11.2 Potentiometric Sensors and Ion-Selective Electrodes

Potentiometric sensors measure potential differences between electrodes, with ion-selective electrodes (ISEs) responding to specific ions. These tools are crucial for environmental monitoring, clinical diagnostics, and industrial process control, offering precise measurements of ion concentrations in various solutions.

ISEs come in different types, including membrane-based, solid-state, and gas-sensing electrodes. The Nernst equation provides the theoretical basis for their function, relating electrode potential to ion activity. Performance factors like selectivity, sensitivity, and response time determine an ISE's effectiveness in real-world applications.

Potentiometric Sensors and Ion-Selective Electrodes

Principles of potentiometric sensors

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• Potentiometric sensors measure the potential difference between two electrodes
  • Working electrode (indicator electrode) responds to the analyte concentration by generating a potential that varies with the analyte's activity
  • Reference electrode maintains a constant potential unaffected by the sample composition, providing a stable reference point for the measurement (Ag/AgCl electrode)
• Ion-selective electrodes (ISEs) are a type of potentiometric sensor designed to selectively respond to the activity of a specific ion in solution
  • Selectivity achieved through the use of ion-selective membranes or materials that preferentially interact with the target ion (K+ ISE, Ca2+ ISE)
• Applications of potentiometric sensors and ISEs include
  • Environmental monitoring: water quality assessment (pH, dissolved oxygen), soil analysis (nutrient levels)
  • Clinical diagnostics: blood electrolyte analysis (Na+, K+, Cl-), urine analysis (pH, creatinine)
  • Industrial process control: pH monitoring (wastewater treatment), food quality control (salt content, acidity)

Types of ion-selective electrodes

• Membrane-based ISEs
  • Consist of an ion-selective membrane, internal filling solution, and internal reference electrode
  • Ion-selective membrane allows selective passage of the target ion based on size, charge, or specific interactions
    • Glass membrane: pH electrode, selective to H+ ions due to the composition of the glass
    • Crystalline membrane: fluoride ISE, using a LaF3 crystal that selectively binds F- ions
    • Polymer membrane: potassium ISE, incorporating a potassium-selective ionophore in a PVC matrix
  • Potential difference across the membrane is proportional to the target ion activity, as described by the Nernst equation
• Solid-state ISEs
  • Consist of a solid-state ion-selective material in direct contact with the sample solution, eliminating the need for an internal filling solution
  • Chalcogenide glass electrodes: selective to heavy metal ions (Cu2+, Ag+) based on the composition of the glass
  • LaF3 crystal electrode: selective to F- ions due to the crystal structure and lattice defects
• Gas-sensing electrodes
  • Measure the partial pressure of a dissolved gas in solution, which is proportional to the gas concentration
  • Consist of a gas-permeable membrane, internal pH electrode, and internal reference electrode
  • CO2 electrode: measures dissolved CO2 by detecting the pH change in the internal solution
  • NH3 electrode: measures dissolved NH3 by detecting the pH change in the internal solution

Nernst equation in potentiometry

• The Nernst equation relates the electrode potential to the activity of the target ion, providing a quantitative basis for potentiometric measurements
  • $E = E^0 + \frac{RT}{zF} \ln a_i$
  • $E$: measured electrode potential (V)
  • $E^0$: standard electrode potential (V)
  • $R$: gas constant (8.314 J mol-1 K-1)
  • $T$: absolute temperature (K)
  • $z$: charge of the ion
  • $F$: Faraday constant (96,485 C mol-1)
  • $a_i$: activity of the target ion (dimensionless)
• The Nernst equation allows for quantitative determination of ion concentration by relating the logarithm of ion activity to the electrode potential
  • At 25℃, the electrode potential changes by 59.2/z mV per decade change in ion activity
  • For a monovalent ion (z=1), a tenfold increase in activity results in a 59.2 mV increase in electrode potential
• Nernstian response is essential for accurate and reliable potentiometric measurements, ensuring a predictable and reproducible relationship between ion activity and electrode potential

Performance of potentiometric sensors

• Selectivity refers to the ability of an ISE to respond preferentially to the target ion in the presence of interfering ions
  • Selectivity is quantified by the selectivity coefficient (Kij), which compares the electrode's response to the target ion (i) and the interfering ion (j)
  • Lower selectivity coefficients indicate higher selectivity for the target ion, minimizing the influence of interfering species (Kij < 1)
• Sensitivity is the change in electrode potential per unit change in analyte concentration, reflecting the electrode's ability to detect small changes in ion activity
  • Determined by the slope of the calibration curve, which plots electrode potential against the logarithm of ion activity
  • Nernstian sensitivity is 59.2/z mV per decade change in concentration at 25℃, indicating ideal electrode performance
• Response time is the time required for the electrode to reach a stable potential after a change in analyte concentration, affecting the speed and temporal resolution of measurements
  • Depends on factors such as membrane thickness (thinner membranes respond faster), sample volume (smaller volumes equilibrate faster), and stirring (enhances ion transport)
  • Faster response times are desirable for real-time monitoring applications, such as process control and dynamic systems (response times < 1 min)