logNH3NH4

LogNH3NH4 refers to the base-10 logarithm of the ratio between ammonia and ammonium concentrations in an aqueous system. It is a dimensionless quantity defined as log10([NH3]/[NH4+]), where NH3 is unprotonated ammonia and NH4+ is the ammonium ion.

In water, the NH4+/NH3 pair follows an acid–base equilibrium: NH4+ ⇌ NH3 + H+. The standard pKa for this pair is about 9.25 at 25°C. This leads to a convenient relation with pH via the Henderson–Hasselbalch equation: pH = pKa + log10([NH3]/[NH4+]). Consequently, logNH3NH4 = pH − pKa. For example, at pH 7 and pKa 9.25, logNH3NH4 ≈ −2.25, indicating a much smaller fraction of NH3 relative to NH4+.

Temperature and ionic strength influence the pKa and therefore logNH3NH4, since pKa shifts with temperature and activity coefficients can differ from ideal behavior. In practice, logNH3NH4 is often estimated from measured pH and total ammonia (or TAN, total ammoniacal nitrogen) using the current pKa at the system’s temperature, or directly from measurements of NH3 and NH4+.

Applications of logNH3NH4 include assessing ammonia speciation, toxicity risk in aquatic environments, and modeling processes in wastewater treatment and soil chemistry. It is a useful parameter for understanding ammonia availability, volatilization potential, and the balance between the two species under varying pH and temperature conditions.