Potentiometric determination of fluoride

We did an experiment of potentiometric determination of fluoride ions. In several beakers we added several concentrations of NaF solutions and then we added water. We calculated with the electrode the potentials.


Then in different beakers we added again NaF solutions, but now we added [total ionic strength adjustment buffer (TISAB)](https://en.wikipedia.org/wiki/Total_ionic_strength_adjustment_buffer). We calculated again the potentials. I see that the potential values in the beakers with TISAB are bigger than those with water. Why?


I know that TISAB works in order to adjust the total ionic strength and to work against the interfering ions, but at this point why is there a difference? Does the hydroxide ion play such a big interference role such as the ferric ions, for example?


