The dissociation of carbonic acid in NaCl solutions as a function of concentration and temperature

Potentiometric measurements of the stoichiometric constants for the dissociation of carbonic acid in NaCl solutions (K-1(*) = [H+][HCO3-]/[CO2] and K-1(*) = [H+][CO32-]/[HCO3-]) have been made as a function of molality (0-6 m) and temperature (0- 50 degrees C). The results have been fitted to the equations pK(i)(*) - pK(i) = A(i) + B-i/T + C-i ln T The values of pK(i) in pure water are taken from the literature and the adjustable parameters A(i), B-i and C-i are a function of molality A(1) = 35.2911 m(0.5) + 0.8491 m - 0.32 m(1.5) + 0.055 m(2) B-1 = -1583.09 m(0.5) C-1 = -5.4366 m(0.5) A(2) = 38.2746 m(0.5) + 1.6057 m - 0.647 m(1.5) +0.113 m(2) B-2 = -1738.16 m(0.5) C-2 = -6.0346 m(0.5) (sigma = 0.013 for PK1* and u = 0.020 for PK2*, N=603). The values determined in this study are in good agreement with the 25 degrees C literature values. Our results have been combined with previous measurements to derive equations that are valid from 0 to 250 degrees C and 0 to 5 m. This large data set has been used to determine the Pitzer parameters (beta((0)), beta((1)) and C-phi) for the interactions of Na+ with HCO3- and CO32- from 0 to 250 degrees C. These results extend the carbonate system Pitzer model to hydrothermal brines containing high concentrations of NaCl. (c) 2006 Elsevier Inc. All rights reserved.