Potentiometric and spectrophotometric study of the stability of magnesium carbonate and bicarbonate ion pairs to 150 °C and aqueous inorganic carbon speciation and magnesite solubility

Abstract

The formation constants of magnesium bicarbonate and carbonate ion pairs have been experimentally determined in dilute hydrothermal solutions to 150°C. Two experimental approaches were applied, potentiometric acid–base titrations at 10–60°C and spectrophotometric pH measurements using two pH indicators, 2-naphthol and 4-nitrophenol, at 25 and 80–150°C. At a given temperature, the first and second ionization constants of carbonic acid (K1, K2) and the ion pair formation constants for MgHCO3+(aq) (KMgHCO3+) and MgCO3(aq) (KMgCO3) were simultaneously fitted to the data. Results of this study compare well with previously determined values of K1 and K2. The formation constants of MgHCO3+(aq) and MgCO3(aq) ion pairs increased significantly with increasing temperature, with values of logKMgHCO3+=1.14 and 1.75 and of logKMgCO3=2.86 and 3.48 at 10°C and 100°C, respectively. These ion pairs are important aqueous species under neutral to alkaline conditions in moderately dilute to concentrated Mg-containing solutions, with MgCO3(aq) predominating over CO32-(aq) in solutions at pH >8. The predominance of magnesium carbonate over carbonate is dependent on the concentration of dissolved magnesium and the ratio of magnesium over carbonate. With increasing temperature and at alkaline pH, brucite solubility further reduced the magnesium concentration to levels below 1mmolkg−1, thus limiting availability of Mg2+(aq) for magnesite precipitation.