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