Electrochemistry: Types of Electrodes

Electrodes Classification

• Electrodes are classified into two classes:
  • Electrodes where redox reaction (electron transfer) takes place at electrode surface, e.g. metallic electrodes
  • Electrodes where charge (ions) exchange takes place at specific membrane surface, e.g. ion selective electrodes (or specific ion electrodes)

Indicator Electrodes

• Indicator electrodes used for different reactions:
  • Electrodes used for redox reactions:
    • Inert electrodes that acquire the potential of the system in which they are dipped
    • Examples: platinum or gold in the form of coils or foils or plates to expose a large surface area to the solution
  • Electrodes used for precipitimetry and complexometry:
    • Electrodes of first type (Kind or order):
      • Examples: Ag° for determination of Ag+, Cu, Pb, Cd, and Hg
      • Potential: EAg° = Eo + 0.059 log [Ag+]
    • Electrodes of second type:
      • Used for determination of anions
      • Examples: Ag° coated with a layer of AgCl immersed in Cl- sample
      • Potential: EAg° = Eo - 0.059 log [Cl-]
  • Electrodes used for neutralisation reactions:
    • Hydrogen electrode
    • Antimony electrode
    • Advantages: easy to use, cheap, and durable
    • Disadvantages: can't be used in presence of oxidising and reducing agents, and only used between pH 2-8

Standard Electrode Potential

• Standard electrode potential is the EMF produced when a metal is immersed in 1 M solution of its ions
• Eo is a quantitative measure of readiness of metal to lose electrons or gain electrons by non-metal giving its ions
• The sign of the potential is similar to the charge on the metal electrode

Electrochemical Cells

• Galvanic cells:
  • Voltalic cell: Zn°/ Zn2+// Cu°/ Cu2+
  • Concentration cell:
    • Each half cell contains the same metal and its ions, but the concentration of metal ion is different
    • Produces EMF of the cell
  • Salt bridge:
    • Permits the passage of electric current between the solutions present in the electrodes
  • Liquid junction potential:
    • Developed between the two boundaries of the junction at the two ends of the salt bridge
    • Produced due to the difference in the rates of migration of both cations and anions of the salt bridge
    • To reduce liquid junction potential, choose the electrolyte of the salt bridge that its cations and anions have nearly the same mobility

Potentiometry

• Potentiometry is a method of analysis concerned with the determination of an ion by dipping a suitable sensor in its solution
• The potential of the indicator electrode is measured relative to a reference electrode possessing a constant potential
• The concentration of the ion is determined using the Nernst equation
• When a rod of metal is dipped in a solution of its ions, it may have:
  • Tendency to lose electrons and convert to its ions (high solution pressure)
  • Tendency of metal ion to accept electrons and convert to element (high ionic pressure)
• Calculation of electrode potential:
  • E25°C = Eo + 0.059 log [Mn+] (Nernst equation)
  • Where: E25°C = electrode potential at 25°C, Eo = standard electrode potential, n = number of electrons gained or lost, [Mn+] = molar concentration of metal ion
  • E25°C is a function of ionic concentration