CHAPTER 11 • Acid-Base Equilibria in Saline Media: Application of the MSA
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11.3
pK vs.lonic Strength Equations
In Table 11.1, it is shown that virtually all the expressions proposed over the years are
of the type:
T
*
pK =pK + f( I,parameters)
(11.8)
The different equations refer to the equilibria types I and II but based on different
models for the activity coefficients logy; and logYneutral: Pitzer, Guggenheim, or other
semi-empirical modifications of the Debye-Huckel equation. Also, a non-Debye-Huckel
based equation is included at the bottom of the Table 11.1. This simple cubic-root equation is based on a classical quasi-lattice model, which exhibits this characteristic dependence.
Different authors(for example Grenthe and Plyasunov 1997; Millero and Pierrot 1998;
Salvatore et al. 1986; Partanen and Juusola 2000; Daniele et al. 1997; Sastre de Vicente
Table 11.1. pK vs. ionic strength dependence for equilibria I and II according to different models for
activity coefficients
Modell (Pitzer)
{(4) = -0.392 [~ + 21n(1 + 1.N,!)1 , {(5) = -1 + ( 1 + 2/1 + 21 )e-2ft
1 + 1.2/1 1.2
J
Model II (Scatchard)
•
T
1.018/1
2
3
pK, = pK, - Z------r. + p, 1 + 0, 1 + R, I
1+1.5,,1
Model III (Guggenheim)
pK;" =pKT +£, I-z 1.018/1
1 + 1.5/1
Model IV (De Stefano et al.)
•
T
2/1
3/2
pK; = pK; - Z----r, + b, 1 + 0; 1
2 + 3,,1
Model V (quasi-lattice)
•
T
3r.
pK; = pK; - z o;vl + b, 1
z=i-1
Taken from (Sastre de Vicente 1997).