Dissociation constants of carbonic acid in seawater as a function of salinity and temperature

Potentiometric measurements of the stoichiometric constants on the seawater pH scale for the dissociation of carbonic acid in seawater (K1 = [H+][HCO3−]/[CO2] and K2 = [H+][CO32−]/[HCO3−]) have been made as a function of salinity (1 to 50) and temperature (0 to 50 °C). The results have been fitted to the equations (T/K) The value of the adjustable parameters Ai, Bi and Ci for pK1 are given by (σ = 0.0054 and N = 466)A1=13.4191S0.5+0.0331⁢S−5.33⁢E−05S2B1=−530.123S0.5−6.103⁢SC1=−2.06950S0.5. The values of pKi0 in pure water are taken from the early work of Harned and Davis (1943) and Harned and Scholes (1941) For pK2 the parameters are given by (σ = 0.011 and N = 458)A2=21.0894S0.5+0.1248⁢S−3.687⁢E−04S2B2=−772.483S0.5−20.051⁢SC2=−3.3336S0.5. The values of pK1 and pK2 determined in this study are in good agreement with the seawater (SW) measurements of Mehrbach et al. (1973) and Mojica-Prieto and Millero (2002) from S = 15 to 45 and 0 to 40 °C. The values of pK1 near S = 35 are also in reasonable agreement with the measurements in artificial seawater (ASW) of Goyet and Poisson (1989) and Roy et al. (1993) from 0 to 35 °C. The values of pK2 in real seawater, however, do not agree with the measurement made in artificial seawater at temperatures above 5 °C. Calculations of pK1 and pK2 near 25 °C using an ionic interaction model (Millero and Roy, 1997) suggest that the pK2 results in SW are more reliable than in ASW. The equations from this study should be valid from S = 0 to 50 and t = 0 to 50 °C for most estuarine and marine waters (check values at S = 35 and t = 25 °C are pK1 = 5.8401 and pK2 = 8.9636).