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)pKi−pKi0=Ai+Bi/T+Ciln⁢T.The values of pKi0 in pure water are taken from the early work of Harned and Davis (1943) and Harned and Scholes (1941)pK10=−126.34048+6320.813/T+19.568224⁢ln⁢TpK20=−90.18333+5143.692/T+14.613358⁢ln⁢T.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−05S2A1=13.4191S0.5+0.0331⁢S−5.33⁢E−05S2B1=−530.123S0.5−6.103⁢SB1=−530.123S0.5−6.103⁢SC1=−2.06950S0.5.C1=−2.06950S0.5.For pK2⁎ the parameters are given by (σ = 0.011 and N = 458)A2=21.0894S0.5+0.1248⁢S−3.687⁢E−04S2A2=21.0894S0.5+0.1248⁢S−3.687⁢E−04S2B2=−772.483S0.5−20.051⁢SB2=−772.483S0.5−20.051⁢SC2=−3.3336S0.5.C2=−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).