Electrolytes and Activity Coefficients

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1. How does OLI calculate pH?

To see how the software calculates the pH, please see the following paper on Mo chemistry, Section 3: Solution chemistry of Mo(III) and Mo(IV): Thermodynamic foundation for modeling localized corrosion.

In short, the chemical potential is calculated for each species (base ions, hydrolyzed species, complexes, etc.) according to the following equation:

Equation 1

The chemical potential contains two contributions – the standard contribution (which is directly linked to equilibrium constants) and the activity coefficient from the MSE model. Then, the equilibrium reactions between all species are written and solved according to Equation 1. As always, the key to accuracy is the determination of parameters, which are also described in the paper referenced above.

2. Activity coefficient from MSE model

In mixed-solvent electrolytes, the non-ideality of these solutions raises from three different forces:

  • Electrostatic forces
  • Chemical forces
  • Physical dispersion forces

In the MSE the Excess Gibbs Energy is constructed as a sum of these three terms, as follows:

Equation 2

Where GLRex, GMRex, GLRex represent the long, middle and short-range contributions respectively. Thus, the activity coefficient will be given by the following equation:

Equation 3

3. Prediction of pH of a metal salt solution

Figure 1 shows the effect of different metal cations in solution on the pH. This pH behavior comes from two factors:

Figure 1. Predicted pH for NiCrFe, NiCrFeMo and NiCrFeMoNb solutions using OLI software

1. Primarily, from the hydrolysis reactions, i.e. the formation of species such as CrOH2+, CrO2+, etc. The formation of these species is accompanied by the release of H+, which reduces the pH.

2. Secondarily, from the interactions between all the ions in the solution, which affect the activity coefficients of all species, including the activity coefficient of the H3O+ ions, which determines the pH.

Editor: Diana Miller Author: Andre Anderko