Calc.  SED & PREDOM / Ionic Strength

The Davies equation is an empirical equation appropiate for ionic strengths up to ≈0.1 M:
   log γi = −0.5 Zi2 (I  /  (1+√I ) − 0.3 I )

where I is the ionic strength; γi is the activity coefficient of an ion “i” with electric charge Zi The last parameter was initially “0.2” (Davies, C.W. (1938) J. Chem. Soc., 2093-2098),  but later Davies changed it to “0.3” (Davies, C.W. (1962) Ion Association. London: Butterworths. p.41).

The Davies equation is derived from the Debye-Hückel model, simplified by setting åBγ = 1 for all electrolytes, and extended with the empirical term: +0.15 Zi2 I.

In SPANA (and SED/PREDOM) the parameter “0.5” is replaced by the temperature-dependent solvent parameter A of the Debye-Hückel model (A=0.509 at 25°C); but the value “0.3” in the last term of the equation is kept temperature-independent.

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, which for the Davies equation it is given by:
    Φ = 1 − (2/3) (ln(10)/∑mk) A I3/2 σ(√I) (ln(10)/∑mk) A 0.3 I2

where
    σ(x) = (3/x3) ((1+x) − 1/(1+x) − 2 ln(1+x))


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