What is the Difference Between Free Ammonia and Total Ammonia?

The difference between free ammonia and total ammonia lies in their chemical forms and toxicity levels in water.

• Free Ammonia (NH3): This is the unionized form of ammonia, which is a toxic, gaseous substance. Free ammonia is highly toxic to aquatic life, including fish and microorganisms in anaerobic digestion processes.
• Total Ammonia: This refers to the sum of ammonia gas (NH3) and ammonium ion (NH4+) species in water. Total ammonia is a combination of toxic ammonia (NH3) and less toxic ammonium (NH4+). The value of total ammonia is always higher than the value of free ammonia for the same water sample, as it includes the amount of ionized ammonia.

In summary, free ammonia is the unionized, toxic form of ammonia, while total ammonia is the sum of both unionized and ionized ammonia species. Test kits for ammonia levels in water, such as those used in aquariums or anaerobic digestion systems, typically measure total ammonia.

Comparative Table: Free Ammonia vs Total Ammonia

The main difference between free ammonia and total ammonia lies in their ionization states. Here is a table comparing the two:

Free ammonia is the unionized form of ammonia, while total ammonia refers to the sum of NH3 molecules and ammonium ions. Free ammonia is highly toxic to aquatic life, and any level above 0.02 mg/L (ppm) is considered harmful. In contrast, total ammonia is less toxic than free ammonia.