Q1(d). Fluoride-selective electrode

Answer in brief

A fluoride-conducting LaF₃ crystal membrane develops a potential controlled by fluoride activity.

Key points

• Eu(II) doping introduces fluoride vacancies into the LaF₃ lattice. Fluoride migration through these defects gives selective ionic conductivity.
• With constant internal fluoride activity, the measured potential is E=K-(RT/F)ln a_(F⁻), or E=K-0.05916log a_(F⁻) at 25°C.
• Measure against a reference electrode and calibrate with fluoride standards. TISAB controls ionic strength and pH and helps release fluoride from metal complexes.
• Excess acidity converts fluoride to HF; high pH introduces hydroxide interference. Operate in the recommended intermediate pH range.