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1. Grenthe
Example 1. This is a comparison of the SIT and Pitzer models for the modelling of the ionic
strength dependence for the reaction COiaq) + H20(l) ¢:::> HCO; + H+ in a NaCl medium.
We have:
log[(l = 10gK + 10glB+ + logIBco 3 -logYc0 2 (aq) -logaH 2 o
From the Pitzer equations, we obtain the following values for the single-ion activity coefficients:
log IB+ = F + mCl(2BH,CI + 2mCICH,CI) + mNa(2if>H,Na + mCI'l'H,Na,CI) + mNamCICNa,CI
log IBC0 3 = F + mNa (2BNa,HC0 3 + 2mNa CNa,HCO) + mCI (2I/>CI,HC0 3 + mNa'l'Na,CI,HC0 3 )
+ mNamCICNa,CI
where
F = - AtfJ J JI k
+ ~ ln~ + bJi:,)l + mnamClB' NaCl
11 + b 1m b
J
logaH 2 o = CPvmMW /1000
where cP is the osmotic coefficient of the electrolyte, Mw the mol mass of water, and
v = 2 for NaCl.
The activity coefficient for CO2(aq) is equal to:
log YC0 2 (aq) = 2(Ac0 2 ,Na + Ac0 2 ,Cl)
where It represents the interaction coefficient between the ionic medium electrolyte
ions and CO2(aq). All the necessary Pitzer parameters for the equations above are
known and listed in Table 10.3.
Note that the various interaction parameters in the example can be determined from
experimental mean activity data of the pure electrolytes. It is not necessary to deterTable 10.3. Pitzer parameters for the equations in Example 1
ti D)
H,CI
0.1775
C~CI
0.00080
C~CI
=0.00060
tiD)
0.0765
l)
0.2664
q,
=0.00127
Na,CI
Na,CI
CNa,CI
tiD)
0.0277
till
0.0411
q,
=0.0
Na,HCO~
Na,HC03
CNa.HC03
°H,Na
0.036
°CI,HC03
0.03
ljIH,Na,CI
-0.004
ljINa,CI,HC03
-0.015
A.C02,Na + A.C02,(1
0.096