262
5 – Applications
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ORJ3 &2 >LQEDU@
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Figure 107 – Change in theoretical emf of the sensor as a function
of the logarithm of the CO 2 partial pressure for three temperatures.
5. a. The electrochemical chain of the sensor with the new reference electrode
is Au1,Ag / AgBr-NaBr / SE / Na 2 CO 3 ,O 2(g) ,CO 2(g) / Au2.
b. To determine the analytic expression of the emf ΔE at the sensor terminals, we consider the following equilibria:
2e Au1 m 2e Ag
2e Ag + 2Ag
+
Ref m 2Ag Ref
2AgBr Ref m 2Ag
+
Ref + 2Br
−
Ref
2Br
−
Ref + 2Na
+
Ref m 2NaBr Ref
2Na
+
SE m 2Na
+
Ref
2Na
+
W m 2Na
+
SE
Na 2 CO 3,W m 2Na
+
W + CO 2 + 2
1
O 2 + 2e W
2e W m 2e Au2
Adding these equilibrium reactions together term by term gives
2e Au1 + 2AgBr Ref + Na 2 CO 3 m 2Ag Ref + 2NaBr Ref + CO 2 + 2
1
O 2 + 2e Au2
and 2(
) 2
2
2
e
Au1
e
Au2
Ag
NaBr
CO
2
1 O
N a CO
A gBr
2
2
2
3
μ
μ
μ
μ
μ
μ
μ
μ
−
=
+
+
+
−
−
u
u
2F (
) 2
2
2
Au2
Au1
Ag
NaBr
CO
2
1 O
N a CO
A gBr
2
2
2
3
ϕ
ϕ
μ
μ
μ
μ
μ
μ
−
=
+
+
+
−
−
By using
ΔE = φ Au2 − φ Au1
we obtain