Solubility of ammonia in water at low concentrations

Abstract

Comparisons of ammonia concentrations in air and in rainwater indicate that a substantial discrepancy exists between observed values and those calculated on the basis of conventional solubility theory, which is based on extrapolation of solubility measurements obtained at higher concentrations. This paper presents the results of ammonia solubility measurements at low concentrations, which have been conducted in an attempt to resolve this anomaly. These results have demonstrated that existing solubility theory, which presumes dissolution to occur via an interphase transport step plus a dissociation of aqueous-phase ammonia to form ammonium ion, provides an adequate description of true behavior at environmental concentrations whenever pure water or pure water plus strong acid is utilized as a solvent. The results are presented in terms of a Henry's Law constant for the interphase transport step NH 3| g⇌ H 1 NH 3| aq which is given as log 10H 1 = −1.69 + 1477.7 T ( dimensionless) , where T is the absolute temperature (K), based on molar units for both gas- and liquid-phase concentrations. Previously-measured values of the ammonia dissociation constant were utilized with the experimental data to derive the above expression for H 1. Limited tests in the presence of carbon dioxide indicate that previous calculations of ammonia solubility in combination with carbon dioxide at atmospheric concentrations are high by about an order of magnitude. These results are not conclusive and should be verified by further experiments. At the present time, however, they suggest that a substantial restructuring of many concepts regarding the aqueous-phase behavior of environmental ammonia may be necessary.