Potentiometry: Principles, Electrodes, and Applications

Electroanalytical Fundamentals

• Common electroanalytical methods
• Electrochemical cell fundamentals
• Potential in electrochemical cells
  • No current: Nernst Equation
  • With current: Ohmic Resistance and Polarization

What is Potentiometry?

• Potentiometry involves measuring the electrochemical potential between two electrodes in a solution. This potential difference (voltage) is related to the concentration of ions in the solution according to the Nernst Equation.

General Principles of Potentiometry

• Reference Electrode:

  • Known, fixed potential
    • Silver/Silver Chloride (Ag/AgCl)
    • Calomel

• Indicator Electrode:

  • Sensitive to concentration of analyte
    • Metallic
    • Ion-Selective
• Measure E_cell (negligible current flows through the cell)
• Correct E_ref and E_j (Junction Potential)
• Compute analyte concentration from E_ind
• Calibration is essential!

Reference Electrodes in Potentiometry

Alkaline and Acid Errors in pH Measurement

• H⁺ is not the only cation that can bind to the glass surface.
• Weak binding of Na⁺, K⁺, etc.
• Most significant when [H⁺] or a_H+ is low (high pH)
  • Usually pH = 11-12
• Errors at low pH (Acid Error) can result in readings that are too high.
  • High proton concentration saturates binding sites.

Liquid Membrane Ion-Selective Electrodes (ISEs)

• Replace the solid-state crystal with a liquid ion-exchanger filled membrane.
• Ions impart a charge across the membrane.
• The membrane is designed to be selective for the ion of interest.

Ion-Selective Field-Effect Transistors (ISFETs)

H⁺ ions interact with ion-selective gate material → Changes surface charge at the gate → Modulates electric field between the source and the drain → Changes current flow between the source and the drain, depending on the concentration of H⁺

• ISFET pH sensors are typically faster and more robust than traditional glass electrode pH meters.
• They can be miniaturized, allowing for applications in small volumes and in situ measurements.