Module 13: Polarography, Stripping Voltammetry, Amperometry

• Voltammetry


  measurement of current after a potential applied to an electrochemical cell.
• Voltammetry


  study of current-potential relations in an electrolysis cell.
• Voltammetry

  there is minimal consumption of analyte.
• Inert Background 


  added to the solut to suppress the migration of electroactive species toward the electrodes by electrostatic attraction.
• Mercury or platinum micro-electrode


  readily polarizable
• Calomel or mercury-pool reference


  non-readily polarizable
• The diffusion current is independent of applied potential and directly proportional to the concentration of electroactive species.
• Limiting Currents


  forms the basis of quantitative analysis. Goes to zero.
• Polarizable


  ability of how easily an electrode cloud is distorted.
• Coulometry


  essentially all of the analyte is converted to another state.
• Polarography


  measurement of difference in current at a constant voltage.
• Electroactive species


  species capable of having electrical activity.
• Polarography


  Uses ILKOVIC EQUATION
• Dropping Mercury Electrode


  the most widely used electrode, and the technique involving its use is known as POLAROGRAPHY.
• Polarogram


  a plot of current flowing in the cell as a function of the applied potential.
• Residual Current


  flows in the cell caused by the reduction of trace impurities in the sample solution and by charging of the mercury drops.
• Decomposition Potential


  point reduction of an electroactive species is initiated.
• Residual Current is known as the Diffusion Current
• Half-wave potential


  the potential at which the diffusion current reaches half the limiting value.
• Amperometry


  measurement of the current flow produced by a redox reaction of the substance to be measured at an electrode held at a fixed potential.
• Amperometry


  Application: glucose, chloride, peroxide determination
• Polarography


  Applications: Quantitative and qualitative determination of metals and organic compounds at trace levels (10^-4 to 10^-8 M); relative precision 2-3%.
• Polarography


  Disadvantages: Measurements very sensitive to solution composition, dissolved oxygen and capillary characteristics.
• Amperometric titrations


  more versatile and more precise than polarography.
• Voltammetry 


  determination of organic compounds.
• Stripping methods

  • Initial step: the analyte is first deposited on a working electrode.
  • Second step: the analyte is redissolved or stripped from the working electrode.
• Anodic Stripping Methods


  the analyte is deposited by reduction and then analyzed by oxidation from the small volume mercury film or drop.
• Cathodic Stripping Methods


  the analyte is electrolyzed into a small volume of mercury by oxidation and then stripped by reduction.
• Adsorptive Stripping Voltammetry (ASV)


  lead and iron determination
• Adsorptive Stripping Voltammetry


  stripping of species spontaneously absorb on the surface of working electrode; without needing a previous electrolysis step.
• Polarography


  used for the determination of trace metals in alloys, ultra-pure metals, minerals, foodstuffs, beverages and body fluids.
• Applications of POLAROGRAPHY