Teledyne 3060e - Ultra Trace oxygen analyzer - Manual (Page 14)

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Model 3060E Model 3060E Model 3060E Model 3060E Model 3060E Chapter 2 Chapter 2 Chapter 2 Chapter 2 Chapter 2
Teledyne Electronic Technologies
Analytical Instruments
The sensing cathode used in the sensor is a high surface-area metal-catalyzed
gas diffusion electrode, with a surface area of 150–180 m
2 /g. A 0.4 mg/cm 2
catalyst load provides an effective cathode surface area of up to 600 times the
geometric area of the cathode. This produces a very high signal output per unit
concentration of O
2
; hence, an excellent signal-to-noise ratio is achieved even
at the highest sensitivity.
The metal-catalyzed gas-diffusion electrode consists of a hydrophobic Teflon-
carbon gas diffusion backing layer and a Teflon-carbon metal catalyst layer
bonded together. The thickness of the catalyst layer is approximately one-tenth
of the gas diffusion layer. The overall thickness of the cathode is approximately
0.5 mm.
The cathode is held against the sensor body by a polyethylene ring. The catalyst
layer of the cathode is exposed to the electrolyte phase; the hydrophobic
backing layer is exposed to the gas to be analyzed. The cathode assembly is
sealed by placing a polypropylene O-ring between the stainless steel plate,
containing stainless steel gas inlet and outlet tubes, and the sensor body.
The electrolyte in the sensor is continuously purged by bubbling the zero gas
(generated by passing the sample gas through the built-in scrubber) through the
fine porous frit mounted in the inner floor of the sensor. Sparging of the
electrolyte removes most of the dissolved O
2
from the electrolyte, reducing the
background signal.
Ultra Trace Sensor Operation
The gas to be analyzed enters the cavity through an inlet tube between the
cathode and the stainless steel plate, and exits through the outlet tube. During
this process, the gas diffuses through the gas wicks of the hydrophobic backing
layer, and reaches the catalyst surface where O
2
present in the gas mixture reacts
by the following mechanism:
O
2
+ 2H
2
O + 4e-
→
4OH- (cathode)
Due to the high surface area of the catalyst, most of the O
2
reacts at the cathode
surface and a very small amount of O
2
dissolves in the bulk electrolyte. The
continuous sparging of the electrolyte with zero gas does not allow the O
2
concentration to build up in the bulk electrolyte. The amount of O
2
that reaches
the catalyst surface is proportional to the partial pressure of O
2
in the gas
mixture.