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c
alibration
c
alibration theory
The sensor detects the partial pressure of H
one component of the total sulfide equilibrium system . If the total
sulfide concentration,
[S
tot], is defined as:
-2
S
= [H
S] + [HS
] + [S
-2
-
tot
2
the H
S concentration will be defined as
2
(
[H
S] = [S
] /
1 +
-2
2
tot
which can be simplified to
(
[H
S] = [S
] /
1 +
-2
2
tot
for pH<9 (Jeroschewski et al . 1996) . Thus it is necessary to know
the pH (i .e . to know [H
calculate [S
] (see Figure 1) .
-2
tot
pH = -log [H
]
+
In solutions with a pH below 4 the equation can be simplified to:
[H
S] ≈ [S
]
-2
2
tot
[H
O
] = [H
] = 10
+
+
-pH
3
K
= 10
-pK1
1
pK
is dependent on temperature and salinity . The literature
1
gives slightly different equations for calculating pK
function of temperature (T) and salinity (S) .
The following equation is derived by Millero et al . 1988:
pK
= -98 .08 + 5765 .4/T + 15 .04555 * LN(T) + (-0 .157 * (S
1
0 .0135 * S
12
]
2-
K
+
1
[H
[H
0
]
+
3
K
)
1
[H
0
]
+
3
O
]) of the sample/calibration solution to
+
3
S gas, which is only
2
K
K
1
2
)
0
]
+
2
3
in water as a
1
)) +
0 .5
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