Solid state internal references

FIGURE 10.8. Examples of electrochemical gauges (potentiometric sensors) (a) laboratory oxygen sensor (air reference), (b) oxygen minisensor with a solid state internal reference (M-MO), (c) chlorine sensor with a solid state internal reference (Ag-AgCl). 

Rosini, S., Siebert, E. (2004) Solid-state internal reference electrode based on quinhy-drone for hydrogen sensor with acid-doped polybenzimidazole. Electrochimica Acta, 49, 525-536. 

Since 1925, The International Commission on Radiation Units and Measurements at Bethesda, Maryland has been publishing reports updating the definitions and units for measurements of various radiation-related quantities. Of these ICRU Reports, special mention may be made of reports no. 19 (1971) [radiation quantities and units], 33 (1980) [radiation quantities and units], 36 (1983) [microdosimetry], 47 (1992) [thermoluminiscent dosimetry], and 51 (1993) [radiation protection dosimetry]. A succinct description of various devices used in dosimetry, such as ionization chambers, chemical and solid-state dosimeters, and personnel (pocket) dosimeters, will be found in Spinks and Woods (1990). In this section, we will only consider some chemical dosimeters in a little detail. For a survey of the field the reader is referred to Kase et at, (1985, 1987), McLaughlin (1982), and to the International Atomic Energy Agency (1977). Of the earlier publications, many useful information can still be gleaned from Hine and Brownell (1956), Holm and Berry (1970), and Shapiro (1972). 

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. 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

Previous reviews on solid-state infra-red spectroscopy are, e.g. by Vedder and Hornig I7) which contains a relatively complete bibliography of all the most important work up to 1960 and by Mitra 18), while Wilks and Hirsh-feld 19) have written a review of internal reflection spectroscopy. Orville-Thomas 20) wrote a review on I. R. spectroscopy as a diagnostic tool, and Annual Reviews of Physical Chemistry 21) often contain review papers on infra-red spectroscopy which also refer to the solid state. 

The solid state high resolution n.m.r. spectra were run on a CXP 200 BruKer spectrometer in which the 31P nucleus resonates at 81 MHz. Samples used were finely powedered and hand-tamped in glass tubes. The spectra were recorded using the Proton Enhanced Nuclear Induction Technique (6) on the same basis of a one shot cross polarization and high power decoupling during acquisition. A capillary tube of trimethylphosphate inserted in the powder sample is used as internal reference. 

Characterization of materials in the solid state, often loosely referred to as materials characterization, can be a vast and diverse field encompassing many techniques [1-3]. In the last few decades, revolutionary changes in electronic instrumentation have increased the use of highly effective automated instruments for obtaining analytical information on the composition, chemistry, surface, and internal structures of solids at micrometer and nanometer scales. These techniques are based on various underlying principles and cannot be put under one discipline or umbrella. Therefore, it is important first to define the scope of techniques that can be covered in one chapter. 

We now turn to the experimental method of measurement of Em. The potential on the membrane exterior is measured by an Ag AgCl or SCE reference electrode. The interior potential is very difficult to measure through a direct metal contact (only in some solid state and in hybrid sensors, Section 13.10) and one opts for another reference electrode called the internal reference. Thus a typical cell would be... 

Fig. 13.7. Forms of ion-selective electrodes with solid state membranes (a) with internal reference electrode (b) with ohmic contact (c) with ohmic contact and combined reference electrode.

In these sensors the technology developed for ISFET construction is used in conventional electrodes. Links between the membrane and internal reference are metallic (ohmic contact), by deposition of the metal on the membrane (solid state membranes), or by deposition of an ion-selective membrane on a metal. This latter is an integrated sensor. 

Recendy original all solid-state elearodes for NH4 were successfully combined with urease for the assay of urea (70,71). These electrodes consist of a conductive resin (epoxy -t- graphite) covered by a nonactin-PVC matrix. They offer interesting commercial advantages over membrane electrodes with internal solution and reference electrode. 

As was stated above, ferrocene is considered to be a potential internal reference for chloroaluminate systems. There are only a few studies that mention the redox potential of ferrocene in nonchloroaluminate systems. Table 4.5 gives the potential data corrected with the redox potential of ferrocene in each RTIL, if available. Figure 4.7 shows the EW of the RTILs. Shown in the figure are the EWs of both chloroaluminate (solid line) and nonchloroaluminate (dotted line) systems. It is interesting to note that both the cathodic and anodic limiting potentials of the RTILs based on the same cation are the same whether the system is chloroaluminate... 

Figure 21-20. Experimental and predicted internal concentration profiles for the separation of l-phenoxy-2-propanoi using operating point 1 from Figure 21-19. Experimental data ((-)-enantiomer, (-I-)-enantiomer, A) were taken at half-time period in the cyclic steady state. Theoretical data ((-)-enantiomer, thick lines (-t)-enantiomer, thin lines) illustrate the band profiles along the columns just after switching (dashed lines) and just before switching (solid lines). (From reference 124, with permission.)...

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