Geology Reference
In-Depth Information
For example, a mixed acidity constant is frequently used where pH has
been measured according to the IUPAC convention as the activity of
hydrogen ions but the concentrations of the conjugate acid-base pair are
used.
The relationship between K and
c
K or K and K
0
as defined above can
be used to calculate the effect of ionic strength of solution on the true
equilibrium constant, K.
c
K and K
0
can be calculated using the Equa-
tions (3.3)-(3.5) together with experimentally determined values of K
(Example 3.3).
Example 3.3: Calculation of the mixed acidity constant, K
0
, using the
Gu¨ntelberg approximation and the Davies equation.
For an acid-base equilibrium:
HA
H
1
þ
A
K
¼
{H
1
}{A
}/{HA}
"
K
0
¼ð
g
HA
=
g
A
Þ
K
Taking logs:
logK
0
¼
log g
HA
=
g
A
ð
Þ þ
log K
pK
0
¼
pK
þ
log g
A
log g
HA
Using the Gu
¨
ntelberg approximation:
p
p
pK
0
¼
pK
þ
0
:
5 z
2
HA
z
2
A
I
=
1
þ
I
Using the Davies equation:
p
p
0
:
3I
pK
0
¼
pK
þ
0
:
5 z
2
HA
z
2
A
I
=
1
þ
I
Now calculate the mixed acidity constant, K
0
, at ionic strength values
of (i) 0.05 mol L
1
using the Gu¨ ntelberg approximation and (ii) 0.4 mol
L
1
using the Davies equation for a monoprotic acid, HA, with K
¼
6.8
10
8
:
(i)
pK
0
¼
7
:
17
þ
0
:
5
ð
0
ð
1
Þ
2
Þð
p
:
05
=ð
1
þ
p
:
05
ÞÞ
¼
7
:
17
0
:
0914
¼
7
:
08
K
0
¼
8
:
34
10
8
