What happens to the toxicity of ammonia when the pH is raised?

When the pH of water is raised, the toxicity of ammonia increases. This is due to the chemistry of ammonia in aquatic environments, which exists in two forms: ammonium (NH4+) and free ammonia (NH3). At lower pH levels, the majority of ammonia is present as the less toxic ammonium ion, while at higher pH levels, the equilibrium shifts towards free ammonia, which is significantly more toxic to fish and other aquatic organisms.

As the pH exceeds 7.0, the proportion of free ammonia increases, leading to a higher overall toxicity. This is particularly critical because free ammonia can easily penetrate biological membranes and interfere with cellular functions, leading to harmful effects on the gills and overall health of aquatic organisms.

Understanding this relationship is crucial for managing water quality in aquaculture and natural water bodies, particularly in environments where pH can fluctuate significantly.