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 (K-1*=[H+][HCO3-]/[CO2] and K-2*= [H+][CO32-]/[HCO3-]) have been made as a function of salinity (1 to 50) and temperature (0 to 50 degrees C). The results have been fitted to the equations (T/K) pK(i) - pK(i)(0) = A(i) + B-i/T + C(i)lnT. The values of pK(i)(o) in pure water are taken from the early work of Harned and Davis (1943) and Flamed and Scholes (1941) pK(1)(0) = - 126.34048 + 6320.813/T + 19.568224lnT pK(2)(0) = - 90.18333 + 5143.692/T + 14.613358lnT. The value of the adjustable parameters A(i), B-i and C-i for pK(1)* are given by (sigma = 0.0054 and N = 466) A(1) = 13.4191S(0.5) + 0.0331S - 5.33E - 05S(2) B-1 = -530.123S(0.5) - 6.103S C-1 = -2.06950S(0.5). For pK(2)(*) the parameters are given by (sigma=0.011 and N=458) A(2) = 21.0894S(0.5) + 0.1248S - 3.687E - 04S(2) B-2 = -772.483S(0.5) - 20.051S C-2 = - 3.3336S(0.5). The values of pK(1)(*) and pK(2)(*) 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 degrees C. The values of pK(1)(*) 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 degrees C. The values of pK2* in real seawater, however, do not agree with the measurement made in artificial seawater at temperatures above 5 degrees C. Calculations of pK(1)(*) and pK2* near 25 degrees C using an ionic interaction model (Millero and Roy, 1997) suggest that the pK(2)* 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 degrees C for most estuarine and marine waters (check values at S=35 and t=25 degrees C are pK(1)(*)=5.8401 and pK(2)*=8.9636). (c) 2006 Elsevier B.V. All rights reserved.