Frezchem

Cold Aqueous Planetary Geochemistry with FREZCHEM

Written by two of the best US researchers in the field, this text investigates issues of astrobiological relevance in the context of cold aqueous planetary geochemistry. At the core of the technical chapters is the FREZCHEM model, initially developed over many years by one of the authors to quantify aqueous electrolyte properties and chemical thermodynamics at subzero temperatures. FREZCHEM is of huge relevance to scientists in a number of fields, including biogeochemists.

FREZCHEM: A Chemical-Thermodynamic Model for Aqueous Solutions at Subzero Temperatures

This report documents a FORTRAN version of the Spencer-Moller-Weare chemical thermodynamic model for aqueous electrolyte solutions at sub-zero temperatures (FREZCHEM). FREZCHEM is structured to predict the chemical composition and unfrozen water of aqueous solutions between -60 deg and +25 deg C at atmospheric pressure (0.101325 MPa). FREZCHEM includes two reaction pathways: freezing at variable temperature and fixed total water and evaporation at variable water and fixed temperature. Activity coefficients and the activity of water are calculated using the Pitzer equations, which are valid to high solution ionic strengths (approx. 20 mol k-1). Fifteen chloride and sulfate salts of sodium, potassium, calcium, and magnesium are included in the model. Predicted and experimental measurements of solute molalities and the unfrozen water fraction during sea water freezing are in good agreement. At -50 deg C, 0.3 % of sea water remains unfrozen with 99.7% of Na and 95.5% of Cl having precipitated into one of four salts. FREZCHEM should find many applications in physicochemical studies of aqueous solutions and freezing.

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