Carbonic acid ionization and the stability of sodium bicarbonate and carbonate ion pairs to 200°C - A potentiometric and spectrophotometric study

Carbonic acid ionization and sodium bicarbonate and carbonate ion pair formation constants have been experimentally determined in dilute hydrothermal solutions to 200°C. Two experimental approaches were applied, potentiometric acid-base titrations at 10-60°C and spectrophotometric pH measurements using the pH indicators, 2-napthol and 4-nitrophenol, at 25-200°C. At a given temperature, the first and second ionization constants of carbonic acid (K₁, K₂) and the ion pair formation constants for NaHCO3(aq)(KNaHCO3) and NaCO3-(aq)(KNaCO3-) were simultaneously fitted to the data. Results of this study compare well with previously determined values of K₁ and K₂. The _NaHCO3(aq) and NaCO3-(aq) ion pair formation constants vary between 25 and 200°C having values of logKNaHCO3=-0.18 to 0.58 and logKNaCO3-=1.01 to 2.21, respectively. These ion pairs are weak at low-temperatures but become increasingly important with increasing temperature under neutral to alkaline conditions in moderately dilute to concentrated NaCl solutions, with NaCO3-(aq) predominating over CO32-(aq) in ≥0.1M NaCl solution at temperatures above 100°C. The results demonstrate that NaCl cannot be considered as an inert (non-complexing) electrolyte in aqueous carbon dioxide containing solutions at elevated temperatures.