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 (K1∗=[H+][HCO3-]/[CO2] and K1∗=[H+][CO32-]/[HCO3-]) have been made as a function of molality (0–6 m) and temperature (0–50 °C). The results have been fitted to the equationspKi∗-pKi=Ai+Bi/T+CilnTThe values of pKi in pure water are taken from the literature and the adjustable parameters Ai, Bi and Ci are a function of molalityA1=35.2911m0.5+0.8491m-0.32m1.5+0.055m2B1=-1583.09m0.5C1=-5.4366m0.5A2=38.2746m0.5+1.6057m-0.647m1.5+0.113m2B2=-1738.16m0.5C2=-6.0346m0.5(σ = 0.013 for pK1∗ and σ = 0.020 for pK2∗, 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 (β(0), β(1) and Cϕ) for the interactions of Na+ with HCO3− and CO32− from 0 to 250 °C. These results extend the carbonate system Pitzer model to hydrothermal brines containing high concentrations of NaCl.