Ammonia permeabilities of perfluorosulfonic membranes in various ionic forms

Perfluorosulfonic membranes like Nafion are much more permeable to ammonia than to hydrogen or nitrogen. This permeability is a strong, complex function of temperature. At room temperature, this membrane has a selectivity about three thousand and an ammonia flux about five times greater than that through polyvinylammonium thiocyanate, the other known highly ammonia-selective membrane. This flux depends on the membrane's counterion in the following order: H⁺ > Na⁺ > Ag⁺ NH⁺₄ > Zn²⁺ > Li⁺ > Cu²⁺ > K⁺. As the temperature rises, the ammonia flux and selectivity both decrease, much as they do in the thiocyanate-based membrane. Flux and selctivity pass through a minimum around 150°C. At higher temperatures, the ammonia flux rises and the membrane remains up to sixty times more permeable to ammonia than to nitrogen. The flux now depends on the membrane's counterion in a different order: Ag⁺ > H⁺ > Na⁺ > Li⁺ > Cu²⁺ > NH⁺₄. The variation of flux with ammonia pressure implies that the transport mechanism is different at lower and at higher temperatures.