Question

A fluoride-selective electrode based on a solid-state LaF3 membrane is usually operated in the aqueous solutions in pH range 5 <pH < 8 to avoid significant systematic errors. Explain what the main sources of these errors are.

Answer 1

LaF_{​​​​3} is used widely as fluoride-selective electrode where LaF_{​​​​3} matrix is doped with certain amount of EuF_{​​​​2} to create anions vacancies within the matrix. The electrochemical response is due to migration of F^{​​​​​​-} ions through the anion vacancies. But, the LaF_{​​​​3} membrane is operational in the aqueous solution of pH range (5-8). This is because two reasons as follows :

(a) At low pH, F^{​​​​​​-} is converted into HF (pk_{​​​​​a}​=3.17) and the electrode is insensitive.

(b) At high pH, alkaline error takes place. At high pH, considerable OH^​​- is present in the medium which has similar size as of F^{​​​​​​-} ions. Hence, OH^{​​​​​-} ions interfaces in the movement of fluoride ions and some error in the results happens.