NH4aq

NH4aq denotes the ammonium ion in aqueous solution, NH4+(aq). It is the solvated form of ammonium and the conjugate acid of ammonia (NH3). In water, ammonium participates in the acid–base equilibrium NH4+(aq) + H2O ⇌ NH3(aq) + H3O+(aq). The acid dissociation constant is Ka ≈ 5.6 × 10^-10, corresponding to a pKa of about 9.25 at 25 °C. This means that solutions of ammonium salts are mildly acidic, and the relative amounts of NH4+ and NH3 determine the pH.

NH4+(aq) commonly arises from dissolving ammonia in water or from dissolving ammonium salts such as ammonium chloride, ammonium sulfate, or ammonium nitrate. Most ammonium salts are highly soluble in water. Because NH4+ can convert to NH3, especially at higher pH, ammonia can volatilize from solution, which is an important consideration in applications and environmental contexts.

In solution, the ammonium ion is a small, strongly hydrated cation and does not form strong, well-defined complexes with many common anions. The NH4+/NH3 buffer system is widely used in chemistry and biology, with a pH close to the pKa when the concentrations of NH4+ and NH3 are equal.

Applications and relevance: NH4+(aq) is a major nitrogen source in agriculture (via ammonium salts and fertilizers) and is used in laboratory buffers and various synthesis and separation procedures. Safety considerations are typical for inorganic salts; handling should follow standard laboratory practices, and environmental considerations include potential nitrification of NH4+ to nitrite and nitrate by microorganisms.