The question of whether ammonium hydroxide is an acid or a base can be answered directly: it is a base. This substance, more accurately described as an aqueous solution of ammonia, increases the concentration of hydroxide ions (\(\text{OH}^-\)) when dissolved in water. This increase is the chemical hallmark of a basic solution. Understanding this requires exploring the fundamental definitions of acids and bases, applying them to ammonia’s behavior, and clarifying a common nomenclature issue.
Defining Acids and Bases
The classification of chemical substances relies on two primary frameworks. The Arrhenius definition, which is the most straightforward, focuses on ions produced when dissolved in water. An Arrhenius acid increases the concentration of hydrogen ions (\(\text{H}^+\)), while an Arrhenius base increases the concentration of hydroxide ions (\(\text{OH}^-\)).
The Brønsted-Lowry theory is a more expansive framework defined by the transfer of a proton (\(\text{H}^+\)). A Brønsted-Lowry acid is a proton donor, and a Brønsted-Lowry base is a proton acceptor. This definition is broader because a substance can act as a base by accepting a proton, even if it does not contain a hydroxide ion in its original formula.
The True Nature of Ammonium Hydroxide
The term “ammonium hydroxide” (\(\text{NH}_4\text{OH}\)) is the traditional name, but it is chemically misleading because the compound \(\text{NH}_4\text{OH}\) does not exist as an isolated, stable molecule. When ammonia gas (\(\text{NH}_3\)) is dissolved in water, the resulting liquid is more accurately called an aqueous solution of ammonia, often denoted as \(\text{NH}_3(\text{aq})\). This nomenclature distinction reflects the actual composition of the solution.
The vast majority of the dissolved ammonia molecules remain as unreacted \(\text{NH}_3\) in the water. Only a small fraction reacts with water molecules to form the ammonium (\(\text{NH}_4^+\)) and hydroxide (\(\text{OH}^-\)) ions. The popular name “ammonium hydroxide” is used because the presence of the hydroxide ion gives the solution its characteristic basic, or alkaline, properties.
Classification as a Weak Base
The classification of aqueous ammonia as a base is confirmed by its ability to generate hydroxide ions in water. Specifically, the ammonia molecule acts as a Brønsted-Lowry base by accepting a proton (\(\text{H}^+\)) from a water molecule. This proton acceptance converts the water molecule into a hydroxide ion, \(\text{OH}^-\), thereby increasing the \(\text{OH}^-\) concentration in the solution.
The chemical process is an equilibrium reaction, meaning it proceeds in both the forward and reverse directions simultaneously, represented by the formula \(\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-\). In this reaction, the ammonia molecule accepts a proton from water to become the ammonium ion (\(\text{NH}_4^+\)), leaving behind the hydroxide ion. This continuous back-and-forth reaction is the defining characteristic of a weak base.
The solution is classified as a weak base because the equilibrium strongly favors the reactants, meaning only a small percentage of the ammonia molecules react with water at any given time. This incomplete reaction limits the overall concentration of hydroxide ions produced, resulting in a lower \(\text{pH}\) compared to a strong base like sodium hydroxide, which dissociates completely.
The extent of this partial reaction is quantified by the base dissociation constant (\(\text{K}_b\)), which for ammonia is approximately \(1.8 \times 10^{-5}\). This small \(\text{K}_b\) value confirms the limited degree of ionization, distinguishing it from strong bases that fully ionize.
Practical Applications of Ammonia Solution
The basic properties of aqueous ammonia make it a valuable and widely used substance in both household and industrial settings. In the home, it is a common ingredient in many cleaning products, frequently labeled as household ammonia. Its alkaline nature is particularly effective at hydrolyzing, or breaking down, grease, fats, and organic grime, making it useful for cleaning glass, porcelain, and oven interiors.
Industrially, over 80% of all produced ammonia is utilized in the agricultural sector, primarily as a source of nitrogen for fertilizers. The solution provides an essential nutrient that supports plant growth and increases crop yields. It is also used in chemical manufacturing as a raw material for producing compounds like plastics, dyes, and pharmaceuticals.
The solution is also employed in large-scale water treatment facilities to neutralize acidic waste streams and to form chloramines, which are used as disinfectants in municipal water supplies. Its ability to adjust the \(\text{pH}\) level is leveraged in industrial fermentation processes and in the treatment of flue gas to reduce nitrogen oxide emissions.