Describe the type of solid-state membrane that can be used for fluoride ion determinations by potentiometry?

Question

And also its operating mechanism to help clarify.

Answer 1

In a direct potentiometric measurement, a junction potential across a membrane is the value measured.  In a determination of F-, one uses a single crystal (a crystalline membrane) of LaF3--Doped with a little EuF2 to enhance conduction.  For an ion selective electrode (ISE) such as this one to work, the membrane must: have minimal solubility in the analyte solution, have some (albeit little) electrical conductivity, and selectively react with the analyte (either ion-exchange, crystallization, and complexation).  

Inside the ISE, there is some type of standard reference electrode and around that, a solution of constant concentration of F-.  On the other side of the membrane there is a solution of unknown concentration of F-.  At both interfaces of the membrane the following reaction is occurring:

LaF3<==>LaF2+ + F-

Since the concentrations of F- are different on different sides of the membrane, this above equilibrium will produce different populations of the charged species on each side--This represents a potential, a charge separation!  This potential is called a junction potential and allows for measurement of F- down to approximately 10^-6M.