Environmental Engineering Reference
In-Depth Information
The solubilities of short-chained alkylated phenols at pH ≪ pK
a
in gen-
eral are approximately a factor of 50 higher than that of their parent aro-
matic compounds. In contrast, a comparison of NSO-heterocycles to their
parent hydrocarbons and PAHs presents a different view. The solubilities of
S-heterocycles are comparable to their analog hydrocarbons; a comparable
and in some cases higher solubility is found for O-heterocycles. However,
the solubilities of N-HETs are usually much higher (for instance the solubil-
ity of acridine is about a factor of 1000 higher compared to that of anthra-
cene). Only a relatively few measurements are available for pH-dependent
solubilities. However, the Henderson-Hasselbalch Equation 11.1 is often
used to predict solubilities (
S
= solubility,
S
0
= intrinsic solubility of the neu-
tral molecule) [17]:
(pK H)
a
−
(11.1)
S
=⋅
(
1 0
+
)
0
2-Methylquinoline and 9-methylacridine should be used as instructive
examples. For 2-methylquinoline, the pK
a
= 5.86 was obtained by ultraviolet
(UV) spectroscopy in this study in agreement with a literature value [18].
An estimated intrinsic solubility
S
0
= 3.6 g/L for 2-methylquinoline is avail-
able [16]. The solubilities of 2-methylacridine between pH = 5-9 are shown
in Figure 11.2. Extrapolating these figures, the predicted solubility at pH = 4
is approximately 300 g/L. In contrast to 2-methylquinoline, the solubility of
1000
S
0
= 3.6 g L
-1
pK
a
= 6.1
100
Adjusted
with 2N HCl
S
0
= 3.6 g L
-1
pK
a
= 5.86
10
Buffered
Water
1
3
4
5
6
7
8
9
pH
FIGURE 11.2
The dependence of experimentally determined water solubilities of 2-methylquinoline from
pH (filled circles). Lines shown are calculated solubilities using the Henderson-Hasselbalch
equation for pK
a
= 5.86 (experimental value, solid line) and for a slightly modified pK
a
= 6.10
(broken line).
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