Thermodynamics of the dissociation of ammonium ion in seawater from 5 to 40°C

The dissociation constant of NH ₄ ⁺ and the associated thermodynamic functions H° and S° in synthetic seawaters of salinity 20.31, 35.00, and 44.55 have been determined by emf measurements of cells without liquid junction over the temperature range 5 to 40°C. Cells with hydrogen electrodes and silver-silver chloride electrodes, whose standard potentials in seawater media were determined in an earlier investigation, were used. At a given temperaturet (in °C), pK _a varies linearly with the formal ionic strengthI _f (uncorrected for ion pairing) according to pK _a=pK _a ^w +(0.1552–0.0003142t)I _f where pK _a ^w is the pK in pure water. The medium effect of seawater on H° amounts to less than 200 cal-mole^–1 for a change inI _f from 0 to 0.72 mole-kg^–1, and S° is less than 1 cal-°K^–1-mole^–1 in all of the media studied, as it is in water. The observed salt effect on pK _a is in close agreement with values predicted from theories of ion-ion interactions.