Electrochemistry (Redox Titration - Voltammetry)

• Redox Titration - type of titration in which the Ecell are monitored
• Redox Titration Indicators

  A) Iodine (I2)

  B) Starch

  C) colorless to blue

  D) Na2S2o3 (Sodium Thiosulfate)

  E) Starch

  F) Blue to colorless

  G) KMnO4

  H) Self indicating

  I) Colorless to purple

  J) K2Cr2O7

  K) Diphenylamine

  L) Colorless to purple

  
• Auxillary Oxidizing/Reducing Agents - used in analytes prior to titration to convert them into the desired ionic forms
• Reducing Agents:

  • Jones Reductor
  • Walden Reductor
• Jones Reductor – uses metal amalgams for prereduction of the analytes
• Walden Reductor – uses Ag for prereduction of the analytes
• Oxidizing Agents

  • Sodium bismuthate
  • Ammonium peroxydisulfate
  • Hydrogen peroxide
• Sodium bismuthate - oxidizing agent used in converting Mn2+ to MnO4 -
• Standard Reducing Agents

  • Iron (II) solution
  • Sodium thiosulfate
• Standard Oxidizing Agents

  • Potassium permanganate
  • Cerium (IV) solution
• Potentiometry - when the pair of electrodes is placed in the sample solution it shows the potential difference by the addition of the titrant or by the change in the concentration of the ions.
• Potentiometry works with these electrodes:

  • Reference Electrode
  • Indicating Electrode
• Electrodes - are mainly used to measure the voltages.
• Reference electrode – these are mainly used for the determination of the analyte by maintaining the fixed potential.
• Examples of reference electrodes:

  • Standard hydrogen electrode
  • Ag/AgCl electrode
  • Saturated calomel electrode
• Indicator electrode - it is used to measure the potential of the analyte solution comparing with that of reference electrode.
• The potential of indicator electrodes are directly proportional to ion concentration.
• Examples of Indicator electrode:

  • Hydrogen electrode
  • Glass electrode
  • Antimony-antimony electrode
• Reference electrodes have two types:

  • Primary standard electrodes
  • Secondary standard electrodes
• Primary standard electrodes

  • Standard hydrogen electrode
• Secondary standard electrodes

  • silver-silver chloride electrode
  • saturated calomel electrode
• Three classes of indicator electrodes:

  • Metal indicator electrodes
  • Membrane electrodes
  • Ion-selective electrodes
• Metal indicator electrodes:

  • First kind electrodes - metal rods
  • Second kind electrodes - metals coated with precipitates
  • Third kind electrodes - inert electrodes
• Membrane electrodes - are used to determine p-values:

  • pH
  • pCa
  • pNO3
• Voltammetry - an electroanalytical technique that gives information about the analyte. We can even measure the amount of current by varying the voltage.
• Voltammogram - change in current with the varying voltage gives the plot
• Voltammetry is a three-electrode system including:

  • Working electrode
  • Reference electrode
  • Auxiliary electrode
• Three-electrode system:

  A) Working electrode

  B) Auxiliary electrode

  C) Reference electrode

  
• Working electrode - ranges from small mercury to flat platinum disc
• Commonly used working electrode materials:

  • Gold
  • Platinum
  • Glassy carbon
• Depending on the choice of working electrode, the type of voltammetry is decided.
• Dropping Mercury Electrode - is used in Polarography technique
• Platinum electrode - Cyclic Voltammetry
• Glassy Carbon electrode - used in Linear Sweep Voltammetry
• Reference electrode - uses standard electrode where the potential is kept constant. It provides potential to the Working electrode.
• Common reference electrodes:

  • Calomel electrode
  • Ag/AgCl electrode
• Auxiliary electrode - usually a thin platinum wire. It serves merely to carry the current flowing through the cell
• Usually, redox reaction occur simultaneously at the auxiliary electrode.
• Polarography - branch of voltammetry in which a dropping mercury electrode (DME) is used as the working electrode.
• Polarography:

  • Wide cathodic potential range and a renewable surface
  • Widely used for the determination of many reducible species