Limiting current sensor

Fig. 7.19 High temperature limiting current sensor (a) pinhole version and (b) porous diffii-sional barrier version...

The limiting current sensor is suitable for use only in lean-bum gases. It is also critical to maintain a stable operating temperature at near 1100 K. In order to obtain reproducible... 

In case of a limited current sensor with a leak aperture according to Fig. 4 the flux J of the gaseous component to be measured to the working electrode can be calculated from the following equation ... 

We must remember that with amperometric sensors, the analytical information is obtained from the mass transport limiting current. One important consequence of the current-voltage equation is that one can always apply a potential high enough in order to transfer electrons to or from the electrode to a given species of interest. 

Yttrium stabilized zirconia (Zr02-Y203) as an electrolyte for reduction of molecular oxygen at elevated temperatures (400-800°C) has been already discussed in Section 6.23.4. In fact, both the reduction of oxygen and the oxidation of the oxide ion at the Pt/zirconia interface is reversible and the transport of both species in zirconia is so rapid it is possible to construct an electrochemical oxygen pump, which is the heart of the limiting current oxygen sensor described in this chapter (Saji, 1987). The overall electrochemical reaction that takes place at the porous Pt electrode is... 

Fig. 7.20 Response of limiting current oxygen sensor at 700°C to various partial pressures of oxygen (adapted from Pham and Glass, 1997)...

The transduction principle in amperometric sensors is proportionality between the bulk concentration of the analyte and the mass transport limited current. There are two ways to keep the proportionality constant really constant. What are they ... 

In the device of Fig.6a (sensor with distributed cavity), depending on the dimensions of the pores of the diffusion barrier, the oxygen diffusion can be bulk diffusion or Knudsen diffusion(ll). In the former case, the sensor output (limiting current Ig) is given by Eq. (9). Knudsen diffusion occurs when the average pore diameter is much smaller than the mean free path of the gas molecules, in which case collisions between molecules and pore walls are the dominant events. In this case(121. Dq -k T /2,and... 

A single-cell sensor based on the measurement of the limiting current can function properly only if the rates of the electrode reaction (1) and its inverse are higher than the rate by which O2 molecules arrive at the electrode by diffusion. The ability of an electrode to support reaction (1) and its reverse is conventionally expressed(fi) by the so-called exchange current density Jg, which is of the order of 1 A/cm2 for thin porous Pt electrodes at T > 600 C. 

Another important point to emphasize is that the limiting-current type of sensor requires that the impedance of the device (i.e. the resistance of the Zr02 material and of the electrode processes) is sufficiently small so that... 

It is apparent that the double-cell sensor can be operated in the limiting current mode (which corresponds to large values of Vg, of the order of 600 to 800 mV). The advantage of this mode of operation is that errors associated with the measurement of small values of Vg (e.g, less than 100 mV) are avoided. However, limiting current operation is generally undesirable since the problems associated with complete depletion of oxygen at the cathode of the pump cell can seriously limit the performance of the device. It is... 

In consideration of a sensor based on the prewave, the highest ascending part of this wave is not important from an analytical point of view. In a possible analytical application, the pseudo-limiting-current needs to be applied, because the current is higher and because it is less sensitive to variations in the potential of the reference electrode. 

From what is described in this section, it can be concluded that the kinetics of the oxidation reaction of sulphite and dithionite at a platinum electrode in alkaline solution are strongly affected by the nature of the platinum surface. This is important when a platinum electrode is used for a quantitative investigation of the kinetics of the oxidation of sodium dithionite and/or sulphite or as electrode material in the development of a sensor for the measurement and/or control of dithionite and/or sulphite concentrations. However, for sodium dithionite, it has no serious consequences because the limiting-current at 0.45 V vs. SCE does not change as a function of scan number. However, this oxidation is still irreversible (no return peak observed) which means that, in the onset of the voltammetric wave, the current is controlled by charge-transfer kinetics. Therefore, it is possible to investigate and obtain the mechanism of the oxidation of sodium dithionite, which is explained in the next section. 

In the first region, the current is completely independent of rotation rate of the electrode and increases exponentially, which means that in this region the current (or reaction rate) is mainly controlled by electron transfer and not by transport phenomena. This allows a study of the kinetics and the mechanism of the electron-transfer reaction of the oxidation of dithionite. The third region shows a well-defined limiting-current plateau. This indicates that in this region, electron transfer is so fast that the overall reaction rate is controlled by transport only. This is confirmed by a linear relationship between limiting-current and square root of the rotation rate of the electrode. In this region, it is not possible to study the kinetics and the mechanism, but such conditions are suitable for electroanalytical purposes and sensor development (see sections 6.5 and 6.7). 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

Associated apparatus is commonly installed in a safe area. Many applications of intrinsic safety in remote control and monitoring instrumentation are assembled in such a way that an intrinsically safe apparatus, e.g. a sensor or actuator in the hazardous area, is connected with an associated apparatus, e.g. a safety barrier or an Ex i-isolator in the safe area (see Fig. 6.196). With that, the associated apparatus takes over the function to safely limit current and voltage in the intrinsically safe circuit to permissible values. 

Externally, the only difference between this Ex i-isolator (Fig. 6.207) with galvanic isolation and a safety barrier is its increased size. The complex electronics and, of course, the components limiting current and voltage are hidden inside. In this example, a trip amplifier for DIN rail mounting is shown, which is suitable for operating temperature sensors in intrinsically safe circuits. The marking is ... 

Amperometric Sensors A number of voltammetric systems are produced commercially for the determination of specific species of interest in industry and research. These are usually based on measuring the limiting current at a constant applied potential and relating the measured current to concentration. This technique is often called amperometry. Amperometric devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors. Two of these devices are described here. 

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