DES-code: A metacode to aid calculation of the chemical potential of aqueous solutions at elevated temperatures and pressures

The DH-ASF (Debye-Hückel-asymmetric formalism) model is a recently developed activity-composition model that can be used to calculate the thermodynamic effects of mixing in strong electrolyte and mixed solvent supercritical solutions at high pressures (> 3 kbar) and temperatures (> 400 ^{{ring operator}} C). The model uses a mole fraction concentration scale, and calculations are based on a pre-defined independent set of end-members that define speciation within the solution. This differs from the conventional use of molal concentration scales with sets of end-members that define the composition of the solution, but not the speciation (apparent end-members). This work presents DES (dual end-member sets)-code, a code that takes a conventional molal scale description of solution composition and implements the DH-ASF model for that solution. The code converts between apparent and independent end-member sets, and calculates standard state chemical potentials, ideal activities and activity coefficients using the DH-ASF model for molal and mole fraction concentration scales. The code runs in Mathematica^TM 4.1 onwards, but it is written in a general meta-code form so that it can be implemented on a variety of platforms. Inputs to the code can be made manually, read from an auxiliary file, or presented to the input modules as passed variables. The code is provided with thermodynamic data from the Holland and Powell data set, but can be used with any data specified by the user. Outputs are designed to be modified by the user. Calculations on the systems NaCl-H₂ O, NaCl-CaCl₂-H₂ O and NaCl-CO₂-H₂ O are used to demonstrate the utility of the DES-code. Calculations predict that ion association increases with increasing temperature and concentration of salt and CO₂, and with decreasing pressure. This is consistent with experimental observation and the results of molecular simulations. The DES-code is suitable for use as it stands, or for modification and incorporation into existing or new Gibbs energy minimisation or equilibrium solving thermodynamic codes.