AALSO Water Quality Level 1 Practice Test 2026 - Free Water Quality Practice Questions and Study Guide

Ammonia exists mainly in two forms in water: uncharged ammonia (NH3) and its ionized form (NH4+). The toxicity is tied to this balance. NH3 is uncharged and can easily cross cell membranes, including gills and other tissues, where it disrupts cellular processes and is more harmful. NH4+ is charged, so it doesn’t diffuse as readily into cells, making it much less toxic. The proportion of NH3 versus NH4+ depends on pH (and temperature); at lower pH more NH4+ forms, reducing toxicity, while at higher pH more NH3 forms, increasing toxicity. Nitrate and nitrite are different nitrogen species and aren’t forms of ammonia, though nitrite can also be toxic. So the least toxic form among the options is ammonium (NH4+).