Nope, it’s not nail polish. This Is what happens when you soak your feet in potassium chloride. Well, a solution of it that is. This patient was attempting to use an old remedy to resolve a chronic foot pain by soaking his foot in a solution of potassium chloride and water. He temporarily stained his toenails as you can see. Although the acid has left a black discoloration, it is only temporary and will grow out.