Calc.  SED & PREDOM

The Ionic Strength corresponds to the concentration of electric charge in a solution:

I = ½ ∑ mi Zi2

where mi is the molality of an aqueous species i with charge Zi. For strong 1:1 electrolytes (like NaCl) the ionic strength is equivalent to the total concentration. The ionic strength is used to calculate activity coefficients in dilute aqueous solutions.

The activity coefficient, γi, gives the relationship between concentration and thermodynamic activity:

γi = ai / mi

where ai and and mi are the activity and molality, respectively, of the aqueous species i”. Note that in equations concentrations are usually written with brackets, for example “[H+]”, while activities are often written with braces, “{H+}”.

Activity coefficient calculations in SPANA (using SED/PREDOM): If the equilibrium constants in the input file are given for zero ionic strength (standard conditions) then it is possible to give a value for the ionic strength, I, and the calculations will include the activity coefficients calculated using the the given values for temperature and ionic strength.

Models for activity coefficients

In in SPANA the user may choose a model to evaluate activity coefficients if the advanced program level selected in the menu “Preferences / General options”. The following models may be selected:

In the normal program level of SPANA activity coefficients are calculated using the simplified HKF model.

Comparison between the Davies eqn., the simplified HKF model, the SIT, and some experimental data