Ammonia or Ammonium? What’s the difference?

Ammonia or ammonium? What’s the difference? In water, ammonia (NH₃) and ammonium (NH₄⁺) exist together in an equilibrium that depends on pH and temperature. As the temperature or pH rise, ammonia (NH₃) becomes more abundant. This is important because ammonia is more toxic than ammonium. The two are usually measured together in a test for total ammonia nitrogen or TAN, in mg of nitrogen per liter, which is the same as parts per million.