The determination of fluoride is a classic one. It was the only experiment we did on crystalline membrane ISEs. (We also used TISAB, a helpful solution for the experiment, to make more accurate measurements). I didn't even mention crystalline membrane ISEs...
There have been many discussions around fluoride use in water hahahhaha. Here in Europe it's not present. But as far as I know, fluoride is still placed on other common products, like table salt.