2.3. Activity coefficients
Many different approaches are available for the estimation of
activity coefficients in an attempt to account for the influence
of ionic interactions on the effective concentration of chemical
species. There are semi-emperical approaches such as the Davies
equation (Davies, 1962), approaches with a more complete derivation
from theory such as the Debye–Hückel equation (Debye and
Hückel, 1923), and extensive and complex systems such as Pitzer
equations (Pitzer, 1995) which are applicable even in saturated
salt solutions. There are many adaptations and extensions of these
methods. However, what is often not asked is what is the simplest
activity model available that adequately represents the data?
Between the years 2000 and 2005 the IUPAC project “Ionic Strength
Corrections for Stability Constants” (IUPAC project number 2000-
003-1-500) attempted to address this question. Their conclusion
was that for most applications specific ion interaction theory (SIT)
(Guggenheim and Turgeon, 1955) was the method of choice for
ionic strengths up to 6 mol dm−3. The form of SIT as used here is
given in Eq. (9) and has been modified to be in mol dm−3 rather
than mole fraction. It consists of the extended Debye–Hückel equation
with the product of the constant B and ionic radius å replaced
with a constant of value 1.5. The equation also includes interaction
parameters ε that are only relevant ( / = 0) for interactions
between ions of opposite charge. SIT was chosen for this work
due to the recommendation from IUPAC and for its simplicity of
implementation.