ZIROX SGM7.2, Руководство пользователя

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Oxygen Monitor SGM7.2 10 Attachment
platinum layers on ceramic bodies of stabilized ZrO
2
(stabilized against breaking),
electrode reactions with the oxygen ion vacancies V
O
are possible :
1/2 O
2
(gas) + 2 e
-
(platinum) + V
O
(solid electrolyte)
=
O
2-
(solid electrolyte) ,
H
2
O(gas) + 2 e
-
(platinum) + V
O
(solid electrolyte)
=
O
2-
(solid electrolyte) +
H
2
(gas) .
Oxygen atoms which are splitted off from molecular oxygen or water vapour, take
up electrons at the surface of the platinum and move to oxygen vacancies of the
solid electrolyte where they form oxide ions. This process however quickly comes
to a stand-still if the electrode is in an open circuit, and neither electrons nor
oxygen ions can flow. In this state, the output of chemical work from the particle
transfer equals the effort that has to be made in terms of electric work. An
electrochemical equilibrium exists in this case, which is a dynamic equilibrium.
The electrode reactions still occur, but equally fast in both directions. The larger
the so-called exchange current density, the less sensitive the electrode is against
disturbances.
At electrochemical equilibrium, the platinum has either given off electrons and is
positively charged, or it has taken up electrons and is negatively charged. The
first can be expected in oxygen, the second in hydrogen.
If two oxygen electrodes are exposed to different oxygen concentrations, on
opposite sides of a gastight sintered ZrO
2
solid electrolyte, then the electrode
exposed to the higher oxygen concentration will be charged more positively than
the electrode exposed to the lower oxygen concentration if the system is in
electrochemical equilibrium. A cell potential can be measured between the
electrodes, that is higher the more the oxygen concentrations at the two
electrodes differ.
In 1889 NERNST was the first to describe the quantitative connection between
the cell potential and the particle concentrations at the electrodes with the so-
called NERNST-equation. In electrochemical thermodynamics, this equation can
be deduced from the chemical potentials (consisting of energy and entropy
components) of the particles participating in the cell reaction (i.e. sum of the
electrode reactions). The chemical potential of the oxygen is given by
μ
(O
2
) =
μ
(O
2
)
,
+ R
⋅
T
⋅
ln p(O
2
) .
For a solid electrolyte cell with two oxygen electrodes, the cell reaction is merely
the transfer of oxygen from higher to lower partial pressure. The chemical work in
cell reactions is described with the molar free reaction enthalpy (Gibbs free
energy)
Δ
R
G, which equals the difference in chemical potentials :
Δ
R
G =
μ
(O
2
)' -
μ
(O
2
)" = R
⋅
T
⋅
ln [p(O
2
)'/p(O
2
)"] .
In isotherm cells, the standard potentials
μ
(O
2
)
,
on both sides are equally high,
and thus drop out.
Δ
R
G equals the maximum work that can be won for an
infinitely slow reaction, i.e. at extremely slow current flowing through the external
circuit. It can be calculated using the equilibrium cell voltage U
eq
, the molar
charge F (Faraday’s constant), and the amount of electrons that are exchanged
in the cell reaction (4 electrons in case of O
2
) :
W
electric
= 4
⋅
F
⋅
U
eq
.
From this follows the NERNST-equation for the equilibrium cell voltage :
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 analysed. For dry air at normal pressure at the reference
electrode, inserting the values for R and F into the equation above and converting
it into the lg-form, the following equation is obtained:
HB_SGM72_Rehm_eng.DOC
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