Thermodynamics and Kinetics of the Solid Solution of HCl in Ice

Abstract

The equilibrium solubility of HCl in ice I h has been measured as a function of temperature and HCl partial pressure, between -8 and -35°C by doping large ice single crystals with gaseous HCl for several weeks. Results indicate that the solubility of HCl in ice is very low, much less than found in many previous studies. Its temperature and HCl partial pressure dependences are found to be X HCl ) 6.13 × 10 -10 e 2806.5/T (P HCl ) 1/2.73 where X HCl is the solubility in mole fraction, P HCl is the HCl partial pressure in Pa, and T is the temperature in kelvin. The diffusion coefficient of HCl in ice is also found to be very low, about 10 -12 cm 2 /s at -15°C. Extrapolations of these data yield the solidus in the temperature-composition phase diagram. The determination of the solid phase composition in equilibrium with a given gas phase composition allows the calculation of the partial enthalpy of sublimation of HCl from ice, ∆h HCl s ) 63.7 ( 7.6 kJ/mol, and of the activity of HCl in ice. Possible mechanisms of HCl incorporation in ice are discussed. Atmospheric implications concerning ozone depletion and the understanding of snow composition are examined.

FTIR spectroscopic studies of the simultaneous condensation of HCl and H2O at 190 K – Atmospheric applications

“…Livingston et al (2000), using very high HCl concentrations, have measured diffusion coefficients of HCl in ice of the order of 5 × 10 −11 cm 2 /s at 190 K. This is several orders of magnitude higher than what could be reasonably envisaged for a solid so far from its melting point, and is much higher than other measured values (Dominé and Xueref, 2001;Thibert and Dominé, 1997;Wolff et al, 1989). Dominé and Xueref (2001) have demonstrated that the high values measured by Livingston et al (2000) were caused by the high HCl concentrations, that led to ice amorphization.…”

“…However, as already mentioned by Banham et al (1995), and as visible on available phase diagrams (e.g. Vuillard, 1955;Thibert and Dominé, 1997), the tetrahydrate discussed by Delzeit et al (1993) does not exist, and the solid in question is certainly the trihydrate.…”

“…HCl adsorbs onto stainless steel (Thibert and Dominé, 1997) and conditioning of the stainless steel tubing was required to ensure that the gas composition in the condensation cell was the same as in the bulb. Two methods were used, that both gave satisfactory results.…”

“…An interesting dopant is HCl. First of all, it is soluble in ice (Thibert and Dominé, 1997). Second, it can form numerous solid HCl : H 2 O mixtures.…”

“…Considering this fact, it is relevant to ask which phases HCl and H 2 O will form together. HCl−H 2 O phase diagrams (Thibert and Dominé, 1997) show five solid phases at 190 K: four hydrates (mono-, di-, tri-and hexa-hydrates) and a solid solution of HCl in ice. But since HCl is incorporated in excess in the co-condensation process, H 2 O and HCl are not in thermodynamic equilibrium and therefore they could form: 1) a supersaturared homogeneous solid solution of HCl in ice, 2) an heterogeneous solid made of one or several hydrates and of a solid solution of HCl in ice, and/or 3) the HCl in excess could diffuse to the surface and then escape into the surrounding air, thus leading to a solid solution in thermodynamic equilibrium.…”