Question

Method. Ion-selective electrodes (ISEs) exist for many common anions and cations, and can be purchased from various manufacturers. They can then be connected to any voltmeter or pH meter with mV display and calibrated with appropriate standard solutions of that ion. The fluoride electrode is a solid-state membrane electrode composed of lanthanum fluoride (LaF3). The potential difference that is created across this membrane is logarithmically related to the concentration of the fluoride ion. In a combination electrode system, the reference electrode is built into the probe, and is similar in concept to the pH combination electrode. The reference electrodes (inside probe) are either Ag/AgCl or Hg/Hg2Cl2 (calomel) electrodes.

Hypothesize the chemical equilibrium that exists for fluoride at the interface of the electrode membrane surface and the analyte solution. Hint: read the above method section to identify what the membrane is constructed of. Then write the solubility equilibrium that exists for this compound. This equilibrium occurs stepwise, and only the first step applies.

Answer 1

In the case of the F ISE, the ion-selective membrane is a single crystal of Lanthanum Fluoride (LaF₃) doped with Europium Fluoride (EuF₂) which produces holes in the crystal lattice through which F ions can pass. When immersed in a fluoride solution and connected via a voltmeter to an AgCl/KCl external reference electrode immersed in the same solution, the negative F ions in the solution pass through the crystal membrane by normal diffusion from high concentration to low concentration until there is an equilibrium between the force of diffusion and the reverse electrostatic force due to repulsion between particles of similar charge. On the other side of the membrane there is a corresponding build-up of positive ions.

The build up of negative F ions on the inside of the membrane is compensated for by Cl ions in the internal reference solution becoming neutralised by combining with the Ag/AgCl wire, and electrons are thus forced through the external wire to the voltage measuring device (ion meter or computer interface). The other terminal of the voltmeter is connected to the Ag/AgCl wire of the external reference electrode. Here, the influx of electrons causes Ag ions in the filling solution to accept electrons and deposit on the silver wire and, consequently, Cl ions to flow out into the sample solution.

Note that, in general, depending on the concentrations inside and outside the membrane and which ion is being measured, all the reactions described above could occur in the opposite direction.