Electrodes coated-wire

TABLE 5-1 Characteristics of Solid-State Crystalline Electrodes  

In contrast, in most ion-selective membranes the charge conduction is done by ions. Thus, a mismatch between the charge-transfer carriers can exist at the noble metal/membrane interface. This is particularly true for polymer-based membranes, which are invariably ionic conductors. On the other hand, solid-state membranes that exhibit mixed ionic and electronic conductivity such as chalcogenide glasses, perovskites, and silver halides and conducting polymers (Lewenstam and Hulanicky, 1990) form good contact with noble metals. 

In summary, it is perfectly legitimate to design CWEs with solid-state internal contact and to expect electrochemical performance comparable to the conventional ISE. However, the design of the membrane/solid interface has to be done with the understanding of the electrochemical processes at such an interface. The problems include the drift scale with the surface area and the length of the internal contact, specifically with its parasitic capacitance and resistance. From this consideration alone, it can be concluded that such problems can be minimized by decreasing the 

The contact problems are mitigated in the hybrid ion sensor by making the internal conductor shorter and shorter until it is more natural to talk about its thickness rather than its length. The material of this internal contact has not changed during this transition and neither has the electrochemistry at the interface. Thus, the only difference between the coated wire and the hybrid sensor is the length (or thickness) of the contact. We therefore skip it and go directly to the solid-state ISFET, in which the thickness of the internal contact is zero. In other words, the ion-selective membrane is placed directly at the input dielectric of the field-effect transistor (Fig. 6.20). 

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Chronocoulometry, 62 Clark electrode, 190 Coated wire electrodes, 160 Cobalt, 82, 85 Cobalt phthalocyanine, 121 Collection efficiency, 113, 135 Collection experiments, 113 Combination electrode, 148 Compact layer, 19 Composite electrodes, 47, 114, 133 Computer control, 80, 106 Concentration profile, 7, 9, 11, 29, 36, 87, 132... 

The first and very simple solid contact polymeric sensors were proposed in the early 1970s by Cattrall and Freiser and comprised of a metal wire coated with an ion-selective polymeric membrane [94], These coated wire electrodes (CWEs) had similar sensitivity and selectivity and even somewhat better DLs than conventional ISEs, but suffered from severe potential drifts, resulting in poor reproducibility. The origin of the CWE potential instabilities is now believed to be the formation of a thin aqueous layer between membrane and metal [95], The dominating redox process in the layer is likely the reduction of dissolved oxygen, and the potential drift is mainly caused by pH and p02 changes in a sample. Additionally, the ionic composition of this layer may vary as a function of the sample composition, leading to additional potential instabilities. 

Selig reported a potentiometric titration method for the analysis of procaine and some other organic cations precipitated by tetraphenylborate [67]. The development of ion selective coated-wire electrodes, and their application in the titration of procaine and other pharmaceutically important substances, was reported [68]. 

Satake et al. reported the use of a coated wire electrode sensitive to procaine and other local anesthetic cations, and their application to potentiometric determination [73]. Electrodes were constructed from a copper wire (0.8 mm diameter), coated with a PVC membrane comprising a mixture of the drug-tetraphenylborate ion-pair, dioctyl phthalate, polyvinyl chloride, and tetrahydrofuran. Potential measurement was made with respect to a Ag-AgCl reference electrode. The electrodes showed linear responses with a Nemstian slope for procaine over the concentration range investigated. The method was used for analyses of the drug in pharmaceutical preparations. 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

In our laboratory, these disadvantages have been overcome with the development of the coated wire electrode (CWE). This sensor, having response characteristics equal to and occasionally better than conventional types, is only 1-2 mm in diameter (further size reduction can be easily achieved), can be used at any angle, and costs only a few pennies to make. Indeed, they can be considered "disposable", though with proper handling lifetimes of over six... 

Interferent Orion Electrode Coated Wire Electrode... 

Selectivity Coefficients, k °j, for the Coated Wire Electrodes Compared to Those for the... 

Fig. 30.8. (A) Classical ion-selective electrode. (B) Coated-wire electrode.

Besides the standard screen-printed or coated-wire electrodes, other architectures can be used. Examples include a rotating-disc electrode coated... 

Coated-Wire Electrodes and Solid-State Electrodes Without an Internal Filling Solution... 

Coated-wire electrodes (CWEs), introduced by Freiser in the mid-1970s, are prepared by coating an appropriate polymeric film directly onto a conductor (Fig. 5.18). The ion-responsive membrane is commonly based on poly(vinyl... 

The integration of chemically sensitive membranes with solid-state electronics has led to the evolution of miniaturized, mass-produced potentiometric probes known as ion-selective field effect transistors (ISFETs). The development of ISFETs is considered as a logical extension of coated-wire electrodes (described in Section 5.2.4). The construction of ISFETs is based on the tech-... 

Promising new analytical tools are further enzymatic analysis, coated wire electrodes, high-pressure liquid chromotography, sulfide electrode use, and use of computers for data storage and retrieval and for monitoring laboratory instrumentation. 

Acetylcholineesterase Enzyme was covalently immobilized on a bovine serum albumin-modified H +-selective coated wire electrode. The sensor was used in 5mM phosphate buffer pH 7 at 30°C. The response time was 3-10 min for O.l-lOmM ACh. pH change of 6-8 had little effect. Coefficient of variation was 5.7 and 5.8%. [72]... 

Coated-wire electrode — A polymer film containing an ion-responsive material and a binder, e.g., poly(vinyl)chloride, is coated onto a conductor (e.g., a metal wire or graphite). They show useful response to solution concentrations of measured species in the range 10-5 < c < 0.1 mol dm-3. The processes at the metal polymer interface are still not understood. 

Among other factors its performance depends on the -> membrane properties. Its potential is equal to the potential across the membrane. It can be measured with high - input impedance voltmeters versus an outer -> reference electrode present in an outer - electrolyte solution. In some cases - inner reference electrode and inner electrolyte solution can be eliminated, see for example -> coated-wire electrodes. This term is very often used as synonym of - ion-selective electrode. 

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