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⁺][CO₃^2-]/[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){A formula is presented}. The values of pK_i⁰ in pure water are taken from the early work of Harned and Davis (1943) and Harned and Scholes (1941){A formula is presented}{A formula is presented}. The value of the adjustable parameters A_i, B_i and C_i for pK₁^* are given by (σ = 0.0054 and N = 466){A formula is presented}{A formula is presented}{A formula is presented}. For pK₂^* the parameters are given by (σ = 0.011 and N = 458){A formula is presented}{A formula is presented}{A formula is presented}. 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).