A chemical model of seawater including dissolved ammonia and the stoichiometric dissociation constant of ammonia in estuarine water and seawater from −2 to 40°C

Abstract

The calculation of the percentage of un-ionised ammonia in estuarine water and seawater requires values of the stoichiometric dissociation constant of ammonia, defined by: K * a/mol kg −1 = mNH 3 mH +/ mNH + 4, where m denotes molality. A thermodynamic model of seawater, including dissolved NH 3 and NH + 4, is developed using an extended Pitzer formalism parameterised from available data. The model is validated using emf measurements for cells containing artificial seawater with added HCl, and NH 4Cl, and NH 3 over a range of temperatures and salinities. Calculated values of K * a are tabulated from 0 to 40 ppt salinity and −2 to 40°C, on both a free ( mH +) and total ( mH + + mHSO − 4) hydrogen ion basis for use with pH measurements made on the corresponding scales. Accuracy (in K * a) is likely to be better than 5% at all temperatures and salinities.