Sensor potentiometric

With these assumptions in mind, we now complete the outline of the solution of the diffusion-reaction problem as it applies to the most difficult case, the pH-based enzymatic sensors (potentiometric or optical). We assume only that there is no depletion layer at the gel/solution boundary (7), and that there is no fixed buffer capacity (4). The objective of this exercise is to find out the optimum thickness of the gel layer that is critically important for all zero-flux-boundary sensors, as follows from (2.26). 

Fig. 8.5 Time courses of output signals of semiconductor-type oxygenate sensors, potentiometric CO sensor and ND-IR C02 sensors. The delay of CO gas sensor was due to the resistance of active carbon filter which removes oxygenate compounds (reproduced by permission of Elsevier from [19]).

An electrochemical sensor is generally an electrochemical cell containing two electrodes, an anode and a cathode, and an electrolyte. Electrochemical sensors in general are classified, based on the mode of its operation, and they are conductivity sensors potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered as a special type of voltammetric sensors. The fundamentals of these sensors operational principles are described exceptionally well in several excellent electro-analytical books. In this entry, only the essential features are included. 

There are three types of electrochemical sensors potentiometric, amperometric, and potentiodynamic sensors. 

Electrochemical sensors can be classified according to their mode of operation, e.g. conductivity/capacitance sensors, potentiometric sensors, and voltammetric sensors. Amperometric sensors can be considered a specific type of voltammetric sensor. The general principles of electrochemical sensors have been extensively described in other chapters of this volume or elsewhere. This chapter will focus on the fabrication of electrochemical sensors of micro or miniature size. 

An electrochemical sensor is essentially an electrochemical cell consisting of two or more electrodes in contact with a solid electrolyte. They can be classified according to their operation mode, e.g., conductivity/impedance sensors, potentiometric sensors, and amperometric sensors. 

A.H. Kamel, ETC. Moreira and M.G.F. Sales, Biomimetic sensor potentiometric system for doxycycline antibiotic using a molecularly imprinted polymer as an artificial recognition element. Sensor Lett., 9 (5) 1654-1660, 2011. 

Potentiometric sensors — Potentiometric biosensors are miniaturized by reducing the dimensions of the transducer on which the enzyme is deposited. Once the transducer is miniaturized, the enzyme is immobilized by immersing the transducer in a solution of the enzyme and a cross-linking agent, glutaraldehyde, as described above. An acetylcholinesterase electrode was constructed in this way for the determination of acetylcholine [25] using a glass microelectrode... 

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. 

The most obvious sensor for an acid-base titration is a pH electrode.For example, Table 9.5 lists values for the pH and volume of titrant obtained during the titration of a weak acid with NaOH. The resulting titration curve, which is called a potentiometric titration curve, is shown in Figure 9.13a. The simplest method for finding the end point is to visually locate the inflection point of the titration curve. This is also the least accurate method, particularly if the titration curve s slope at the equivalence point is small. 

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

Amperometry is a voltammetric method in which a constant potential is applied to the electrode and the resulting current is measured. Amperometry is most often used in the construction of chemical sensors that, as with potentiometric sensors, are used for the quantitative analysis of single analytes. One important example, for instance, is the Clark O2 electrode, which responds to the concentration of dissolved O2 in solutions such as blood and water. 

Martinez-Eabregas, E. Alegret, S. A Practical Approach to Ghemical Sensors through Potentiometric Transducers Determination of Urea in Serum by Means of a Biosensor, /. Chem. Educ. 1994, 71, A67-A70. 

Selig, W. S. Potentiometric Titrations Using Pencil and Graphite Sensors, /. Chem. Educ. 1984, 61, 80-81. 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

Heterocycles as ligands in ionophores for potentiometric and optical sensors 98CRV1593. 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

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... 

FIGURE 6-17 Schematic of the potentiometric sensor for carbon dioxide. (Reproduced with permission from reference 60.)... 

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