
  2. Ultimately leads to the disappearance of the gradient

  Diffusion
• Rate of diffusion is reciprocal to the concentration difference
• In diffusion, as the applied potential becomes more and more negative, the [A] near the electrode surface becomes smaller and smaller.
• The electrode is said to be completely polarized since its potential can be varied widely without affecting the current

  Limiting current region
• 1. An electrostatic process by which ions moves under the influence of an electric field (attraction)
  2. Undesirable in most electroanalytical methods = minimized by using a supporting electrolyte

  Migration
• Rate of migration increases as electrode potential increases.
• 1. Reactants are transferred to or from an electrode by mechanical means
  2. Has 2 types: Forced and Natural

  Convection
• 1. Uses stirring or agitation
  2. Decrease thickness of diffusion layer or decreases concentration polarization

  Forced convection
• Uses temperature or density gradients


  Natural convection
• As temperature increses, [P] decreases, Ionic strength increases, electrostatic forces decreases, rate of diffusion increases
• Factors that affect concentration polarization

  1. Raising temperature increases rate of diffusion and decreases [P]
  2. Mechanical stirring is very effective in transporting species through the cell
  3. Increasing ionic strength decreases the electrostatic forces between ions and the electrode
  4. The greater the electrode surface area, the more current can be passed without polarization
• measures at 0 current and 0 polarization

  Potentiometry
• 1. Measures at minimal polarization
  2. All analytes are consumed

  Coulometry
• 1. Measures at complete polarization
  2. Minimimal analytes are consumed

  Voltammetry
• 1. Usually for gases
  2. The magnitude of the current is limited by the rate of one or both of the electrode reactions rather than the rate of the mass transfer
  3. Negligible for deposition/dissolution reactions of metals but significant for transition metals
  4. It decreases with increasing temperature and decreasing current density
  5. Also causes the potential of a galvanic cell to be smaller than calculated from the Nernst equation and the IR drop


  Kinetic Potential
• Coulometric methods

  1. Electrogravimetry
  2. Potentiostatic - uses 3 electrode system
  3. Amperostatic - used 2 electrode system
• The selectivity of electrolytic methods

  1. Feasibility of separation and determination of several ions can be derived from computations based on the standard reduction potentials of the species of interest.
  2. Should also consider the effects of IR drop and overpotential.
  3. A three-electrode setup is most commonly employed to prevent inconsistencies caused by two-electrode systems.
• Electrogravimetry could be in two forms:

  1. Cathodic deposition - analyte is reduced
  2. Anodic deposition - analyte is oxidized
• For electrogravimetric without potential controls, you need to apply very negative electrode relative to SRP to induce electrolytes reaction
• Nature of electrolytic deposit:

  • Dense
  • Smooth
  • Strongly adherent to the electrode
  • No powdered or flakey
• Metallic deposits that are powdery and/or flakey are likely to be less pure and less adherent
• Factors affecting the physical property of the deposit

  • Current density - should be small (<~0.1A/cm^2); large surface area
  • Complexing agents - many metals form better deposits when electrolyzed from solutions in which their ions are complexed (ammonia and CN- often used) = improves quality of ppt
  • Stirring - improves quality of deposit
  • Temperature - unpredictable/determine empirically
• Other practices of electrolytic deposition

  • Addition of selective complexing agents
  • Stabilize other metal ions
  • Shifts reduction potential to a more negative value
  • Addition of substances that reduced at a potential less negative than H
• • A chemical that is easily reduced or oxidized
  • Helps maintain the potential of the WE at a relatively small and constant value
  • Prevents interfering that would occur under more reducing or oxidizing conditions

  Depolarizer
• • Preferred for only one species in solution
  • Current and IR decrease steadily with time; cathode potential shifts negatively to offset decrease in IR drop
  • Advantage when analyte is almost pure

  Application of electrogravimetry without potential control
• maintains the working electrode potential at a constant value relative to a reference electrode automatically

  Potentiostats
• attached to a high impedance digital voltmeter

  Control circuit WE-RE
• • Current flows between WE and CE
  • CE has no effect on the WE reaction
• Eapplied has to be decreased with time to maintain Ecell constant with respect to RE
• Hg cathode as WE:

  • Useful for removing easily reduced elements as a preliminary analytical step
  • Amalgam formation
  • Large overvoltage of Hg prevents H2 generation
• WE:

  typically Pt
• • Preferred for solutions with more than one electroactive species; potential is adjusted on the plateau of the voltammetric wave corresponding to the metal ion being deposited
  • It is possible to eliminate interferences due to more than one deposited metal

  Application of gravimetry with potential control
• Mixed solution (usually metals + Hg electrode)

  Amalgam