Environmental context. Quantifying ammonia concentrations in natural waters is important for our understanding of environmental processes that relate, in particular, to aquaculture toxicity and to the transfer of gaseous ammonia into the atmosphere where it plays a role in new particle formation and climate regulation. The proportion of ammonia present in natural waters is determined in part by variations in temperature and salinity. This work identifies that a previous equation for predicting ammonia concentrations over natural temperature and salinity ranges is incorrect and suggests alternative, more appropriate equations. A more accurate estimation of environmental ammonia concentrations is essential if improved estimates are to be made of the flux of ammonia into the atmosphere and the level of ammonia toxicity within aquacultures.

Abstract. The equilibrium between ammonia (NH₃) and ammonium (NH₄⁺) in aqueous solution is a function of temperature, pH and the ionic strength of the solution. Here we reveal a 30-year-old error in published work on the thermodynamics of ammonium dissociation in seawater, which has propagated throughout the literature. The work in question^[1] [K. H. Khoo, C. H. Culberson, R. G. Bates, J. Solution Chem. 1977, 6, 281] presents an incorrect expression for the variation of the acid dissociation coefficient (K_a) of ammonium with temperature and ionic strength. We detail the error and reveal that it can lead to as much as a 500% overestimation in calculated NH₃ concentration under environmental conditions. This finding is highly relevant, particularly for studies of ammonia toxicity and air–sea ammonia exchange. In addition, we recommend two expressions that better reproduce previous experimental work: (i) taken from the work of Johansson and Wedborg,^[2] and (ii) our own derivation using the dataset of Khoo et al.^[1]