Thermodynamic modeling of salt solutions

At Aqueous Solutions ApS our main expertise is thermodynamic modeling of salt solutions. We develop software that makes it easy to perform calculations to design and optimize processes with salts. A fully functional free demo version of the software is made available for download at this site. Our software uses Microsoft Excel macro-enabled workbooks as user interface. The thermodynamic model is programmed in FORTRAN and is delivered as a dll file which can be called from for example an Excel macro.

  • Thermodynamic modeling of salt solutions based on data from the open literature or your proprietary data
  • Software solutions for equilibrium calculations including solid-liquid equilibrium, vapor-liquid equilibrium, and liquid-liquid equilibrium for systems with salts and sour gases. The software is delivered as dll files that can be used with interfaces such as Microsoft Excel®, MATLAB from MathWorks®, and SysCAD Plant Simulation Software (
  • Calculation of phase diagrams based on model prediction/correlation
  • Equilibrium simulations/optimizations of processes with salt solutions such as closed loop simulation of fractional crystallization processes.
  • Equilibrium based simulations of processes for CO2 capture or similar.

All calculations and all software we offer are based on the Extended UNIQUAC thermodynamic model for electrolytes. Please inquire for details on the above services or related services that we might be able to provide by emailing us at

About Aqueous Solutions Aps

Aqueous Solutions ApS was started by Kaj Thomsen in 2002 to provide phase diagram calculation and software development for commercial and academic purposes. The software is based on thermodynamic modeling of salt solutions. The modeling encompasses a wide array of ions, amines, gases and other species that are relevant for aqueous solutions. Experimental data published in the open literature during more than a century are being used for the modeling.

As a result of thermodynamic modeling of salt solutions phase diagrams like this can be calculated
Phase diagram for the magnesium sulfate- potassium sulfate - water system as a function of temperature

Phase diagram for the magnesium sulfate - potassium sulfate - water system​

The phase diagram for the MgSO4 – K2SO4 – H2O system is shown to the left. The fractional amount of the two salts is shown on the vertical axis. Temperatures between -10 and 120 °C are shown on the horizontal axis.

The water content is not depicted in the phase diagram. That would require a third dimension.

This kind of phase diagram gives an overview of the different solid phases appearing in a system over a temperature range. A more detailed phase diagram can be calculated at individual temperatures to show the water content of the solution. One such isotherm can be seen here.

The solid phases appearing in this system are ice, potassium sulfate, schoenite, epsom salt, leonite, hexahydrite, langbeinite, and kieserite.

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