454
Chapter 10
where the equilibrium constants are
K, =
KJ63 M and
K2
= 10'10 25 M. From these equilibria the total
carbon dioxide concentration in the medium is found to be
[ c o ,L = [c o ,L ,
f
1
+
K.
K ,K 2
(1032)
[C02]mt is a strong function of the medium pH (see Fig. 10.9). For pH < 5, nearly all carbon dioxide
is present as dissolved C 0 2, while bicarbonate dominates when 7 < pH < 9 and carbonate for pH >
11. Thus, in neutral to alkaline media there is a coupling between chemical reaction and mass
transfer.
To quantify the gas liquid mass transfer of other components one may use Eq. (10.33), which
relates the volumetric mass transfer coefficients for two different components to the corresponding
molecular diffiisivities:
{^!a )o2 _ D q2
( H
D A
(1033)
This is based on the assumption that the mass transfer coefficient for a certain component is
proportional to its diffusion coefficient This assumption is reasonable if the film theory can be
applied (see Section 10.1.1). Often the volumetric mass transfer coefficient for oxygen is known,
and equation (1033) can then be used to find
kft.
for other components, e.g., carbon dioxide. A list
of diffusion coefficients for different solutes in dilute aqueous solutions is given in Table 10.10.
Figure 10.9. Equilibrium concentrations of dissolved CO: , HC03" and C 032' . [C02],0, is the total dissolved
concentration of all forms.
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