Ammonia vs Ammonium
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. However, the high similarity between these two, and the use of the word ammonia more frequently as a general term for both the pure ammonia and the ammonium compounds is what brings about confusion. The basic understanding of their differences and the context of using them is looked at in this article to some detail while having a rough glance at their industrial and scientific uses.
The fundamental understanding required is that ammonia 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. This pure form of ammonia is also called anhydrous (water-free) ammonia. Ammonium on the other hand, is a positively charged ion that could exist as free ions in solution, or as an ionic salt compound forming a lattice structure with ananion, for example, ammonium chloride. The word ammonium, therefore, is generally not used as a word by itself, but followed by the 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.
Ammonia, like water, is polar (but not charged) due to its unequal electron distribution. This polarity makes it soluble in water. An important point to note would be that solubilized or aqueous ammonia is in the form of ammonium hydroxide, which, further dissociates to form ammonium ion and hydroxide ion, the dissociation of which depends on the temperature and pH of the solution (dissociation increasing with the increase in temperature and the decrease in pH).
Ammonia gas is colorless and has a sharp, intensely irritating smell. Ammonium ions do not have characteristic smells; however, ammonium salts when in aqueous solution, with slow dissociation give odors characteristic of ammonia.
Looking at their uses, although ammonia is the general word used when talking about fertilizers, explosives, cleaning solutions, detergents, dyes, etc., these products do not contain ammonia in its pure anhydrous form, but rather as derivatives, as ammonium compounds; ammonium nitrates, ammonium hydroxides and other ammonium salts. The common use of ammonia in its pure form in our daily life, other than to convert it to the above ammonium compounds, is as a cooling refrigerant in its liquid state due to its very low melting and boiling temperatures.
Another point of confusion is the toxicity of ammonia and ammonium ions. Ammonia is toxic whereas, free ammonium ions by themselves are not (though they can be converted by certain bacteria in aquatic environments to nitrates, which in turn are toxic). What has to be clearly understood is that ammonia is toxic because of the formation of ammonium hydroxide when it dissolves in the moist inner linings of tracts. This ammonium hydroxide is what is caustic due to its alkalinity. Therefore, for excretion, the ammonia formed by animals and birds has to be converted to a lesser toxic substance as like urea and uric acid.
Finally, remember that ammonia is a molecule by itself and has its own set of characteristics. However, ammonium ions form compounds with other anions; therefore, their characteristics depend on both the parent compounds and the degree of dissociation of the compound.
Difference Between Ammonia and Ammonium
• 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.
• Ammonium hydroxide solution is also called aqueous ammonia.
• The “ammonia” present in fertilizers, cleaning solutions, detergents, dyes, etc. are actually derivative ammonium compounds; however, anhydrous ammonia in its pure form is used as cooling refrigerants.
• Ammonia is a toxic gas, but free ammonium ions by themselves are not.
• Ammonia has a set of characteristics by itself, but ammonium compounds’ characteristics depend on the associated anion, as well.