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 degrees C. Two experimental approaches were applied, potentiometric acid-base titrations at 10-60 degrees C and spectrophotometric pH measurements using the pH indicators, 2-napthol and 4-nitrophenol, at 25-200 degrees C. At a given temperature, the first and second ionization constants of carbonic acid (K-1, K-2) and the ion pair formation constants for NaHCO3(aq)(K-NaHCO3) and NaCO3(aq)(K-NaCO3(-)) were simultaneously fitted to the data. Results of this study compare well with previously determined values of K-1 and K-2. The NaHCO3(aq) and NaCO3 (aq) ion pair formation constants vary between 25 and 200 degrees C having values of log K-NaHCO3 = -0.18 to 0.58 and logK(NaCO3)(-) = 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 >= 0.1 M NaCl solution at temperatures above 100 degrees C. The results demonstrate that NaCl cannot be considered as an inert (non-complexing) electrolyte in aqueous carbon dioxide containing solutions at elevated temperatures. (C) 2013 Elsevier Ltd. All rights reserved.