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
-