274
1. Grenthe
concentration/ionic medium dependence of equilibrium constants in ionic media is
described in the following section.
For a chemical reaction with reactants/products Qi, we have
I, PiQi + rH20(l) = 0
i
and
(10.23)
InKO = I, Pi lnmi + I, Pi In Yi + rlnaH 0 = InK + I, Pi In Yi + rlnaH 0 (10.24)
.
.
2
.
2
I
I
I
Most experimental studies of complex formation reactions have been performed
in ionic media (Rossotti and Rossotti 1961), with a concentration that is much higher
than that of the reactants/products. In this way, the "trace" activity coefficients of reactants/products remain very near constant, even when the total concentrations of
reactants/products are varied. At "trace" concentrations of reactants and products, all
summations in Eqs. 10.11 and 10.12 reduce to terms that include only the molality of
the ionic medium electrolyte; the others are negligible. Hence, the Pitzer model in a
1:1 electrolyte ionic medium, NX, results in the following analytical statement:
InKO = InK + rlnaH20 + I,PiZ;(jY + m2B~x) + 2m'i;.PiBij + 2m2I,PiCij
I
I
I
+ 2mI,PilPu' + m 2 I,Pill'ii'j + m2I,pdZdCNX
i i i
2
r
.
2 I I
= InK + rlnaHzo + f).Z (f + mBNX ) + m I1ZCNX
+ 2m(/1B + M) + 2m2(I1C + 11",)
2
(10.25)
where the index i refers to species i, and i' and j stand for ionic medium ions, having
the same and opposite charge signs, respectively, as i. The definitions of f).Z2, I1B, etc.
are clear from Eq. 10.25; m is the molality of ionic medium 1-1 electrolyte. The ionic
strength dependence of parameter /1B is
/1B = 11[3(0) + 11[3(l) g(aI 1I2 )
(10.26)
where
11[3(0) = I, Pi[3~O) and 11[3(1) = I, Pi[3~I) .
i
i
Equation 10.25 shows that the coefficients for the terms in m and m 2 for the reactants/products contain 11/1°) and 11f!>, and I1C and 11 "', respectively. Hence, it is not pos-