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 thats 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?