Calc.  SED & PREDOM / Ionic Strength

The Specific Ion interaction Theory (SIT) was adopted by the OECD-NEA thermodynamic database project, and described in its published reviews and books, see list of references.

In the SIT the activity coefficient of an ion “i” with electric charge Zi is given by:
    log γi = A Zi2 (I  /  (1+BI )) + ∑ ε(i,j) mj

where I is the ionic strength; A is the Debye-Hückel slope (a solvent parameter); B is a temperature-dependent parameter:

t (°C) A B
0 0.4911.48
25 0.5091.50
50 0.5341.52
100 0.6001.56

and ε(i,j) is the specific interaction parameters between the ion “i” and another ion “j” of opposite charge sign. The summation is taken over all ions j such that Zi×Zj > 0 (opposite electric charge sign). Often the ε-values are assumed to be independent of the ionic strength. A fundamental principle of the SIT is that ε(i,j) = ε(j,i), that is, the specific ion interactions are symmetrical.

ε-values in SPANA (and SED/PREDOM):

The activity of water (H2O) is given by
    log aH2O = − Φ ∑mk / (ln(10) 55.508)

where “k” are all solute species in the aqeuous solution, and “Φ” is the osmotic coefficient, calculated using the SIT as follows:
    Φ = 1 − (2/3) (ln(10)/∑mk) A I3/2 σ(B I) (ln(10)/∑mk) ij ε(i,jmi mj

where “i” are cations or neutral species and “j” are anions, and
    σ(x) = (3/x3) ((1+x) − 1/(1+x) − 2 ln(1+x))

References


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