Pick Our Brain - March 2009

What’s the difference between ammonia and ammonium?

Recent CALFED Science Program and Interagency Ecological Program (IEP) funded research has shown that ammonium and ammonia may contribute to some of the ecological problems in the Delta. Most articles refer to ammonia in the waters of the Delta, but care needs to be taken to accurately present what is actually in the water. Ammonia is a gas that is dissolved in water much like the gases oxygen and nitrogen dissolve in water. Ammonia is quite toxic to animals living in water. The chemical formula for ammonia is NH3. Because ammonia is a gas, it is very hard to measure unless procedures for measuring gases are used.

Ammonium is an important nutrient for plants and algae and a source of energy for some bacteria. Ammonium is dissolved in water like a salt dissolves in water. The chemical formula for ammonium is NH4, and this form has a positive charge. The chemical methods used on water samples measure mainly ammonium because the water samples are filtered before analysis and the gaseous ammonia escapes to the atmosphere. It is more accurate to refer to the ammonium concentration of a water sample because this is what is measured.

Collectively, ammonium and ammonia are often referred to as total ammonia, or simply (but somewhat misleadingly) ammonia. In practice, total ammonia is rarely measured. Both ammonium and ammonia are often present in high concentrations in effluent from wastewater treatment plants that employ secondary treatment methods and in some types of agricultural run-off. It is important to be clear on the form (ammonia and ammonium) when reporting concentrations or studying toxicity.