Geology Reference
In-Depth Information
A second ligand may then be co-ordinated as
M
þ
L
"
ML
2
f
ML
2
g
f
ML
gf
L
g
K
2
¼
ð
4
:
17
Þ
The equilibrium constant for ML
2
can be expressed solely in terms of
the activities of the M and L
f
ML
2
g
f
M
gf
L
g
2
b
2
¼
ð
4
:
18
Þ
where ß
2
is the product of K
1
K
2
and is known as the stability constant.
The case can be extended to include n ligands as
f
ML
n
g
f
M
gf
L
g
n
b
n
¼
ð
4
:
19
Þ
Worked Example 2
Assuming all g
0
s
¼
1, calculate the speciation of mercury in typical
seawater (35 psu at 25 1C) given the following values for stepwise
stability constants for successive chlorocomplexes (K
1
¼
10
6.74
, K
2
¼
10
6.48
, K
3
¼
10
0.85
, K
4
¼
10
1.00
). Note that [Cl
]is0.559mmolL
1
and
that it is not necessary to know the mercury concentration in seawater.
The total mercury concentration is given as the sum of all contribut-
ing species. Thus
[Hg]
T
¼
[Hg
2 1
]
þ
[HgCl
1
]
þ
[HgCl
2
]
þ
[HgCl
3
]
þ
[HgCl
4
2
]
From the definition of the stability constants, we know that
[HgCl
1
]
¼
K
1
[Hg
21
][Cl
] and [HgCl
2
]
¼
b
2
[Hg
21
][Cl
]
2
, etc. Thus,
[Hg]
T
¼
[Hg
21
](1
þ
K
1
[Cl
]
þ
b
2
[Cl
]
2
þ
b
3
[Cl
]
3
þ
b
4
[Cl
]
4
).
Now let D
¼
(1
þ
K
1
[Cl
]
þ
b
2
[Cl
]
2
þ
b
3
[Cl
]
3
þ
b
4
[Cl
]
4
)
1
,and
note that this is a constant for a stipulated chloride concentration.
Thus, at the seawater chloride concentration ([Cl
]
¼
10
0.25
)thisgives
D
¼
(1
þ
10
6.74
10
0.25
þ
10
6.74
10
6.48
10
0.25
þ
10
6.74
0
6.48
10
0.85
10
0.25
þ
10
6.74
10
6.48
10
0.85
10
1.00
10
0.25
)
1
¼
7.27
10
15
The fractional (or percentage) contribution of each species can be
determined using
