Electroanalysis - Ion Selective Electrodes

Electroanalysis - Ion Selective Electrodes

Probe Construction

Reference Electrode

• Completes the electrical circuit.
• Exhibits a stable potential difference, necessary for accurate readings.

Ion-Selective Electrode

• Definition: A metal wire immersed in a solution containing specific ions.
• Components:
  • Internal Solution: Contains ions specific to the electrode's selectivity.
  • Ion-Selective Membrane: Separates the internal solution from the sample. It interacts selectively and reversibly with the ion of interest.
• Membrane Properties:
  • Mechanically stable, nonporous, and water-insoluble.
  • Potential difference across the membrane is dependent on the concentration of ions in the sample.

Measurement of Potential Difference

• All other potential differences are negligible or remain constant during measurement.
• Requires a sensitive voltmeter, capable of measuring differences as small as 0.2 mV.

Membrane Types

• Various types of ion-selective membranes exist, categorized into four main types:
  • Glass Membranes
  • Sparingly Soluble Inorganic Salt Membranes
  • Liquid Membranes
  • Polymer-Immobilised Ionophore Membranes

Glass Membranes

• Example: Glass pH probe.
• Composition: Made from sodium silicate glass through fusing aluminum oxide ($ext{Al}<em>2 ext{O}</em>3$), sodium oxide ($ext{Na}<em>2 ext{O}$), and silica ($ext{SiO}</em>2$).
• Structure: Typically formed into a bulb of 0.3 to 0.4 mm thickness.

Electrode Representation Convention

• The ion-selective electrode is placed on the right, and the reference on the left.
• A single vertical line indicates the solid/liquid interface, while double lines represent the junction between two liquid phases:
  • $ext{Ag,AgCl} | ext{sat KCl} || ext{sample} | ext{membrane} | ext{0.1 M HCl} | ext{Ag,AgCl}$

Conditioning of Glass Membrane

• The glass membrane must be soaked in $0.1 ext{M HCl}$ for several hours, leading to an ion exchange that forms silanol groups ($− ext{SiO}− ext{H}^+$). This creates a hydrated layer of thickness between 5 to 500 nm.

Important Considerations

• The glass has high electrical resistance; hence the membrane should be thin to minimize interference, especially from sodium ions.
• An “alkaline error” is noted around pH 12 and above.

Sparingly Soluble Inorganic Salt Membranes

• Must be ionic conductors at room temperature and exhibit ion-exchange interactions at their solid-water interface.
• Examples: Lanthanum fluoride ($ext{LaF}<em>3$), silver sulfide ($ext{Ag}</em>2 ext{S}$), copper sulfide ($ext{CuS}$).
• Lanthanum Fluoride Electrode:
  • Configuration: $ext{Ag,AgCl} | ext{sat KCl} || ext{sample} | ext{LaF}_3 | ext{0.1M NaF/NaCl} | ext{Ag,AgCl}$
• Considerations:
  • Doping with europium fluoride ($ext{EuF}_2$) for better ionic conduction.
  • Must control interference from hydroxide ions using a buffer (pH 5.5).
  • Detects fluoride ions at a concentration range of $0.1$ to $2000 ext{ppm F}^-$.

Electrochemical Principles

Nernst Equation

• Denotes the relationship between the electrical potential and ion concentration. E = - rac{RT}{nF} ext{ln} rac{[Ox]}{[Red]} where:
  • $R = 8.314 ext{ J mol}^{-1} ext{ K}^{-1}$
  • $T = ext{temperature in K}$
  • $F = 96487 ext{ C mol}^{-1}$
  • $[Ox]$ and $[Red]$ represent oxidized and reduced ion concentrations respectively.

Liquid Junctions

• Formed via a salt bridge or porous plug, allowing for electrical conduction through diffusion of ions while preventing mixing.
• Common material includes sintered alumina ceramic, which features an open morphology to facilitate ion transport.

Liquid Junction Potentials

• Ions diffuse from areas of high to low concentration, but at varying rates.
• H+ ions diffuse rapidly using hydronium ion transfer, whereas Cl− ions diffuse slowly due to their interactions with solution molecules.
• Minimize junction potential in reference electrodes via electrolytes with similar cation and anion transport numbers, e.g., potassium chloride.

Silver/Silver Chloride Electrode

• Establishes an electroneutral solution and maintains a reversible electrochemical equilibrium:
  $ext{Ag}^+ + ext{Cl}^- + e^- <br /> ightleftharpoons ext{Ag} + ext{Cl}^−$
• This can create charge separation between the wire and solution, where equilibrium is established at the boundary between silver and silver chloride.

Creation of Porous Coating

• Electrolysis serves to develop a suitable AgCl coating layer on silver wires by immersing them in $ext{KCl}( ext{aq})$.

Types of Reference Electrodes

• Standard Hydrogen Electrode
• Calomel Electrode
• Silver/Silver Chloride Electrode:
  • Requires one concentration to maintain a constant potential difference with minimal liquid junction potential.
  • Simple, inexpensive, and non-toxic, it serves as a convenient point of reference for ion-selective electrodes.

Ion-Selective Electrode Characteristics

• Needs a fixed, stable potential difference between the metal wire and internal solution.
• Ion concentrations in equilibrium play a crucial role in the overall potential difference sensitivity.