Proton-Coupled Transport of Ammonia in Aqueous Hydrochloric Acid

Abstract

Ternary diffusion in aqueous ammonia + hydrochloric acid solutions has been measured at 25°C. It is shown that the transport coefficients can be successfully predicted from the mobilities of H⁺, Cl ^-, NH₄⁺, and un- ionized ammonia species present in the solutions. When an ammonia concentration gradient is prepared in a solution containing excess acid, the region of the solution with the higher ammonia concentration is depleted in H+ by formation of ammonium ion: NH₃+H⁺ ↔ NH₄⁺. As highly mobile H⁺ diffuses up the ammonia gradient, there is counterflow of NH₄⁺ in order to maintain electroneutrality . This mechanism leads to rapid diffusion of the ammonia component as NH₄⁺. Binary diffusion coefficients of aqueous ammonia have also been measured. Extrapolation gives 2.08(±0.01) × 10^-2m² s^-1 for the limiting diffusion coefficient of un-ionized ammonia, a value 6% greater than the limiting diffusion coefficient of the ammonium ion.