Solid-state reference electrodes

A potentiometric electrochemical cell consisting of a reference electrode, solid-state electrolyte(s), and an indicator electrode can provide information about the partial pressure of gas in the same way as the cells utilizing ion-selective electrodes and liquid electrolytes can. The general mechanism is as follows. A sample gas, which is part of a redox couple, permeates into the solid-state structure usually through the porous metal electrode and sets up a reversible potential difference at that interface according to the reaction... 

Holzinger, M., Maier, J. and Sitte, W. (1996) Fast CO2-selective potentiometric sensor with open reference electrode. Solid State Ionics, 86-88, 1055-62. 

Figure 13.9 Schematic of reference electrode, (a) Gas-phase reference (b) Solid-state reference (c) Reference with electrolyte chain.

Liquid junction problems can be avoided by using as reference a solid-state lanthanum fluoride membrane electrode in conjunction with spiked fluoride samples [9j. On details, Moore [46,53] has highlighted the need for careful sampling and subsequent processing by his studies on technique, selectivity, calibration, sample freezing and pH. 

Eor a lot of potentiometric indicator electrodes solid-state embodiments are available widely equivalent to the electrolyte containing systems concerning the electrode performance. Because this does not apply for all measuring electrodes (e.g., for pH glass electrodes) and for reference electrodes to the full extent, there is also still a need for development in this field. 

Similarly to aqueous electrochemistry, potentials in solid state electrochemistry utilizing YSZ are expressed in terms of the potential of a reference metal electrode exposed to P02 = 1 atm at the temperature T of interest. Thus a standard oxygen electrode scale (soe) can be defined. Similarly to equation (7.2) one has ... 

The use of the soe scale is more convenient than the she scale in solid state electrochemistry since YSZ (an O2 conductor) is the most commonly used solid electrolyte and the metal/02 (po2 =1 atm)/YSZ (soe) electrode is the most commonly used reference electrode in solid state electrochemistry. 

Figure 7.12 shows the relationship between the standard oxygen electrode (soe) scale of solid state electrochemistry, the corresponding standard hydrogen electrode (she) scale of solid state electrochemistry, the standard hydrogen electrode (she) scale of aqueous electrochemistry, and the physical absolute electrode scale. The first two scales refer to a standard temperature of 673.15 K, the third to 298.15 K. In constructing Figure 7.12 we have used the she aqueous electrochemical scale as presented by Trasatti.14... 

What is the practical usefulness of the soe scale of solid state electrochemistry As in aqueous electrochemistry, it is limited but not trivial. When a potential Uwr of, e.g. -300 mV is measured in an YSZ solid state cell at 673 K vs a reference electrode at p02=l atm, the implication is that the work function of the reference electrode is 5.14 eV and that of the working electrode 4.84 eV regardless of the material of the two electrodes. 

The microarray electrodes used for solid state electrochemistry are a slight variation of the transistor decribed in Sect. 5.2.2 The most appealing feature is the location of all the necessary electrodes on a single microchip, the reference electrode being provided by the application of a droplet of silver epoxy to one of the gold micro electrodes (Fig. 7). 

Electrode potentials are relative values because they are defined as the EMF of cells containing a reference electrode. A number of authors have attempted to define and measure absolute electrode potentials with respect to a universal reference system that does not contain a further metal-electrolyte interface. It has been demonstrated by J. E. B. Randles, A. N. Frumkin and B. B. Damaskin, and by S. Trasatti that a suitable reference system is an electron in a vacuum or in an inert gas at a suitable distance from the surface of the electrolyte (i.e. under similar conditions as those for measuring the contact potential of the metal-electrolyte system). In this way a reference system is obtained that is identical with that employed in solid-state physics for measuring the electronic energy of the bulk of a phase. 

Rao et al. [140] described a study using a commercial pH sensitive radio-telemetry capsule (RTC) to evaluate small bowel and colonic transit time in athletes with gastrointestinal symptoms. The RTC (type 7006 Remote Control Systems, London, UK) consists of a glass electrode with an integral reference cap and battery. RF transmissions from the capsule are detected by a solid-state receiver worn on the belt of the patient. The recorder samples the pH from the capsule at 6 second intervals for a period of 24 hours. They used pH changes as an indication of the pH capsule s movement. A sharp rise in pH from around pH 2 to pH 6 indicates that the capsule has moved into the duodenum from the stomach. Then the pH progressively rises to a plateau around pH 8, which indicates that the capsule has moved into the terminal ileum. Another commercially available wireless pH sensor (Bravo) from Medtronic Inc. has been used to measure esophageal pH for a period of 48 hours and it will be discussed later. 

H.J. Yoon, J.H. Shin, S.D. Ixe, H. Nam, G.S. Cha, T.D. Strong, and R.B. Brown, Solid-state ion sensors with a liquid junction-free polymer membrane-based reference electrode for blood analysis. Sens. Actuators B. 64, 8-14 (2000). 

J. Gabel, W. Vonau, P. Shuk, and U. Guth, New reference electrodes based on tungsten-substituted molybdenum bronzes. Solid State Ionics 169, 75-80 (2004). 

G. Valdes-Ramirez, G.A. Alvarez-Romero, C.A. Galan-Vidal, P.R. Hernandez-Rodrguez, and M.T. Ramrez-Silva, Composites a novel alternative to construct solid state Ag/AgCl reference electrodes. Sens. Actuators B. 110, 264-270 (2005). 

A disadvantage of the described reference electrode is its maintenance porous frits may dry out, Cl solution needs refreshment from time to time. Therefore the Hewlett-Packard HP 1049A Electrochemical Detector cell contains a solid state, in situ Ag/AgCl reference electrode as a standard with the refillable electrode described above as an option. 

The solid state reference electrode has no internal Cl solution but uses Cl ions in the mobile phase to keep its potential constant. This means that a certain amount (typically 1 to 10 mM) KC1 must be dissolved in the mobile phase. 

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