Reference electrode oxygen partial pressure

Description of the cell composition is based - as far as possible - on the Stockholm convention (1953), i.e. the left-hand electrode constitutes the negative terminal of the cell. Cells are listed according to the metallic constituent of the electrode mentioned first which is involved in the electrode reaction establishing the respective electrode potential. Contact materials and conductive additives may be mentioned first before the actual element of interest only for the sake of correct materials sequence. The sequence of electrode components is stated as reported in the original publications. When an oxygen electrode is used as reference electrode an oxygen partial pressure of 0.21 atm is assumed. 

While a number of designs have been used, most oxygen sensors for automotive applications consist of a hollow, closed end tube, a schematic of which is shown in Figure 2. As shown, the interior of the closed end tube is open to the atmosphere which serves as a constant or reference oxygen partial pressure while the exterior is exposed to the exhaust gas. The voltage signal produced by the electrolyte is sensed by electrodes on the inner and outer surface of the sensor. These, in turn, are connected to the electronics package of the closed loop system. 

A major factor in the selection of a reference electrode is the reference potential established. If the reference potential is close to the potential at the sensing electrode, the cell voltage will be small and thus tend to be more stable than a large voltage. For example, the oxygen partial pressure established by a Pd + PdO reference electrode... 

The basic principle of the potentiometric ZrOz oxygen concentration cell is explained in Section 5.63.1, Fig. 5.6.2. The sensor consists of a Zr02 solid electrolyte body coated with two Pt electrodes, a measuring electrode exposed to the exhaust gas with an oxygen partial pressure of p 02, and a reference electrode exposed to a reference atmosphere with a constant oxygen partial pressure of p f2, typically ambient air. 

On the other hand, Ni,NiO mixtures that are contained in a limited volume or in an inert gas, deliver at constant temperature a definite constant oxygen partial pressure, which can be used in reference electrodes. The electrode reaction can run without molecular oxygen (cf. Equation (25-9)) ... 

Comparison of Equation (26-10) with Equations (26-11 a) and (26-1 lb) shows that the reference electrode (hereafter often zirconia electrode ) functions as a platinum electrode in a melt with defined oxygen partial pressure, since Uo -, zr02(m) = o2-, zro w because of the uniform solid electrolyte [1]. This is also seen from Equation (26-12) for its galvanic potential ... 

Figure 26-15. Relative potentials of a Zr02 electrode with 1 bar oxygen partial pressure in glass-forming melts with temperature-independent (A, B) and temperature-dependent (C, D) standard Seebeck coefficients. Reference potential 9, = 0. (a) Fiolax klar (b) (Na20)oo7 (K20)o 78(CaO)o, (Si02)o.737 (c) (Na20)o,s6(CaO)ojB7(Si02) .737 (d) BK7 (e) phos-phate-ba optical glass.

This working principle is the one in the lambda probes used to control combustion in engines. If the two electrodes are under different oxygen partial pressures, then an electromotive force is produced following the Nemst law (Equation [16.4]). In practical devices, one of the electrodes is in permanent contact with a reference atmosphere (see Fig. 16.2). 

The wide range of oxygen partial pressures makes such cells an excellent analytical instrument for measurements of gases. It should be mentioned that the measured value is obtained almost instantaneously. In such measurements a reference atmosphere, for example, air or a metal-metal oxide mixture, is located on one side of the zirconium dioxide, so that the oxygen partial pressure is already known at one electrode the oxygen partial pressure at the other electrode is then measured by the emf in accordance with Eq. (47). In the measurement the gas is passed to the sensor. 

It can be concluded therefore that at 1273°K, a cathodic overpotential of 1.228V (with reference to an electrode maintained at an oxygen partial pressure of 1 atm.) should be sufficient to initiate a reaction between the platinum electrode and the zirconia based electrolyte... 

In actual practice, the most often used reference media in commercial probes are air with or without circulation and metal-oxide couples M0-M0O2 or Cr-Cr202 for which equivalent oxygen partial pressure at l600 C are respectively lO and 10 "13 atm, (22), Internal electrodes are either platinum or molybdenum. 

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