The dissociation of carbonic acid in NaCl solutions as a function of concentration and temperature Frank Millero * , Fen Huang, Taylor Graham, Denis Pierrot Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA Received 16 May 2006; accepted in revised form 31 August 2006 Abstract Potentiometric measurements of the stoichiometric constants for the dissociation of carbonic acid in NaCl solutions (K  1 ¼½H þ ½HCO 3  =½CO 2  and K  1 ¼½H þ ½CO 3 2 =½HCO 3  ) have been made as a function of molality (0–6 m) and temperature (0– 50 °C). The results have been fitted to the equations pK  i  pK i ¼ A i þ B i =T þ 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 ¼ 35:2911 m 0:5 þ 0:8491 m  0:32 m 1:5 þ 0:055 m 2 B 1 ¼1583:09 m 0:5 C 1 ¼5:4366 m 0:5 A 2 ¼ 38:2746 m 0:5 þ 1:6057 m  0:647 m 1:5 þ 0:113 m 2 B 2 ¼1738:16 m 0:5 C 2 ¼6:0346 m 0:5 (r = 0.013 for pK  1 and r = 0.020 for pK  2 , N = 603). The values determined in this study are in good agreement with the 25 °C literature values. Our results have been combined with previous measurements to derive equations that are valid from 0 to 250 °C and 0 to 5 m. This large data set has been used to determine the Pitzer parameters (b (0) , b (1) and C / ) for the interactions of Na + with HCO 3  and CO 3 2 from 0 to 250 °C. These results extend the carbonate system Pitzer model to hydrothermal brines containing high concentrations of NaCl. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Carbonate equilibria over a wide range of ionic strength and temperature are important in a number of geochemical systems such as seawater, brines, and hydrothermal fluids. The solubility of CaCO 3 minerals in natural waters also re- quire reliable dissociation constants for carbonic acid CO 2 þ H 2 O () K 1 H þ þ HCO 3  ð1Þ HCO 3  () K 2 H þ þ CO 3 2 ð2Þ Measurement of the stoichiometric dissociation constants for carbonic acid 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.08.041 * Corresponding author. Fax: +1 305 361 4144. E-mail address: fmillero@rsmas.miami.edu (F. Millero). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 46–55