The chemical dynamics of ammonia volatilization from aqueous solution

Abstract

Ammonia volatilization from aqueous solutions was studied by computer simulation using stagnant-films models. A version of the model which predicts a gradient in the hydrogen ion concentration through the liquid-phase diffusion barrier agreed with the rate of ammonia volatilization loss experimentally observed in carbonate-free solutions. At moderate pH the ammonia-diffusion flux in the liquid phase is augmented by ammonium ion diffusion. The existence of a pH gradient in the liquid-diffusion barrier offsets this flux augmentation and may enhance the influence of solution buffer capacity upon differential volatilization rate. The experiments used for model verification employed a small volatilizing chamber connected to an airflow system. The solution composition was continually monitored by measuring pH. At moderate pH values this technique can provide a precise determination of the ammoniacal nitrogen solution concentration. The effective thicknesses of the liquid- and gas-phase stagnant films calculated from ammonia volatilization and water evaporation rates in the chamber are similar to corresponding parameters found in larger scale wind tunnel experiments.