Oxygen partial pressure, measurement

Fig.5 Oxygen activity corresponding to equation 4 and the measured oxygen partial pressure as a function of the modified residence time...

Methanol oxidation on Pt has been investigated at temperatures 350° to 650°C, CH3OH partial pressures, pM, between 5-10"2 and 1 kPa and oxygen partial pressures, po2, between 1 and 20 kPa.50 Formaldehyde and C02 were the only products detected in measurable concentrations. The open-circuit selectivity to H2CO is of the order of 0.5 and is practically unaffected by gas residence time over the above conditions for methanol conversions below 30%. Consequently the reactions of H2CO and C02 formation can be considered kinetically as two parallel reactions. 

Figure 6. Tip of the 20-pm tip of a fiber optic oxygen microsensor. The tip is coated with a ormosil-type of sol-gel doped with a ruthenium indicator for oxygen that display red luminescence. The sensor measures its decay time as a function of oxygen partial pressure.

Lubbers D.W., Opitz N., The pC02-/p02 optode, a new probe for measurement of partial pressure of carbon dioxide or partial pressure of oxygen in fluids and gases, Z. Naturforsch. 1975 30C 532. 

Measurements of partial pressure of oxygen (pC>2) and of carbon dioxide (PC02) are described in the following sections. 

Peterson J.I., Fitzgerald R.V., Buckhold D.K., Fiber-optic probe for in vivo measurement of oxygen partial pressure. Anal. Chem. 1984 56 62. 

Such an electrochemical arrangement can also be used to transport oxygen from one electrode to the other by the imposition of an externally applied potential. This technique, known as coulometric titration , has been used to prepare flowing gas mixtures of oxygen/argon with a controlled oxygen partial pressure, to vary the non-stoichiometry of oxides, to study the thermodynamics of dilute oxygen solutions in metals, and to measure the kinetics of metal oxidation, as examples. 

The voltage measured will now appear to drift as the composition range of the nonstoichiometric oxide is crossed. The voltage will become constant above and below the composition range of the oxide. Note that this is closely related to the variation of oxygen partial pressure over a nonstoichiometric oxide (see Sections 7.3, 7.4). 

The high pressure p"0i is taken as a reference pressure, such as the oxygen partial pressure in air. A measurement of the cell voltage, together with knowledge of the temperature allows the unknown pressure p 0i to be determined. 

J. 1. Peterson, R. V. Fitzgerald, and D. K. Buckhold, Fiber-optic probe for vivo measurements of oxygen partial pressures, Anal. Chem. 56, 62-67 (1984). 

Several oxygen sensors based on oxygen pumping with stabilised zirconia have been reported (Hetrick, Fate and Vassell, 1981). This type of oxygen sensor is able to measure the oxygen partial pressure in the exhaust gas from the engine in lean burn. The operating principle of the... 

Equation 18 can be used to write the ORR rate in eq 13 or 15 in terms of the limiting current density, an experimentally measurable quantity, rather than the oxygen partial pressure. 

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