Specific Sensors: Potentiometric Ion-Selective Electrodes Quiz

Unlike metallic electrodes that rely on redox reactions, ISEs develop a potential due to a selective ion exchange process or a phase boundary potential across a semi permeable membrane.

The Nikolsky Eisenman equation is an extension of the Nernst equation. It accounts for the electrode response to both the primary ion and interfering ions using selectivity coefficients.

For a glass electrode to function, the surface must be hydrated. This creates a thin gel layer where ion exchange between sodium or lithium ions in the glass and hydrogen ions in the solution can occur.

This is true. A smaller K value means the electrode is less sensitive to the interfering ion. For example, if K is 0.001, the electrode is 1000 times more sensitive to the primary ion.

According to the Nernst equation, a tenfold change in activity (one decade) for a monovalent ion results in a potential change of 59.2 millivolts at standard temperature.

Ionophores are ion carriers (like Valinomycin for Potassium) that specifically bind to the target ion and transport it across the hydrophobic membrane, creating the potential difference.

This is false. Like all potentiometric methods, ISEs measure the activity of the free, uncomplexed ions in the solution, not the total analytical concentration.

ISEs are categorized by their membrane type: Glass (pH), Solid State (Fluoride), and Liquid Polymer (Potassium or Calcium). Paper is not a selective electrochemical membrane.

At high pH (low H+ concentration), the electrode begins to respond to sodium ions as if they were hydrogen ions, leading to a measured pH value that is lower than the true value.

This is true. This solid state electrode is one of the most successful ISEs, allowing for the direct measurement of Fluoride ions with very little interference from other anions.

TISAB ensures that the activity coefficient of the analyte remains constant across samples and standards. It also adjusts pH to prevent interference from H+ or OH- ions.

As the membrane degrades or becomes coated with proteins or fats, the response becomes sluggish and the slope of the calibration curve drops below the theoretical Nernstian value.

This is false. Response times are usually faster when moving from low to high concentrations. Moving to a lower concentration requires the ions to diffuse out of the membrane gel layer, which takes longer.

Temperature affects the Nernst slope, complexing agents reduce the free ion activity, and interfering ions add to the measured potential. The beaker material is usually irrelevant unless it leaches ions.

Calcium electrodes typically use a liquid ion exchanger (like calcium dialkyl phosphate) immobilized within a PVC (polyvinyl chloride) polymer matrix.