Severinghaus-type electrodes

Because the electrical circuit is closed inside the sensor, no external reference electrode is necessary and the Severinghaus-type electrode can be used for measurement in either gaseous or liquid samples. It is important to remember, however, that the potential of the internal reference electrode must remain constant. In principle, it would be possible to use a liquid junction but it would add to the complexity of the design. Because the counterion resulting from the dissociation equilibrium is the only interfering ion, and because it is present in a very low concentration, it is possible to ascertain the constancy of the reference potential by careful choice of the internal electrolyte. Thus, for example, in the CO2 electrode the internal electrolyte is O.lMNaHCOs and 0.1 M NaCl and Ag/AgCl is used as an internal reference element. 

To date, the majority of enzyme-based potentiometric sensors do not involve detection with an ionophore-doped selective membrane and fall outside of the scope of this chapter. The same is also true for most Severinghaus-type gas sensors, where a gas-permeable membrane covers an inner solution in which the gaseous analyte is determined with an ISE. Most Severinghaus-type electrodes use a pH-sensitive glass electrode to monitor the pH of this inner filling solution. However, ammonia has been detected indirectly with an ammonium-selective ionophore-based ISEs upon protonation in that inner solution, and the use of other ionophore-based ISEs for the more selective detection both in enzyme-based and Severinghaus-type ISEs is readily conceivable. 

The electrolytes Na", and Cl are second only to glucose in being the most frequently run hospital tests. Many clinical chemistry analyzers now contain an ISE module for electrolyte analysis. Most commonly the module will consist of a Na -glass electrode, a valinomycin/PVC electrode, a Ag/AgCl pellet or a quaternary ammonium ion/PVC electrode and a reference electrode. A selective electrode for the bicarbonate ion continues to elude workers in the field. An indirect measurement of HCOf must be made. The sample is usually reacted with acid to evolve carbon dioxide gas which is measured with a traditional Severinghaus type CO2 electrode. Alternatively, the sample is treated with base to convert HCO to CO3 and a carbonate ion-selective electrode is used In this manner, the complete primary electrolyte profile is obtained electrochemically. 

L. C. Clark first suggested in 1956 that the test solution be separated from an amperometric oxygen sensor by a hydrophobic porous membrane, permeable only for gases (for a review of the Clark electrode see [88]). The first potentiometric sensor of this type was the Severinghaus CO2 electrode [150], with a glass electrode placed in a dilute solution of sodium hydrogenocarbonate as the internal sensor (see fig. 4.10). As an equilibrium pressure of CO2, corresponding to the CO2 concentration in the test solution, is established in the... 

Table 6.6 Performance characteristics of Severinghaus-type gas electrodes...

A large number of ISEs for a variety of analytes has been described in the literature. Only a few of them have found real routine use (Table 2). The glass pH ISE is unrivalled in fact, the overwhelming majority of pH determinations in all fields of human activity is done potentiometrically with the glass electrode. Moreover, the pH ISE is very often used in more complex sensors, such as the Severinghaus-type gas probes and enzyme sensors. 

Compact gas probes In compact gas probes (combination electrodes), the ion-selective sensor is covered with a gas-permeable membrane enclosing a thin film of electrolyte containing the ion sensed (the Severinghaus-type probe. Figure 1). A gas from the test medium passes through the membrane, dissolves in the electrolyte film, and changes the activity of the ion sensed. The type of ion-selective sensor. 

Hgure 1 A simplified scheme of the CO2 membrane gas probe (Severinghaus type). 1, Detector body 2, indicator electrode (pH glass electrode) 3, reference electrode 4, internal electrolyte 5, gas-permeable membrane 6, medium analyzed and 7, voltmeter (pH-meter). (The components of the probe are not drawn to a real scale.)... 

Figure 4. Schematic diagrams of (A) a Severinghaus-type SO2 gas sensor and (B) a SO2 gas sensor based on a HSO -selective electrode.

Severinghaus electrodes have found wide application in clinical analysis. It is pertinent to mention here that the general principle of permeation of the gas through a hydrophobic membrane followed by its detection (with or without its solvolysis) has been used with different types of internal sensors, for example, optical, ampero-metric, conductimetric, or a mass sensor. The choice of the internal sensing element depends on the circumstances of the application in which the gas sensor would be used, such as the required time response, selectivity considerations, complexity of instrumentation, and so on. 

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

In this section the basic design and operation of these types of sensors are described. Examples of such sensors in common use are the Clark oxygen electrode [10] and the Severinghaus carbon dioxide electrode [11]. 

Gas sensors and biosensors are obtained by fixing an auxiliary chemical or biochemical system over the ISE membrane. The analyte reacts with the auxiliary system with production or consumption of the ion that is sensed by the ISE. Two basic types of gas sensor, the Severinghaus electrode and the air-gap electrode, are described elsewhere in this encyclopedia. 

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