AMMONIA 113
4. CHEMICAL AND PHYSICAL INFORMATION
4.1 CHEMICAL IDENTITY
Data pertaining to the chemical identity of ammonia are presented in Table 4-1. These data are for
ammonia in its pure gaseous state (i.e., anhydrous ammonia). Ammonia is also commercially and
commonly available as an aqueous solution; the most common commercial formulation is 28–30% NH3
(Weast et al. 1988). At this concentration, ammonia forms a nearly saturated solution in water. Data on
ammonia in aqueous solution, ammonium hydroxide, and ammonium ion are also included in Table 4-1
where appropriate.
4.2 PHYSICAL AND CHEMICAL PROPERTIES
Ammonium hydroxide is a weak base that is partially ionized in water according to the equilibrium:
NH3 + H20 º [NH4OH] º NH4
+ + OH-
The dissociation constant, Kb, is 1.774x105 at 25 °C (pKb is 4.751) and increases slightly with increasing
temperature (Weast et al. 1988). At pH 9.25 half of the ammonia will be un-ionized (NH3) and half will
be ionized (NH4
+). At pH 8.25 and 7.25, 90, and 99% of the ammonia will be ionized, respectively.
Therefore, at most environmentally significant pHs, ammonia will be largely ionized; the fraction of un-
ionized ammonia will become increasingly more important at pHs above 7. As a result, many physical
and chemical properties will be a function of pH. For example, the solubility of ammonia in water will
increase with decreasing pH. The volatility of ammonia increases with increasing pH; therefore, it
volatilizes freely from solution at high pH values. Ammonium salts such as chloride, nitrate, and sulfate
are strongly dissociated and very soluble in water (Weast et al. 1988); therefore, changes in pH will not
normally result in the formation of ammonium precipitates.
The physical and chemical properties of ammonia are presented in Table 4-2. Also included are some
chemical and physical properties of ammonia in solution. Ammonia in solution is widely available, and it
is often referred to as ammonium hydroxide and has been also historically referred to as “spirit of
hartshorn” (Windholz 1983).
