Question

Flouride selective electrodes are created by replacing some of the La^3+ ions in a membrane made of a LaF3 crystal with Eu^2+. A. What is the process of intentionally contaminating the crystal with Eu^2+ called, how do it make the membrane selective for F^-? B. Which of the following species do you expect to interfere with this ion-selective electrode and why? Species: Na+, Cl-, S^2-

Answer 1

A. The F^- ion selective electrode is basically prepared of doping of EuF₂ in LaF₃ lattice. However there are many processes available for doping on the type of electrode or material required.

Explanation

The mismatch in lattice parameters and ionic valencies for the crystal results in ionic conductivity due to mmigration of F^- ions. As the vacancy in the lattice, allows the hopping or movement of F^- ions from one side to other. The mechanism which is reponsible for movement of ions is diffusion from a higher concentration gradient to lower concentration gradient. So when electrode is imersed in a F- solution. The F- ion is moved from solution to electrode by process of diffuion and counter positive ion is moved to other electrode. The process of diffusion continued till the chemical potential of both the sides becomes equal. and this movement results in flowing of charge through external circuit and can be measured by voltmeter.

B. The ion which can interfere with this electrode can be S2^- or Cl^-. However interference can be occur when the selectivity coefficient of these ion is high. which depends on many factors like temperature, pH, concentration of interfering ion. Similarly if we are using a cross/reference electrode of AgCl/KCl. there would be no effect of Cl ions in F^- ISE.

e.g While measuring the F^- concentration with F^- ISE, the formation of sufides or chlorides with other present ions like Ag, Fe etc. can shift the chemical euilibria of solution mixtures.

These ions can also accumulate at the surface of electrode (if present at higher concentrations) and form dielectric boundary which can stop or slow down the diffusion process.

Sulfide can destroy the selectivity of ISE by forming the stronger complex with Eu or La ions. Which results in stopage or passage of mobile F^- ions.