RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Conversion to zero ionic strength


Protolysis constants (pK1/2) values of surface complexation reactions are usually determined at a constant ionic strength of a certain background electrolyte. Note, that the protolysis constants in RES³T are handled for the corresponding deprotonation reactions:

≡XOH2+ ↔ ≡XOH + H+   pK1
≡XOH ↔ ≡XO− + H+   pK2

That might be different to the way as it is given in the original reference. To use the collected SCM data in speciation codes that support Surface Complexation Modelling (e.g. PHREEQC or Geochemist's Workbench), a conversion procedure has been integrated into RES³T, to calculate the pK1/2 values at zero ionic strength using the Davies equation (1,2) (Davies 1962). This equation is valid up to an ionic strength of I = 0.5 mol L−1.


Davies equation

IS conversation formula for pK values

The parameters A and b (A = f(T,p), b = free parameter) are set to:

  • A298.15 K = 0.5093 kg1/2 mol−1/2
  • b298.15 K = 0.3 kg1/2 mol−1/2
to be consistent with the currently most widely used surface complexation speciation code PHREEQC. Note that the surface species are not considered within the Σνi·zi2 term!

The pK1/2 conversion to zero ionic strength and the pK1/2 normalisation to a SSD = 2.31 nm−1 are seperate and not cummulative.