Isolation of 4-methoxybenzoic acid

OK, Let's isolate 4-methoxybenzoic acid* from the solution which contains it.

• A basic aqueous solution contains the salt* of 4-methoxybenzoic acid, sodium 4-methoxybenzoate*.
• You want to change sodium benzoate (salt of an organic acid) to the water-insoluble 4-methoxybenzoic acid (organic acid) by converting the salt to a protonated, non-ionic* form.

Which solution will you add?

Organic Web Chem University of Alberta