280
I. Grenthe
method; in order to be confident about the mixing terms, they should be confirmed
by an independent experiment.
The first example concerns a problem where we have a simple equilibrium system
and where the equilibrium constant at zero ionic strength can be determined by a
simple extrapolation method. The situation is very different if both equilibrium constants and Pitzer parameters have to be determined from experimental concentration
equilibrium constants. The second example illustrates this:
Example 2. This example refers to the determination of IOgIO[(l and the Pitzer and SIT
parameters from the experimental values of loglOK for the first protonation constant
of the sulphate ion, H+ + SO~- ¢::} HS0 4 , studied in NaCI0 4 media. References to the original data are given in Grenthe et al. (1997), where more details about the procedure are
given.
The experimental data are fitted to an equation of type Eq. 10.23, which in this case
is equal to:
InKo = InK + rlnaH 2 0 +t.z 2 (F + mB~x) + m2t.IZICNX + 2mXI
+ 2mg(Im)X 2 + 2m2X3
where
t,.Z2 = L viii; ~ Z I = LVi I Zi I
Xl = Ll/fo l + t.cfJ = LVi/fOlij + LVicfJij'
X 2 = t.lfll = LVif3~l)
X3= t.C + 112t.1fI= LViCij+ 1l2LVilflii'i
It is obvious that one cannot determine individual Pitzer parameters from these
data, even if they are precise enough to permit the evaluation of all fitting terms. The
results of different fitting models are given in Table 10.4 and Fig. 10.6.
• Model I. Determination of the whole set of parameters IOglo[(l, Xl' X2 and X 3 •
• Model II. Determination of the parameters IOglo[(l, Xl' and X2, neglecting ternary
interactions.
• Model III. Determination of the parameters IOglO[(l,X 1 and X 3 , assuming Ifl) = 0 for
all species.
• Model IV. Determination of the whole set of parameters IOglO[(l and X I, the smallest
number of parameters in the Pitzer model. The quantities Xl' X 2 and X3 are in this
case known by independent activity data. This is a situation that is rare.
The results of the various fitting models are shown in Fig. 10.6.
The parametrization of the SIT and different choices of the Pitzer model are illustrated below for the reaction H+ + SO~- ¢::} HS0 4 in NaCI0 4 ionic media at 25°C.
I. Grenthe
method; in order to be confident about the mixing terms, they should be confirmed
by an independent experiment.
The first example concerns a problem where we have a simple equilibrium system
and where the equilibrium constant at zero ionic strength can be determined by a
simple extrapolation method. The situation is very different if both equilibrium constants and Pitzer parameters have to be determined from experimental concentration
equilibrium constants. The second example illustrates this:
Example 2. This example refers to the determination of IOgIO[(l and the Pitzer and SIT
parameters from the experimental values of loglOK for the first protonation constant
of the sulphate ion, H+ + SO~- ¢::} HS0 4 , studied in NaCI0 4 media. References to the original data are given in Grenthe et al. (1997), where more details about the procedure are
given.
The experimental data are fitted to an equation of type Eq. 10.23, which in this case
is equal to:
InKo = InK + rlnaH 2 0 +t.z 2 (F + mB~x) + m2t.IZICNX + 2mXI
+ 2mg(Im)X 2 + 2m2X3
where
t,.Z2 = L viii; ~ Z I = LVi I Zi I
Xl = Ll/fo l + t.cfJ = LVi/fOlij + LVicfJij'
X 2 = t.lfll = LVif3~l)
X3= t.C + 112t.1fI= LViCij+ 1l2LVilflii'i
It is obvious that one cannot determine individual Pitzer parameters from these
data, even if they are precise enough to permit the evaluation of all fitting terms. The
results of different fitting models are given in Table 10.4 and Fig. 10.6.
• Model I. Determination of the whole set of parameters IOglo[(l, Xl' X2 and X 3 •
• Model II. Determination of the parameters IOglo[(l, Xl' and X2, neglecting ternary
interactions.
• Model III. Determination of the parameters IOglO[(l,X 1 and X 3 , assuming Ifl) = 0 for
all species.
• Model IV. Determination of the whole set of parameters IOglO[(l and X I, the smallest
number of parameters in the Pitzer model. The quantities Xl' X 2 and X3 are in this
case known by independent activity data. This is a situation that is rare.
The results of the various fitting models are shown in Fig. 10.6.
The parametrization of the SIT and different choices of the Pitzer model are illustrated below for the reaction H+ + SO~- ¢::} HS0 4 in NaCI0 4 ionic media at 25°C.