Why Does the Potential Become Constant Below 1x10^-6 M NaF?

[higherme]

In this potentiometry lab, serial dilutions were made from stock 0.1M solution of NaF that ranged from 1E-2M to 1E-6M. Then these solutions were measured with a fluoride ion-selective electrode for response. The question is why does the potential become constant when the concentration of NaF is below 1x10^-6 M

From the book, I found out that the fluoride electrode is made with a single crystal of an insoluble pure rare-earth fluoride such as LaF3, NdF3 or PrF3 and when the concentration of NaF is below 10^-6, the lanthanum fluroride (LaF3) contributes more fluoride ion to the solution than is originally present. so that's why the potential is constant. I don't really get this... when it says the potential is constant, does that mean the potential is equal to 0? if it is equal to 0, aren't there still some F- from the LaF3 that can contribute to the potential?

 

[nate808]

Great question! The potential becoming constant in this situation does not necessarily mean it is equal to 0. It means that the potential is no longer changing significantly with changing concentrations of NaF. This is because at concentrations below 1x10^-6 M, the amount of fluoride ions contributed by the lanthanum fluoride (LaF3) is greater than the amount of fluoride ions originally present in the solution. This results in a steady state where the potential is not significantly affected by changes in concentration. However, there may still be some contribution from the LaF3, but it is not enough to significantly alter the potential. This is why the potential is considered constant in this scenario.

It is important to note that the potential may not necessarily be 0, as there could still be some contribution from the LaF3. Additionally, other factors such as temperature and ionic strength may also affect the potential and should be taken into consideration when interpreting the results. I hope this helps clarify the concept for you. Let me know if you have any other questions!

 

[Blueshift5]

The potential becoming constant at concentrations below 1x10^-6 M of NaF is due to the equilibrium between the fluoride ions in solution and the fluoride ions in the electrode. As the concentration of NaF decreases, the number of fluoride ions available in the solution also decreases. However, the electrode is made of a pure rare-earth fluoride crystal, which can release fluoride ions into the solution to maintain the equilibrium. This is why the potential remains constant, as there is a continuous supply of fluoride ions from the electrode.

The potential being constant does not necessarily mean it is equal to 0. It means that there is no further change in potential, as the equilibrium is maintained. There may still be some fluoride ions from the LaF3 contributing to the potential, but the overall concentration of fluoride ions in the solution is too low to cause any significant change in potential. Therefore, the potential remains constant.