ZIROX SGM7, Руководство пользователя

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Oxygen Monitor SGM7 10 Appendix
HB_SGM7_eng.docx
34
From this the NERNST equation for the equilibrium cell voltage follows :
U
eq
= (R ⋅ T / 4 ⋅ F) ⋅ ln [p(O
2
)'/p(O
2
)"] .
In gas potentiometry, one of the electrodes is fed with a gas of known
composition (reference electrode), and by measuring U
eq
and T the gas at
the measuring electrode is analyzed. Inserting the values for R and F into the
equation above and converting it into the lg-form, the following equation is
obtained for dry air under normal pressure at the reference electrode:
U
eq
/mV = 0.049606 ⋅ T/K ⋅ lg [p(O
2
) / (0.2093 ⋅ 1013.25 mbar)] .
In practice, the oxygen concentration ϕ (O
2
) in vol% is often used for
calculations, which relates to the partial pressure as p(O
2
) = ϕ (O
2
) ⋅ p / 100.
If the total pressure does not vary much from the normal pressure (1013.25
mbar), the following equations are used for calculations :
U
eq
/mV = 0.049606 ⋅ T/K ⋅ lg [ ϕ (O
2
) / 20.93 vol%] ,
ϕ (O
2
)/vol% = 20.93 ⋅ 10
(U
eq
/mV)/(0.049606 ⋅ T/K)
.
If a gas is present at the measuring electrode that mainly consists of reducing
components, the chemical standard potentials do not drop out when the cell
voltage equation is derived. In this case, a NERNST equation is obtained for
reaction cells with concentration-independent terms, e.g. cells with hydrogen
and water vapor at one of the electrodes and air at the other electrode the
equation, valid in the range 400...1000 °C,
U
eq
(H
2
,H
2
O-air)/mV = -1280.6 + { 0.3165 + 0.0992 ⋅ lg [ ϕ (H
2
O)/ ϕ (H
2
)] } ⋅ T/K
+/-1.
In several technical processes the quotient Q = ϕ (H
2
O)/ ϕ (H
2
) has to be
known. It can be calculated from this equation.
If the measuring electrode is the negative pole of the cell, the cell voltage is
signed negatively.
In the diagram plotted below the dependence of the equilibrium cell voltage
on
λ , the oxygen concentration, and an excess CH
4
in methane combustion
is shown.
The cell voltages calculated with the above-mentioned equations and
presented in the diagram are only valid for isothermal cells with equal
temperature at both electrodes. Such isothermal cells have been realized
very carefully in the ZIROX products. Unlike these, the well-known lambda
probes used in automobiles with catalysts in the exhaust tube are not
isothermal. Their main purpose is indicating
λ > or < 1, and they are less
suitable for precise gaspotentiometric analysis.