Chemical Potential Sensors

Different types of sensor based on solid electrolytes have been developed following a report by Kiukkola and Wagner (1957). These sensors are based on one of two principles (a) the chemical potential difference across the solid electrolyte (potentiometric sensor), or (b) the charge passed through the electrolyte (amperometric sensor). In the following galvanic cell,... 

Generally, in solid electrolytes, ionic conductivity is predominant (( = 1) only over a limited chemical potential. The electrolytic conductivity domain is an important factor limiting the application of solid electrolytes in electrochemical sensors. 

Figure 12-9. Chemical potential of silver during u- /8 phase transformation of Ag2+

In most sensing situations, the analyte molecules are transported to and from the sensor by diffusion. This is particularly true for sensors in which the analyte is chemically transformed, such as sensors that rely on catalysis or in amperometric sensors. Diffusion is another activated (see (B.5)) process in which time and temperature have to be considered. It is driven by the gradient of chemical potential (A. 19). Two kinds of diffusion are most relevant. These are the Fickian diffusion, which depends on molecular properties of both the diffusing medium and of the diffusing species, and the Knudsen diffusion which depends on the size of the vessel. 

This anodic reaction provides sodium ions and electrons to the solid electrolyte and the inert Pt counter electrode, respectively, at the source side. Both the sodium ions and electrons will then travel through the solid electrochemical cell along previously-mentioned ionic and electronic paths to sustain the PEVD cathodic reaction for Na COj product formation at the sink side. Eurthermore, based on anodic reaction 60, the chemical potential of sodium is fixed by the vapor phase at the source side. Under open circuit conditions, this type of source can also serve as the reference electrode for a CO potentiometric sensor. 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

Plachrritsa, V.V, and Miura, N. (2005) Effects of different additives on the sensirrg properties of NiO electrode used for rrrixed-potential-type YSZ-based gas sensors. Chemical Sensors, 21 (Suppl. B). 94-6. 

If the SE and RE of the zirconia-based sensor are exposed to different oxygen partial pressures, P02 (gas) and 7 02 (reference), this induces different chemical potentials of oxygen ions in zirconia at the interfaces with gas phases. In order that the electrochemical potential remains constant, the electrical potential has to be different. Therefore, the output emf of the electrochemical cell (3.2), represented as a difference between potentials on the RE and SE, obeys the well-known Nemst s law ... 

The situations depicted in Figs. 7.18a and 7.19c are very similar in that in both cases a driving force exists for mass transport. The origin of this force is the chemical potential gradient dji/dx that exists across the growing oxide layer in one case and the solid electrolyte or sensor in the other. 

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