Severinghaus-type Probes

Selective potentiometric sensors are used as base sensor elements in modified probes. Two types of these are mentioned here briefly the potentiometric gas sensors and the potentiometric enzyme electrodes. 

As the dissociation equilibrium is achieved, a constant pH and cell voltage are achieved. Ionic species will not interfere with the electrode function that is not able to passing through the membrane. Volatile acids can interfere [28]. The pH-sensitive glass electrode can be replaced by the more robust IrO sensor [29]. A very similar ammonia gas sensor can be fabricated. 

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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. 

In 1954, the first pC02 sensor was introduced by Stow and Randall [18]. Severinghaus and Bradley [19] have optimized this sensing concept and pubhshed a CO2 sensor suitable for routine laboratory use one year later. These Severinghaus-type probes are still the sensors of choice for PCO2 monitoring in modem blood gas analyzers. 

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. 

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.)... 

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