Potentiometric Sensors Equilibrium

Both potentiometric (equilibrium) and amperometric (dynamic) modes of operation, however, require facile electron transfer between the biomolecule and the surface of an electrode. For most large, electroactive biomolecules such direct heterogeneous electron transfer is extremely slow and even for the measurement of equilibrium potentials, which requires only very small currents, a suitable mediator must be added to the solution. Clearly, provision for charge transfer mediation is essential for the successful construction of sensors of both types. The selection of an appropriate mediator is usually based on the fulfillment of certain criteria. Its heterogeneous and homogeneous redox reactions must take place rapidly at a well-defined potential in the medium of interest and involve the transfer of a definite number of electrons to and from stable redox species. In addition the mediator should be adequately soluble, usually in aqueous media at pH 7, and should not inhibit the reaction of the biomolecule with its substrate. Very few substances meet all these demands. 

Dq is not negligible [446]. In complete contrast to the bulk conductivity sensors, redoxactive impurities may now be of considerable advantage (see Section 6.6.1). The third type of sensor, in which a pure ion conductor is used, is the EMF sensor (potentiometric sensor), as represented by the A-probe, dealt with in detail in Section 7.2. Again, only local equilibrium is set up there (see Table 7.2), thus, in the oxide V/xo 0, but now conversely = 0 Vpe- (see previous section). Here... 

A solid electrolyte is a functional material in which single ions migrate through a solid structure, and these materials are expected to be applied as components of compact gas sensors. There are two sensing mechanisms which may be exploited in solid electrolyte sensors potentiometric and amperometric. Potentiometric sensors may be further divided into equilibrium and non-equilibrium potential (mixed potential) types, although typically only the equilibrium potential type has been investigated for use in CO2 sensors. 

By using different membranes, it is possible to obtain potentiometric sensors for gases such as sulfur dioxide or nitrogen dioxide. Such sensors employ similar (acid-base) or other equilibrium processes. These devices, along with their equilibrium processes and internal electrodes, are summarized in Table 6-2. Membrane coverage... 

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... 

Potentiometric sensors for free oxygen and equilibrium oxygen are very common in gas phases with established thermodynamical equilibria (e.g., p(H20)/p(H2) or, generally, the ratio of partial pressures of burnt and non-burnt components). For that purpose highly porous platinum is used as an electrode material (Fig. 3, left). Such cells symbolized by... 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

Potentiometric sensors are based on the measurement of the voltage of a cell under equilibrium-like conditions, the measured voltage being a known function of the concentration of the analyte. Potentiometric measurements involve, in general, Nernstian responses under zero-current conditions that is, the measurement of the electromotive force of the electrochemical cell. 

Sensors for which the output is an open-circuit voltage are referred to as potentiometric sensors, and can be used for a wide variety of species [13-18]. The measured voltage can be established by a thermodynamic equilibrium or by a nonequilibrium steady state between electrochemical reactions at the electrode. 

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

A heterogeneous EIA coupled with a potentiometric electrode permitted the assay of BSA down to 10 ng/ml and cAMP down to 10 nmol/1 (Meyerhoffand Rechnitz, 1979). Urease was used as the marker enzyme and its activity was measured by means of an ammonia gas-sensing electrode. The equilibrium of the immunological reaction at the sensor was reached rather slowly. The advantage of the rapid response of biosensors could not therefore be exploited. 

The use of potentiometric gas sensors without calibration by realizing equilibrium cell potential differences demands that above all the following is guaranteed ... 

Electrodes are used as sensors in either a potentiometric mode or an amperometric mode. As the names imply, potentiometric electrodes measure electrochemical activity by relating it to a potential (voltage). Amperometric electrodes measure electrochemical activity by relating it to a quantity of current (amperes). Both modes have found wide application. Ion-selective electrodes generally operate in the potentiometric mode. Amperometric sensors, conversely, generally use nonselective electrodes which can be made selective by electrochemical and nonelectrochemical modification. Potentiometric electrodes operate via a number of presently ill-defined mechanisms. However, regardless of the mechanism, the measured potential is due to an interfacial chemical equilibrium that does not involve a bulk transfer of material. Amperometric electrodes, on the other hand, do involve the bulk transfer of material. 

---