Waltron 9072, Руководство пользователя

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Waltron User Manual 101-054-A.1
9072 Hydrazine Analyzer
OPERATING PROCEDURES
3
OPERATING PROCEDURES
3.1 PRINCIPLE OF MEASUREMENT
The 9072 Hydrazine Analyzer electrochemical hydrazine sensor functions in a
potentiostatic mode with an open three-electrode system. The measuring electrode is
made of solid titanium, coated with gold. The counter electrode is made of stainless steel
and the reference electrode is made of silver/silver-chloride. The use of open measuring
electrode means, that there is a direct contact between the sample and the measuring
electrode. A membrane is not required. For the measurement of dissolved hydrazine
(N2H4) the fluid medium flows into the coaxial measurement cell between the platinum
measuring electrode and counter electrode. If the measuring electrode has reached the
correct potential, hydrazine is oxidized in the interphase boundary layer. The
electrochemical reaction may represented qualitatively as:
4OH-

N
2
+ H
2
O + 4e-
The characteristic potential for this reaction the potential at which hydrazine oxidation occur
in preference to other competing reactions is measured by holding the potential of the
measuring electrode steady with a potentiostatic and comparing its value with that of the
reference electrode. Under these conditions the rate of the electrochemical reaction is limited
by the rate of diffusion of hydrazine molecules to the electrode. As long as we remain within
this ‘diffusion-limiting-current’ area, the electrochemical cell delivers an electric current,
whose size depends linearly on the concentration of dissolved hydrazine. This current, which
is measured by the instrument’s electronic processing unit, is known as the diffusion-limiting
current,
I
diff
: cN2 H4 = k • I
diff
This diffusion-limiting current depends mainly on the mass transport of the
electrochemically active components, and hence on the hydrodynamics in the measuring
cell. It therefore changes significantly with the temperature and flowrate through the
measuring cell. In practice the flowrate and temperature vary over wide ranges, but the
instrument corrects these changes by using empirical correlations.
A second important influence on the measurement is the condition of the electrodes. In
particular the adsorptive and oxidative state of the electrode surface layers. This is
independent of the flow conditions; it depends instead on the voltage between the
electrode and the electrolyte, and on the material of which the electrode is made
Appearing flow- and temperature effects of the measuring signal are recorded with the
aid of the installed flow meter and temperature sensor and are compensated, if necessary.