Oxygen partial pressure calibration

Under equilibrium conditions the electrical conductivity of many oxide phases, e.g., CugO, FeO, CoO, NiO, or ZnO at elevated temperatures is a function of the oxygen partial pressure in the ambient gas phase 12). The oxygen partial pressure determines the metal excess or deficit in the metal oxide and thereby the concentration of electrical carriers especially excess electrons and electron holes. Thus, after proper calibration, the steady-state oxygen activity ao(st) may be deduced from measurements of the conductance of a metal oxide foil used as catalyst while an oxygen transfer reaction, e.g., CO2 + H2 = CO -)- HjO or 2N2O = 2N2 + O2 proceeds at the surface of the metal oxide 13). 

One of the principal objections centers on the nonlinear calibration curve for the determination of blood oxygen partial pressures. This resulted from the formation of an oxygen-depleted boundary layer near... 

An inexpensive, electrochemical probe, sensitive to oxygen partial pressure is used to measure the oxygen composition in a gas permeate stream following gas separation. Separation of air by a barrier material, poly(ethylene carbonate), is shown as an example of probe operation. Separation and calibration techniques demonstrate probe accuracy and reproducibility. 

Figure 4.28 Calibration curves of a glucose-sensitive electrode, based on the PO2 transducer, as a function of oxygen partial pressure in the sample, between 100 and 800 mm Hg.

The specimen was suspended from a thin Pt wire in a vertical tube furnace equipped with a Zr02/Y2O3 oxygen transfer cell, which was placed next to the sample. The emf (electromotive force) of the transfer cell was used to monitor to oxygen partial pressure achieved with a CO2/CO gas mixture circulated over the sample. The unit was calibrated against the oxygen partial pressure in equilibrium with the Ni/NiO phase boundary. The appropriate CO2/CO content was then selected by reference to Fig. 4. The composition of... 

It is very interesting to examine the behaviour of the membrane oxygen electrode with a lowered partial pressure of gaseous oxygen in the inner space of the electrode (inside the YSZ test-tube) in the melts. The peroxide function of gas oxygen electrodes (and of the gas membrane oxygen electrode, as well) is caused by the formation of stable peroxide ions at the interface boundary. Therefore, the reduction of the partial pressure of 02 in the inner electrode space should result in a shift of the inflection point of the E-pO calibration plot to lower pO values. It follows from the above-considered investigations... 

Fig. 2.4.14. -pO calibration plots of the cells with Pt(02)lYSZ membrane oxygen electrodes with different partial pressures of 02 in the inner electrode space in molten Csl at 700 °C (a) 1, 02 2, CulCu20 3, NilNiO (b) TilTi02. 

Calibration of the oxygen electrode is done with the help of an oxygen free sample (5 mg sodium sulphite in 5 ml 0.01 M borax, or water through wnlch nitrogen gas has been bubbled) and another sample with a known pO equUihratlng a liquid with air with a known ojqrgen content). The response of the calibratea electrode is exactty linear to the partial pressure in the sample. 

Calibration Procedure. The probe potential exhibits a linear relationship with the partial pressure of the oxygen for the pressure range explored in this study. Thus, calibration at varying pressures and varying compositions was required for system operation. Addtionally, the probe baseline tends to drift with time, as shown in Figure 3. Therefore, the probe must be recalibrated occasionally to ensure accuracy. 

System Calibration. Calibration lines are shown in Figures 4 and 5. A linear relationship exists between the detector reading and the partial pressure of oxygen in the system. Because the meter reading is proportional to the system pressure and the precision of the multimeter is a constant 0.001 mV, the accuracy of the probe increases with an increasing system pressure, as detailed in Table 1. Therefore, the larger the pressure in the permeate volume, the more accurate the result, as predicted by the Nemst equation. A linear relationship exists between the detector rea ng and the partial pressure of oxygen in the system for the low pressures examined in this study. 

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