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⁺] [CO₃^{2 -}] / [HCO₃^-]) have been made as a function of molality (0-6 m) and temperature (0-50 °C). The results have been fitted to the equationsp K_i^* - p K_i = A_i + B_i / T + C_i ln TThe 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 molalityA₁ = 35.2911 m^0.5 + 0.8491 m - 0.32 m^1.5 + 0.055 m²B₁ = - 1583.09 m^0.5C₁ = - 5.4366 m^0.5A₂ = 38.2746 m^0.5 + 1.6057 m - 0.647 m^1.5 + 0.113 m²B₂ = - 1738.16 m^0.5C₂ = - 6.0346 m^0.5(σ = 0.013 for p K₁^* and σ = 0.020 for p K₂^*, 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 C^φ{symbol}) for the interactions of Na⁺ with HCO₃^- and CO₃^2- from 0 to 250 °C. These results extend the carbonate system Pitzer model to hydrothermal brines containing high concentrations of NaCl.