Iron-Selective Electrodes 1

Electroanalysis

Conductimetry

  • Measures electrical resistance of solution.
  • Indicates total ion concentration.

Potentiometry

  • Relates to electrochemical cells.
  • Allows selective response using ion-selective electrodes (ISE).
  • Example: pH electrode measures hydrogen ion concentration.

Amperometry

  • Involves electrolysis with a fixed potential applied.
  • Resulting current is proportional to concentration when suitable potential is chosen.

Voltammetry

  • Similar to amperometry but uses varying applied potential.
  • Enables determination of several species in the same experiment if they react at different potentials.

Ion-Selective Electrodes (ISE)

  • A type of potentiometric technique and form of electroanalysis.
  • Available in a wide variety to respond selectively to specific ions (analytes).
Examples of ISEs
  • pH probe: Detects changes in hydrogen ion concentration.
  • Other examples include:
    • CN⁻
    • F⁻
    • S²⁻
    • Cl⁻
    • NO₃⁻
    • NH₄⁺
    • K⁺
    • Ca²⁺
    • I⁻
    • NO₂⁻
    • Ba²⁺
    • Hg²⁺

Advantages of ISE

  • Cost-effective and User-friendly: Suitable for field and laboratory use.
  • Wide Concentration Range: Typical analysis range is from 10−510^{-5} to 10−110^{-1} M (few ppm to thousands of ppm).
  • Unaffected by Sample State: Performance remains consistent regardless of color or turbidity.
  • Monitoring: Useful for environmental pollution or water quality tracking.
  • Analyzing Changes: Ideal for observing shifts in ion concentration, reaction rates, or nutrient uptake.

Limitations of ISE

  • Ionic Strength: Must be similar across all solutions for accurate analysis.
  • Potential Drift: Variations in liquid junction potential; may necessitate recalibration. Reversal of standards order can mitigate this.
  • Temperature Consistency: All solutions including standards need to be at a uniform temperature.
  • Selectivity Issues: Other ions might interfere with the readings.
  • Accuracy and Precision: Generally ±10%, but can reach ±2% (1σ) under optimal conditions.

Calibration of ISE

  • Necessary to calibrate ISE before use.
  • Readings taken with a series of calibrated standards.
  • A calibration graph is plotted.
  • Concentration of unknown determined via the graph or line of best fit.

Propagation of Errors for ISE

  • The value of xx from an ISE calibration graph does not represent final concentration.
  • Calculations require derivatives of the function to propagate errors.

Error Calculation Formula

  • Generalized function representation:
    • z=f(x)z = f(x)
  • Propagation is based on variations of xx in the function.