Carbonate and sulfate scaling

This software is used for calculation of carbonate and sulfate scaling. Phase diagrams and general phase equilibrium calculations for aqueous solutions with the ions Na+, H+, Mg2+, Ca2+, Ba2+, Sr2+, Cl-, OH-, SO42-, HSO4-, CO32-, HCO3- under varying pressure and temperature conditions are performed.

The program also gives information on the speciation of the solution, the enthalpy of formation, the heat capacity, the ionic strength, the bubble point pressure of the solution, and more.

The user interface to this software is Microsoft Excel. After writing the input as grams of each ion, you can right away read the results:

The first part of the results is a statement on the speciation. The amounts of aqueous species are given in grams of each species. Next, the amounts in grams of solids precipitated from the solution are stated. In the following rows, the “Saturation index before equilibrium” is given. Further down, “Saturation index after equilibrium” is given.

The saturation index is defined as the ionic activity product divided by the solubility product. At equilibrium, the saturation index is therefore equal to one. If a salt is unsaturated, its saturation index is less than one. If the salt is supersaturated, its saturation index is larger than one. Any salt with a saturation index higher than one can precipitate if it has favorable kinetic conditions. At thermodynamic equilibrium, which can take ages to achieve, the solids with the lowest gibbs energy form the precipitate. The “Saturation index before equilibrium” can be used to evaluate which salts might precipitate from a certain solution. The “Saturation index after equilibrium” gives the saturation index after thermodynamic equilibrium is achieved.

In the last paragraph, the following properties are given:

  • Enthalpy of liquid, J/g. This is the enthalpy of formation of the aqueous solution at the given composition and temperature.
  • Enthalpy of solid, J/g. This is the enthalpy of formation of the precipitated solids at the given temperature.
  • Heat capacity of liquid, J/(g·K). This is the heat capacity of the aqueous solution at the given composition and temperature.
  • Bubble point pressure, bar. This is the bubble point pressure of the solution in thermodynamic equilibrium.
  • pH – calculated as minus the logarithm base 10 of the hydrogen ion activity (molality scale). The hydrogen ion activity is calculated as the molality of the hydrogen ion times the mean ionic molal activity coefficient. pH-meters are usually calibrated to very dilute solutions. At increasing ionic strength, there will therefore be an increasing difference between the calculated pH and the measured pH.
  • Mean ionic molal activity coefficient.
  • Ionic strength, mol/(kg H2O). This is the ionic strength of the equilibrium solution calculated on the molality scale.
  • Water activity


Kaj Thomsen
Associate Professor
DTU Chemical Engineering
+45 45 25 28 60