Redox Electrode and Nernst Equation

Redox Electrode

• A redox electrode uses two different oxidation states of the same metal in the same half cell.
• Example: Fe2+ and Fe3+ are dissolved in the same container, with an inert electrode of platinum for electron transfer.

Nernst Equation

• The Nernst equation relates electrode potential with the concentration of ions.
• Reduction potential increases with the increase in the concentration of ions.
• The equation is:
• Substituting the values of R and F, we get:

Applications of Nernst Equation

• The Nernst equation can be used to calculate the equilibrium constant for a Daniel cell.
• At equilibrium, the Nernst equation can be applied to calculate the equilibrium constant.

Concentration Cells

• Concentration cells are formed when two electrodes of the same metal are dipped separately into two solutions of the same electrolyte with different concentrations.
• The solutions are connected through a salt bridge.
• Example: Two electrodes of the same metal are dipped into two solutions of the same electrolyte with different concentrations.
• There are two types of concentration cells:
  • Electrode concentration cells
  • Electrolyte concentration cells
• The EMF of a concentration cell at 298 K is given by:

Cases of Electrolysis

• Electrolysis of molten sodium chloride:
  • The reactions occurring at the two electrodes are:
• Electrolysis of an aqueous solution of sodium chloride:
  • The reactions occurring at the two electrodes are:

Batteries

• A battery is formed when Galvanic cells are connected in series to obtain a higher voltage.
• Primary batteries:
  • These batteries can be used until the active materials are present.
  • Once they get consumed, the cell will stop functioning and cannot be reused.
  • Examples: Dry cell or Leclanche cell and Mercury cell.
• Dry cell:
  • Anode: Zn container
  • Cathode: Carbon (graphite) rod surrounded by powdered MnO2 and carbon.
  • Electrolyte: NH4Cl and ZnCl2
  • Reaction:
  • The standard potential of this cell is 1.5 V and it falls as the cell gets discharged continuously.
• Mercury cells:
  • Anode: Zn – Hg Amalgam
  • Cathode: Paste of HgO and carbon
  • Electrolyte: Paste of KOH and ZnO
  • The cell potential is approximately 1.35V and remains constant during its life.
• Secondary batteries:
  • These batteries can be recharged again and again for multiple uses.
  • Examples: Lead storage battery and Ni – Cd battery.
• Lead storage battery:
  • To recharge the cell, it is connected with a cell of higher potential, and this cell behaves as an electrolytic cell.
  • The reactions are reversed, and Pb(s) and PbO2(s) are regenerated at the respective electrodes.

Fuel Cells

• A fuel cell differs from an ordinary battery in the sense that the reactants are not contained inside the cell but are externally supplied from an external reservoir.
• Example: Fuel cell used in space vehicles, where the two gases are supplied from external storages.
• In this cell, carbon rods are used as electrodes with KOH as the electrolyte.

Complete Cell for pH Measurement

• The complete cell is set up to measure pH.
• The glass electrode must be soaked in 0.1 HCl for 24 hours, followed by storing in distilled water.
• The electrode results in the pH range 2-10, and up to 14 with a special glass.

Corrosion

• Corrosion is the loss of materials as a result of chemical or electrochemical reaction with the environment.
• The corrosion process can be classified on the basis of mechanisms.
• Stress corrosion:
  • It is an electrochemical corrosion.
  • Presence of stress in a metal (due to bending) produces strains, which forms anodic part, while the remaining part acts as cathodic part.
  • Corrosion reactions:
    • At Anode: Fe → Fe2+ + 2e-
    • At Cathode: O2 + H2O + 2e- → 2OH-
    • Net reaction: Fe2+ + 2OH- → Fe(OH)2
• Soil or underground corrosion:
  • Factors influencing soil corrosion:
    • Acidity of the soil
    • Moisture and electrolyte contents
    • Presence of micro-organisms and bacterias
    • Content of organic matter
    • Texture (physical properties) of the soil
  • Classification of soil corrosion:
    • Based on the particle size, soils are classified into six types:
      1. Gravel or Sandy soils
      2. Water-logged soils
      3. Intermediate character soils
      • Oxidation (passivation):
        • Steel can be coated with an oxide film by:
          • Heating at high temperature
          • Chemical oxidation by treating steel with hot alkaline nitrate, or persulphate or perchlorate
          • Anodic oxidation by making the steel structure an anode in an electrolytic cell

Corrosion Protection

• Phosphating:
  • Steel is coated with a layer of iron phosphate by dipping in a solution containing phosphoric acid and zinc phosphate.
  • The iron phosphate film is not highly protective because it is porous, so it usually covered with paint.
• Enamels:
  • Enamels are glassy layers applied to the metal by dipping it in a suspension of powdered glass, and then the metal is heated in a stove (furnace) at high temperature.
• Cement coating:
  • It is used to coat the inner side of steel pipelines carrying water or wastewater.
• Organic coating:
  • Paints:
    • Paint consists of a film-forming substance, an organic solvent, and a pigment.
    • Before applying paint to a steel surface, the metal surface should be cleaned of oxides by sandblasting or acid pickling.
    • A primer is a paint containing a pigment such as lead oxide (Pb3O4 red lead) or zinc chromate which oxidizes the steel surface and inhibits its corrosion.
  • Lacquers:
    • A lacquer consists of a thermoplastic polymer dissolved in an organic solvent.
    • Lacquers can be used to line steel tanks holding corrosive chemicals such as acids.
  • Coal tar:
    • Coal tar is a brown or black liquid of extremely high viscosity.
    • It is used to protect underground structures.
  • Temporary coating:
    • It is used to protect metallic structures during shipping and storage by coating the structure with a layer of lubricating oil which can be removed by an organic solvent when the structure is put to service.

Corrosion Inhibitors

• A corrosion inhibitor is a chemical compound that can be added to liquids or gases to decrease the corrosion rate of a given material (usually a metal).
• One method for the inhibition of corrosion is the addition of a coating on the surface of the metal which acts as a passivation layer and disallows access to the surface of the metal.
• Types of corrosion inhibitors:
  • Inorganic inhibitors:
    • Cathodic inhibitors:
      • Examples: Sulfite and bisulfite ions which can react with oxygen to form sulfates.
    • Anodic inhibitors:
    • Mixed inhibitors:
  • Organic inhibitors:
  • Green inhibitors:
  • Vapor phase inhibitors:
  • Volatile inhibitors:
  • Contact inhibitors:
  • Barrier inhibitors:
  • Active inhibitors:
  • Passive inhibitors:
  • Cathodic protection:
    • It is a technique used to prevent corrosion by making the metal act as a cathode.
    • It is used to protect underground structures, ships, and other structures.