Hydrothermal chemistry

The Criss-Cobble correspondence principle is useful for aqueous solutions to about 200 °C. At higher temperatures, the heat capacities Cp of ionic solutes such as NaCP at constant pressure rapidly become strongly negative and appear to be headed toward infinite negative values on approaching the critical temperature (which, incidentally, is somewhat higher for aqueous electrolyte solutions than for pure water). If, however, we examine the heat capacities Cy of aqueous electrolytes at constant volume, 

A further problem is that ion association, that is, the tendency of oppositely charged ions to form pairs or larger aggregates in solution, becomes increasingly important as the temperature rises unless the density is kept constant this is because ion association is inversely related to the dielectric constant (relative permittivity) of the medium, which is correlated with density for a given solvent. Helgeson and co-workers have attacked these problems theoretically for aqueous solutions up to 1000 °C. For our purposes, it is enough to note that quantitative treatment of ionic reactions in sub- and supercritical aqueous solutions is extremely difficult at present, and likely to remain so for some time. 

As an illustration of the effects of temperature, pressure, and ionic strength on ionic equilibria in solution, we can consider the self-ionization (autoprotolysis) of water  

The thermodynamic equilibrium constant K° is given by the activity quotient H+ 0H- / H20, and pK° (= -logAT°) has the value 14.00 (more precisely, 13.998) at 25.0 °C. In practice, H2O is taken to be unity for dilute solutions, and the concentration product K = [H+][OH ] is used. The common assumption that pK, is 14.00, however, is correct only for very dilute solutions at 25.0° C and 0.1 MPa. For seawater at this temperature and pressure, pKyg is 13.76 because the ionic strength is about 0.71 mol L. In pure water at 0.0 °C and 0.1 MPa, pKy, is 14.94 because the ionization of water is endothermic AH° is 55.8 kJ mol at 25 °C but is temperature dependent, so that Eqs. 2.26-2.28 must be invoked to extrapolate Ky, with respect to temperature (see Exercise 2.1). Since AV° for the ionization is -21 cm mol , Eq. 2.31 predicts pKy, = 13.63 at 100 MPa and 25.0 °C if AV° is pressure independent. At the saturated vapor pressure, pKy, passes through a broad minimum of about 11.36 around 240 °C as the temperature is increased, then rises to 16.63 at 374 °C and 22.1 MPa, the critical point. For supercritical water, pKy, decreases (ionization increases) with increasing pressure (density) at a given temperature at the critical temperature, for example, pKy, falls to only 5.70 at 7.1 GPa. Thus, as Hawkes has remarked, pKy, almost never has the traditional value of 14.00. 

---

R. M. Barrer, Hydrothermal Chemistry oJXeolites, Academic Press, Inc., New York, 1982. 

ROLE OF HYDROTHERMAL CHEMISTRY IN THE CRUSTAL-OCEAN-ATMOSPHERE FACTORY... 

The hydrothermal chemistry of methane also provides another buffering control on the global biogeochemical carbon cycle by serving as the site of reactions that act as sources and sinks of methane. Examples of source reactions are... 

Barrer, R.M. (1982) Hydrothermal Chemistry of Zeolites, Academic Press, London. 

Barrer, R.M. and White, E.A.D. (1952) The hydrothermal chemistry of silicates. Part 11. Synthetic crystalline sodium aluminosilicates./. Chem. Soc., 1561-1571. 

Barrer, R.M., Baynham, J.M., and McCallum, N. (1953) Hydrothermal Chemistry of Silicates. Part V. Compounds strucmrally related to Analdte./. Chem. Soc., 4035-4041. 

Barrer, R. M. (1982) Hydrothermal Chemistry of Zeolites, Academic Press, London. Bechgaard, K. Jerome, D. (1982) Scientific American 247, 50. 

P. R. Tremaine, E. E. Isaacs, and J. A. Boon, Hydrothermal chemistry applied to in situ bitumen recovery. Chem. Can. April, 29-33 (1983). 

Barrer RM. Hydrothermal chemistry of zeolites. (1982) Academic Press London and New York. 111-115. 

Barren, R. M. Hydrothermal Chemistry of Zeolites Academic London, 1982. 

---