The key difference between ammonia and ammonium is that ammonia is an uncharged polar molecule existing as a gas at room temperature, whereas ammonium ions are charged and exist as free ions in solution or as crystallized salt compounds.
There are few images and even certain smells that our minds immediately associate with ammonia or ammonium; these include those of fertilizers, nitrogenous wastes, soaps and even explosives. Moreover, most people assume that there is no difference between ammonia and ammonium. The high similarity between these two, and the use of the word ammonia more frequently as a general term for both pure ammonia and ammonium compounds is what brings about this confusion.
CONTENTS
1. Overview and Key Difference
2. What is Ammonia
3. What is Ammonium
4. Side by Side Comparison – Ammonia vs Ammonium in Tabular Form
5. Summary
What is Ammonia?
Ammonia is a compound of nitrogen and hydrogen with the formula NH3. It is uncharged, and a molecule by itself; it exists as a gas at room temperature and atmospheric pressure, and as a liquid at very low temperatures and high pressures. We call this pure form of ammonia anhydrous (water-free) ammonia. Ammonia gas is colourless and has a sharp, intensely irritating smell. Furthermore, it is toxic.
Figure 01: Ammonia Molecule
Ammonia, like water, is polar due to its unequal electron distribution. This polarity makes it soluble in water. An important point to note is that solubilized or aqueous ammonia is in the form of ammonium hydroxide, which further dissociates to form ammonium ion and hydroxide ion. This dissociation depends on the temperature and pH of the solution (dissociation increasing with the increase in temperature and the decrease in pH).
What is Ammonium?
The ammonium cation is a positively charged polyatomic ion with the chemical formula NH4+. This is an ion that could exist as free ions in solutions, or as an ionic salt compound forming a lattice structure with an anion; for example, ammonium chloride.
Therefore, we do not generally use the term ammonium as a word by itself; it always precedes words ‘ion,’ ‘salt,’ or the respective negatively charged ion. For example, it has to be ammonium ion, ammonium hydroxide, ammonium nitrate, ammonium sulfate, etc., and not just simply ammonium.
Figure 02: Ammonium Ion
Ammonium ions do not have characteristic smells; however, ammonium salts when in aqueous solution with slow dissociation, give odours characteristic of ammonia.
What is the Difference Between Ammonia and Ammonium?
Ammonium is the major cation that originates from ammonia. The key difference between ammonia and ammonium is that ammonia is an uncharged but polar molecule existing as a gas at room temperature, whereas ammonium ions are charged and exist as free ions in solution or as crystallized salt compounds. Furthermore, ammonia has a sharp, intensely irritating smell while ammonium ion itself does not have a characteristic smell.
When considering the toxicity, ammonia is toxic but free ammonium ions by themselves are not toxic. However, the compounds of ammonium can be toxic. In addition, ammonia has a set of characteristics by itself, but ammonium compounds’ characteristics depend on the associated anion, as well.
Summary – Ammonia vs Ammonium
In brief, ammonium is the major cation that originates from ammonia. The key difference between ammonia and ammonium is that ammonia is an uncharged but polar molecule existing as a gas at room temperature, whereas ammonium ions are charged and exist as free ions in solution or as crystallized salt compounds.
Reference:
1. “Process Steps of Ammonia Production.” Ammonia, pp. 65–176., doi:10.1002/9783527613885.ch04.
Image Courtesy:
1. “Ammonia-3D-balls” (Public Domain) via Commons Wikimedia
2. “Ammonium-3D-balls” (Public Domain) via Commons Wikimedia