Salt for Arcadia is a side quest given by Farengar. It is given when he is asked if there are any other practitioners of the magical arts in Whiterun. He will ask that some frost salts need to be delivered to Arcadia.
When the Frost Salts are delivered to Arcadia, she mentions that she is attempting to create a powerful love potion. She rewards the Dragonborn with three potions: a Potion of Brief Invisibility, a Potion of Enhanced Stamina and a Potion of Illusion.