CHAPTER 10 • Equilibrium Analysis, the Ionic Medium Method and Activity Factors
279
mine them from experimental concentration equilibrium constants in various ionic
media. However, the fitting is very sensitive even for small parameter variations. Figure 10.5 illustrates this.
The fit is not satisfactory without the mixing terms. The dotted line, obtained by
including the mixing terms 8Cl04,HC0 3 , 8Cl04,C0 3 ' 1i'Na,CI0 4 ,HX0 3 and 1i'Na,CI0 4 ,C0 3 , which have
been fitted to the experimental equilibrium constants, results in a good agreement between model and experimental data. One can discuss whether this is a satisfactory
Fig. 10.4. Comparison of experimental (symbols) and calculated equilibrium constants
log K for the reaction
CO2(aq) + H20(l) H HCO; + H+
in a NaCl medium using the SIT
(dashed line) and Pitzer (julldrawn line) models
Fig. 10.5. Comparison of experimental and calculated ionic
strength dependence of the first
dissociation constant of carbonic acid. The full-drawn curve
has been calculated using the
Pitzer model with available
literature values for the interaction parameters, but without
including mixing terms for
which no data are available. The
dotted curve is obtained using
mixing terms estimated from
the experimental equilibrium
constants. The dashed curve has
been calculated using the SITmodel
6.5
6.4
6.3
:.c:
:= 6.2
C'I
0
'T 6.1
6.0
5.9
5.8
0
8.0
7.9
7.8
:.c: 7.7
o
g;
'T 7.6
7.5
7.4
H 2 0(I) + COztaq) ~ H+ + HC0 3
o [45HARIBON)
t:,. [73DYRlHAN)
• [810HM/FOR)
o [82THU/MIL)
• [87HED/SJO)
• [93HE/MOR)
2
3
4
5
Molality of NaCI (mol kg-1)
6. [58FRY/NIL)
D [76HIElHOG)
o [81C1NFER)
• [82BIUSCH)
• [85SPA)
... [92BRU/STU)
o [92BRUIWER)
r'
;.'
/ / '
/.,'
/ :;.::/ ...
.\.
0/ / ~.::..<"'"
' ' ., - - - -6. ...... .
.. '
...... -
1'*,,,,,
,,'
HzO(I) + C0 2 (9) ~ H+ + HC0 3
•
/0
6
~ ....................
7.3 +I---,-----,----,-----,-----,----,r--,---r--'
o
2
3
4
Molality of NaCI (mol kg-1)