[nexisrocks]

I'm studying, and have a few questions...

The specific solid state electrode I'm studying is the fluoride ion-selective electrode, LaF₃ doped with EuF₂.  I know that the EuF₂ leaves holes in the lattice of the crystal, which F^- can come in and fill.

So... F^- moves through the 'holes' in the crystal, causing a small current.  Is it just that, for higher concentrations of F^-, more F^- goes into the crystal and moves, and thus there is a greater current?  (So that there would be a small current even with a blank, because of the internal reference solution).

And why exactly is the internal reference solution there?  Is it to give a concentration gradient across the membrane (but if that's the case, I'm not sure why you would need it, because if there was no F^- then there would still be a concentration gradient).

Or is it just that the F^- goes into the membrane and helps create a build-up of charge on each side of the membrane, which leads to a boundary potential?


For the Liquid and Polymer Membrane electrode, is it just that the ion-exchanger exchanges with the ions in solution (where it exchanges more in higher concentrations), which gives a boundary potential?  And do we have the reservoir of ion exchanger in organic solvent to replenish the porous plastic disk?

Thanks!

[Borek]

I doubt F^- diffuses through the crystal, it rather adsorbs in the holes on the surface, creating potential difference. Amount adsorbed depends on the concentration.

Internal solution... think in terms of concentration cell. At least that's the way it works for pH electrode, no reasons why this approach may fail here.

[Valdorod]

Your glass electrodes (usually pH), liquid membrane and solid state all work in slightly different manners.

Your typical solid state membrane electrode has a membrane made up of a single crystal, in the case of the fluoride electrode the crystal ismade up of LaF3.  The crystal lattice allows teh fluoride ions to move freely within an immobile framework of La ions, thus the ions do penetrate and move about the crystal lattice.  The response is specific to F and no other ions can penetrate into this crystal.  The electrodes usually have a true Nernsitian response if the concentration is greater than 10 ^-6M.  It becomes less accurate at very low concentrations by the finite solubility of the crystal.

In the case of the liquid membrane electrode, a small hydrophobic filter disk forms the barrier between an inner and outer electrolytes.  This disk is in contact with an organic solvent that is immiscible in water in the perimeter.  Thus there is an inner reference electrode in an aqueous solution surrounded by an organic solvent.  In this solvent you have a soluble salt with the ion you want to measure and a counter ion of high molecular weight whose solubility in the organic phase is much higher than in water.  The solvent is then pulled into the pores of the membrane by capillary action where it is able to make contact with the reference electrode solution and the solution you are measuring.

yes the organic solvent is just an ion exchanger and the potential is measured as the two solutions come to equilibrium for the ion that you are measuring.

However, for measuring fluoride I am not aware of a liquid membrane electrode.  The solid state for fluoride works so good because of its speed and accuracy.

Valdo