The pH of a solution will be weakly basic when a pH probe is administered into a solution comprising the fluoride ion.
The fluoride ion works like a weak base in an aqueous solution, below is the reaction that shows this:
F⁻ + H₂O ⇄ HF + OH⁻
The hydroxide ions produced in the mentioned reaction is the reason for the solution to be weakly basic. 10.8 is the pKb value of the fluoride ion, which signifies that it is a weak base, the stronger bases exhibit lower pKb values.
