OK, Let's isolate 2-naphthol* from the solution which contains it.
A basic aqueous solution contains the sodium salt* of 2-naphthol, sodium 2-naphtholate*.
You want to change sodium 2-naphtholate (salt of a weak organic acid) to the water-insoluble 2-naphthol (weak organic acid) by converting the salt to a protonated*, non-ionic* form.
Which solution will you add?