Calculation of salt precipitation and phase diagrams

The phase diagrams shown on these pages are calculated with the Extended UNIQUAC thermodynamic model. Calculation of phase diagrams and salt precipitation can be done in easy to use software. The software is programmed as FORTRAN DLL and has Microsoft Excel as user interface.

A free demo version can be downloaded below.

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On this page, phase diagrams for the following systems are being shown:

  • Iron bromide-water. The phase diagram covers the temperature range from minus 40 to 140 °C. The solid phases are ice and the nonahydrate, hexahydrate, tetrahydrate and dihydrate of iron bromide.
  • Sodium hydroxide – phosphoric acid – water. The solubility phase diagram at 60 °C is shown.  Many solid phases appear in this phase diagram. The phase diagram includes all compositions from the acidic region to the basic region.
  • Ammonia-phosphoric acid-water. The phase diagram is the 50 °C solubility isotherm for ammonia – phosphoric acid  mixtures. Four solid phases appear on this isotherm: NH4H2PO4·H3PO4, NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4.
  • Sodium chloride – potassium sulfate – water. This is a three dimensional Jänecke diagram valid at 100 °C. Further details can be seen under quaternary diagrams.
  • Calcium chloride – potassium chloride – sodium chloride – water. The phase diagram for this quaternary system is shown at 50 °C. This is a Jänecke projection with salt composition given on dry basis and the water content given at grid intersections. All compositions are in mass percent.
  • Magnesium hydroxide – phosphoric acid – water. The solubility isotherm at 0 °C is shown. Only the acidic region of the phase diagram is shown. At high fractional content of magnesium hydroxide, the solubility in this system is extremely low.
  • Lithium hydroxide – boric acid – water. The phase diagram at 60 °C is shown. Going from the acidic side to the basic side, the solid phases are boric acid, lithium pentaborate pentahydrate, lithium tetraborate trihydrate, lithium metaborate dihydrate, and lithium hydroxide monohydrate.
  • Aluminum sulfate – potassium sulfate – water. The phase diagram at 0 °C is shown. The low solubility og potassium alum dominates this phase diagram.
  • Ammonia – carbon dioxide – water. The diagram shows the CO2 partial pressure in solutions with different ammonia contents at 80 °C. More details about this system can be found under ternary systems.
  • Calcium sulphate – water. This phase diagram shows the solubility of calcium sulfate in water at temperatures between 0 and 200 °C. Gypsum and anhydrite are the stable solid phases in this system.
  • Calcium hydroxide – water. The solubility of calcium hydroxide in water is quite low and it is decreasing with increasing temperature. This phase diagram covers the temperature range 0 – 110 °C.
  • Calcium carbonate – carbon dioxide – water. This 100 °C isotherm shows how the solubility of calcium carbonate drastically increases when the partial pressure of CO2 is increased. This phenomenon results in the precipitation of calcium carbonate when the CO2 partial pressure is lowered.
  • Barium sulfate – water. The 100 °C isotherm at pressures from 1 to 1000 bar is shown. The solubility of barium sulfate increases with increasing pressure. The reason why the solubility increases is that the aqueous ionic species take up less space than the solid salt. The salt therefore prefers to be dissolved at high pressure.
  • Aluminum sulfate – sodium sulfate – water. The phase diagram and experimental data at 42 °C are shown. Soda alum is precipitating on one of the three branches of this phase diagram. The solubility of soda alum is significantly higher than that of potassium alum.
  • Aluminum chloride – potassium chloride – hydrochloric acid – water. The phase equilibrium line for co-precipitation of potassium chloride and aluminum chloride hexahydrate at 40 °C in this system is shown.
  • Aluminum chloride – hydrochloric acid – water. The 85 °C solubility isotherm for aluminum chloride in aqueous hydrochloric acid is shown. Aluminum chloride hexahydrate is the solid phase at HCl contents below 25 mass %. 
  • Aluminum fluoride-water. The phase diagram shows the solubility of aluminum fluoride in the temperature range 0 to 110 °C. The trihydrate of aluminum fluoride is the only stable solid phase in this temperature range.
  • Iso-propanol – potassium sulfate – water. This is the 50 °C solubility isotherm for the solubility of potassium sulfate in iso-propanol water mixtures. Further details can be seen under mixed solvent systems. On that page, solubility istherms for potassium sulfate in other mixed solvent systems is also shown.

Understanding the thermodynamics of aqueous salt solutions

Activity coefficients, osmotic coefficients, calculation of salt precipitation, super-saturation,  phase diagrams, and other aspects of electrolyte solution thermodynamics are described in a compendium by Kaj Thomsen, “Electrolyte Solutions: Thermodynamics, Crystallization, Separation methods”. The compendium can be downloaded at no cost at

The Ph.D. thesis by Kaj Thomsen: “Aqueous Electrolytes Model Parameters and Process Simulation” can be downloaded at

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