What is the Difference Between Ammonia and Ammonium?

Ammonia and ammonium are compounds that contain hydrogen and nitrogen, but they have distinct differences:

1. Structure: Ammonia has one nitrogen atom and three hydrogen atoms (NH3), while ammonium has one nitrogen atom and four hydrogen atoms (NH4).
2. Odor: Ammonia has a pungent smell, while ammonium is odorless.
3. Toxicity: Ammonia can be harmful to aquatic organisms, whereas ammonium is harmless.
4. Ionization: Ammonia is a weak base and is not ionized, while ammonium is ionized.
5. Nitrogen fixation: Although both ammonia and ammonium are sources of nitrogen fixation for plants, ammonium is primarily found in a range of salts, such as ammonium chloride, ammonium carbonate, and ammonium nitrate.

The relationship between ammonia and ammonium is driven by the chemical equation: NH3 + H2O ↔ NH4+ + OH-. The proportion of ammonia or ammonium in water depends on factors such as pH, temperature, and ionic strength. At a pH less than 6, the proportion of ammonia-N as NH3 is very low, and almost all ammonia nitrogen is present as NH4+. At a pH around 8, the proportion as NH3 is 10 percent or less, and at a pH slightly above 9, the proportion is about 50 percent.

Comparative Table: Ammonia vs Ammonium

Here is a table summarizing the differences between ammonia (NH3) and ammonium (NH4+):

Ammonia and ammonium are both compounds containing hydrogen and nitrogen, but they have different chemical properties and toxicities. Ammonia is a gas, while ammonium is an ion that exists in solution or as crystallized salt compounds. Ammonia has a lone electron pair, making it more chemically reactive and toxic to aquatic life, while ammonium lacks any lone electron pairs and is not harmful to aquatic life. The equilibrium between ammonia and ammonium depends on factors such as temperature and pH.