Geology Reference
In-Depth Information
1.0
pH=pK
α 0
0
α 1
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
10
12
14
pH
Figure 1 The influence of pH on a 0 and a 1 for a monoprotic acid
For a solution containing a weak acid, the charge balance relationship
(3.47) is satisfied at the equilibrium pH (Example 3.5). The same log{}
vs. pH plot can also be used to determine the equilibrium pH of a
solution containing the salt of a weak acid, e.g. NaA, by using the
appropriate charge balance expression.
{Na 1 } þ {H 1 } ¼ {A } þ {OH }
(3.48)
Using C ¼ {Na 1 } ¼ {HA} þ {A } gives
{HA} þ {H 1 } ¼ {OH }
(3.49)
Example 3.5: Graphical illustration of the relationship between (i)
log { 2,4,6-TCP } and pH; (ii) log { 2,4,6-TCP } and pH for an organic
pollutant, 2,4,6-trichlorophenol (2,4,6-TCP), that has been released into a
natural water (assume I ¼ 0molL 1 ). Use C ¼ { 2,4,6-TCP TOTAL } ¼ 4
10 4 mol L 1 and K ¼ 10 6.13 .
Graphical representation is best approached by dividing the pH range
into two regions, pH o pK and pH4pK. The slope of the lines in each
of these two regions can be determined by differentiating (3.45) and
(3.46) with respect to pH.
For pH o pK,(K þ {H 1 })
{H 1 } and
d log {2, 4, 6-TCP}/d pH ¼ d log{H 1 }/d pH þ d log C/d pH
d log{H 1 }/d pH ¼ 0
-
 
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