Hi!
When a ph probe is inserted into a solution containing the fluoride ion, the solution will be weakly basic.
The fluoride ion behaves like a weak base in aqueous solution, following the reaction that is shown below:
F⁻ + H₂O ⇄ HF + OH⁻
The OH⁻ ions generated in this reaction will make the solution weakly basic. The pKb value of the fluoride ion is 10,8 meaning that it is a weak base (Stronger bases have lower pKb values).