Severinghaus Electrodes

One could immobilize the urease layer on top of a Severinghaus electrode for CO2 or NH3 (Section 6.3.2) and use the device as an enzymatic-potentiometric gas sensor. The primary disadvantage of such an arrangement would be its slow response time. A more direct way is through the detection of the ionic species resulting from the hydrolysis of ammonia and carbon dioxide. 

Fig. 6.28 Schematic of general Severinghaus electrode for sensing of hydrolyzable gases...

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). 

The theory of operation of the conductometric gas membrane sensor has been experimentally verified in detail for CO2 and SO2, and sensors for H2S and NH3 based on the same principle have also been made. The basic transport and equilibration processes are the same as in the Severinghaus electrode (Section 6.2.2). Upon entering the aqueous solution inside the cell, the gas dissociates to its constituent ions. Because each dissociated species contributes to the overall conductivity, the specific conductance A of the cell is... 

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. 

The oldest clinical appKcations of po-tentiometry involved blood gas analysis, and this method is still used to quantitate dissolved carbon dioxide a glass pH electrode is housed behind a membrane permeable to CO2, and a thin solution layer of hydrogen carbonate separates the pH electrode from the membrane. This device, called the Stow-Severinghaus electrode, remains unchanged in principle from its original version reported in 1958 [18]. The measurement is based on the hydrolysis of CO2 to HC03 andH+ in the thin solution... 

There is also a solid-state version of the Severinghaus electrode which utilizes a Pd/PdO internal pH electrode [102] and Cl2-electrode based on an Ag/AgCl internal electrode [103]. Enzymatic Severinghaus electrodes include, for example, electrodes for the determination of urea [100] and lysin [104]. 

Parameter Severinghaus electrode Conductivity sensor Solid electrolyte sensor NDIR sensor... 

PCO2 can be measured directly by a PCO2 electrode (th< Severinghaus electrode). This is essentially a pH electrodt in contact with a bicarbonate solution. It is separated fron the sample by a membrane which is permeable to gas but no to solutions. CO2 diffuses across the membrane from th sample into the bicarbonate solution. This results in i... 

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