Apparent nitrous acid dissociation across environmentally relevant temperatures in freshwater and seawater

Nitrite is a ubiquitous compound found across aquatic systems and an intermediate in both the oxidative and reductive metabolisms transforming fixed nitrogen in the environment. Yet, the abiotic cycling of nitrite is often overlooked in favor of biologically mediated reactions. Here we quantify the apparent acid dissociation constant (pKa) between nitrous acid and nitrite in both freshwater and seawater systems across a range of environmentally-relevant temperatures (5–35 ºC) using potentiometric-based titration. We find substantial effects of both salinity and temperature on the pKa, with colder and fresher water manifesting higher values and thus a greater proportion of protonated nitrite at any given pH. Because nitrous acid is unstable and decomposes to nitric oxide, a toxic free radical gas but also a potential antioxidant and biological signaling compound, the implications for the nitrous acid pKa on ecosystem function are broad.