RADIOMETER ABL800 FLEX, Справочное руководство

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ABL800 FLEX Reference Manual 1. Potentiometric measuring principles
pCO
2
electrode,
Continued
Nernst equation
(continued) The pH value is related to the partial pressure of CO
2
in the sample by the
following equation:
2
CO 2
-
3
a
CO
HCO
log + pK = pH
α
×
p
c
where:
pK
a
=
−
log K
a
, the equilibrium constant for the dissociation of carbonic acid in
water
α
CO
2
= solubility coefficient for CO
2
in water
The bicarbonate concentration
[ ]
HCO
3
-
is so large compared to
[
that it can be
considered constant. At constant temperatures
]
H
+
α
CO
2
is also constant. So the
equation can be simplified to:
pH = K' - log CO
2
p
where:
K' is a constant incorporating the equilibrium constant for carbonic acid (K
a
), the
bicarbonate concentration, and the solubility coefficient
α
CO
2
.
2
3
CO
HCO H
− + ×
=
c c
K
a
is the equilibrium constant for carbonic acid.
p CO
2
of the sample is then calculated from the equation above.
Designation The following symbols are used:
p CO
2
(Gas1),
p CO
2
(Gas2)
= Pressure of CO
2
in Gas 1 or Gas 2, respectively
F CO
2
(Gas1),
F CO
2
(Gas2)
= Fraction of CO
2
in Gas 1 or Gas 2, respectively
B
Gas 1 or 2
= Pressure inside the measuring chamber during a
measurement on Gas 1 or Gas 2 respectively
p H O
2
= Water vapor pressure (6.2751 kPa at 37
o
C)
E(CO
2
,Gas1),
E(CO
2
,Gas2)
= Potential of the p CO
2
electrode from a measurement on
Gas 1 or Gas 2, respectively
Sens( p CO
2
,theo)
= Theoretical (absolute) sensitivity of the p CO
2
electrode
at 37
o
C
Sens( p CO
2
,prev)
= Relative sensitivity of the p CO
2
electrode from the
previous 2-point calibration
Continued on next page
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