Solid membranes, electrodes with

Dynamic properties of i.s.e.s. differ greatly for various electrode types and constructions. When the capacitance of analyte/active surface interface is the only cause of response delay, then relaxation time (or time constant of first-order step-response characteristic) is in the order of milliseconds. When the transport of ions across the dynamic Prandtl layer to the surface of the i.s.e. is the main factor (i.e., this transport is the slowest process of equilibrium reinstallation), for a mixing velocity of about lOcm/s a relaxation time of several seconds occurs. This is typical of solid-membrane electrodes with the exception of glass ones. On the other hand, the limited rate of the exchange process in the liquid membrane, the small diffusion flux of the tested ions into the membrane, the slow dynamics for the creation of diffusion potential and the solubility of the active component of the membrane in the testing solution are the main reasons for the slow response of liquid ion-exchanger electrodes (time constants 10-30 s or even more). 

Figure 12. Two types of solid-membrane electrodes (with permission from Springer-Verlag. Heidelberg)...

Potentiometric Sensors Inthe field of ion-selective electrodes, considerable progress has been achieved in the last few years. By buffering the primary ions concentration on a low level in the internal solution, ionic fluxes in the membrane are affected [424-426]. Thus, primary ion leakage into sample solution is hindered, resulting in a tremendous shift of detection limits to lower values for Pb +-selective electrodes, the detection limit up to 10 M level has been achieved for internal solution electrodes [424, 427] and below 10 M for all-solid-state electrodes with conducting polymer solid... 

Variation of Reference Electrode Potentials with Temperature pH Values of Standard Solutions Used in the Calibration of Glass Electrodes Temperature vs. pH Correlation of Standard Solutions Used for the Calibration of Electrodes Solid Membrane Electrodes Liquid Membrane Electrodes... 

The theory and application of selective-ion electrodes have been extensively reviewed.143-151 One of the interesting sidelights is the fact that the internal reference electrode may be replaced by an apparent ohmic contact in many instances, as illustrated by Figure 5 Ale for the solid membrane electrode. Thus the glass electrode can be filled with mercury in place of the internal reference electrode,152 or a gold contact that is plated over with copper can be used.153 Likewise, a selective-ion electrode for calcium ion has been described that is coated on a platinum electrode 154 the contact appears to be mainly ohmic. 

Recendy original all solid-state elearodes for NH4 were successfully combined with urease for the assay of urea (70,71). These electrodes consist of a conductive resin (epoxy -t- graphite) covered by a nonactin-PVC matrix. They offer interesting commercial advantages over membrane electrodes with internal solution and reference electrode. 

The membrane is a conducting solid. Both single crystal and peUet-pressed crystalhne substance mixtures can be used in membrane construction. Table 2, accumulated from data available from several electrode manufactmers data sheets, shows information relative to crystal membrane electrodes and then-application capabilities. As can be seen, crystal membrane electrodes, with the exception of that selective to F, involve Ag2S or crystal mixtures where one component is Ag2S and the other the sulphide of the selective ion of interest. The membranes are generally produced by pressing the polycrystalline substance in a pellet press. 

Ion-selective electrodes with a liquid membrane are more reliable than ion-selective electrodes with a solid membrane because of the uniformity of the active material partition in the membrane. For the construction of biosensors the maximum reliability is obtained by using graphite paste as the support. As of the present, for in vivo tests only sensors based on plastic membranes have been used. The main problem associated with using them for in vivo tests is the biocompatibility of the materials.147 149 The membrane biocompatibility, the matrix biocompatibility, and the electroactive material biocompatibility are important factors. The matrix biocompatibility is assured by the biocompatibility of the polymer and by the biocompatibility of the plasticizer. The ratio between the quantity of polymer and quantity of plasticizer affects the response of electrochemical sensors because the matrix of the solid membrane electrodes plays the same role as does the solvent in liquid membrane electrodes. 

Electrodes suitable for the potentiometric determination of surfactants are either specially designed liquid or solid membrane electrodes or ion-selective electrodes that in addition to being selective to a particular ion, also quantitatively respond to surfactants. For example, a nitrate ion-selective electrode responds to anionic surfactants, a calcium ion-selective electrode is sensitive to quaternary ammonium salts, and a barium ion-selective electrode can be used for assaying polyethoxylates [43], In some cases it is possible for one to perform potentiometric determination of a counter-ion, e.g. one can titrate alkylpyridinium chloride or bromide salts with silver nitrate solution using silver wire as an indicator electrode [38]. 

Fig. 3. Ion-selective electrodes. (A) solid-membrane electrode, (B) glass electrode, (C) tip of glass microelectrode, (D) through-flow tubular electrode, (E) liquid ion-exchanger electrode, (F) electrode with restorable heterogeneous membrane layer...

Miniature Shapes. Miniaturization of liquid-membrane ISEs uses the same techniques as are used with solid-membrane electrodes. Coated-wire electrodes are manufactured by dip-coating a metallic wire with a polymer layer of e.g. PVC. The oldest example (Fig. 7.9, left) is a piece of platinum wire soldered to the internal lead of a coaxial cable. The end of the wire was formed into the shape of a ball by melting it in a hot flame. The polymer coating was applied by repeated dipping into the polymer solution (Freiser 1980). 

Sutter, 1., A. Radu, S. Peper, E. Bakker, and E. Pretsch. 2004. Solid-contact polymeric membrane electrodes with detection limits in the subnanomolar range. Anal. Chim. Acta 523 53-59. 

Sutter, 1., E. Lindner, R.E. Gyurcsanyi, and E. Pretsch. 2004. A polypyrrole-based solid-contact Pb -selective PVC-membrane electrode with a nanomolar detection limit. Anal. Bioanal Chem. 380 7-14. 

As the detection limit anticipated on the basis of simple thermodynamic displacement (ion-exchange) becomes on the order of micromolar or lower, counterdiffusion ion fluxes may become important (see below for more detail). For a well conditioned membrane electrode with an optimized inner solution composition or a suitable solid contact material the following relationship may be used to estimate the kinetic detection limit if all ions are monovalent. 

There have been many attempts to develop selective precipitate membrane electrodes for the important sulfate and phosphate anions. Unfortunately they have all failed to date. A sulfate-sensitive solid-state membrane electrode with a pellet of 31.7% Ag2S, 31.7% PbS, 31.7% PbS04 and 5% CU2S has been described [91], but its selectivity over I, HPO4" and SO3 is low. 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

This presentation reports some studies on the materials and catalysis for solid oxide fuel cell (SOFC) in the author s laboratory and tries to offer some thoughts on related problems. The basic materials of SOFC are cathode, electrolyte, and anode materials, which are composed to form the membrane-electrode assembly, which then forms the unit cell for test. The cathode material is most important in the sense that most polarization is within the cathode layer. The electrolyte membrane should be as thin as possible and also posses as high an oxygen-ion conductivity as possible. The anode material should be able to deal with the carbon deposition problem especially when methane is used as the fuel. 

Gel of collodion, PVC, etc. Solution (with low ) of ion exchangers or complex in suitable solvent (apparently dry) "solid-state membrane electrode K, nh , Ca bf4, no3... 

Homogeneous liquid membrane electrodes. This type, which is in limited use, is sometimes considered as a solid ion-exchange electrode as the electroactive species, e.g., calcium dioctylphosphate, after being dissolved in an ethanol-diethyl ether solution of collodion, is left to "dry and can function as an ion-selective pellet in an electrode tip. Orion37 use these electrodes with PVC-gelled membranes for Ca2+, K+, BF4 and N03. ... 

Yoon el al. [112] reported an all-solid-state sensor for blood analysis. The sensor consists of a set of ion-selective membranes for the measurement of H+, K+, Na+, Ca2+, and Cl. The metal electrodes were patterned on a ceramic substrate and covered with a layer of solvent-processible polyurethane (PU) membrane. However, the pH measurement was reported to suffer severe unstable drift due to the permeation of water vapor and carbon dioxide through the membrane to the membrane-electrode interface. For conducting polymer-modified electrodes, the adhesion of conducting polymer to the membrane has been improved by introducing an adhesion layer. For example, polypyrrole (PPy) to membrane adhesion is improved by using an adhesion layer, such as Nafion [60] or a composite of PPy and Nafion [117],... 

Ke and Regier [71] have described a direct potentiometric determination of fluoride in seawater after extraction with 8-hydroxyquinoline. This procedure was applied to samples of seawater, fluoridated tap-water, well-water, and effluent from a phosphate reduction plant. Interfering metals, e.g., calcium, magnesium, iron, and aluminium were removed by extraction into a solution of 8-hydroxyquinoline in 2-butoxyethanol-chloroform after addition of glycine-sodium hydroxide buffer solution (pH 10.5 to 10.8). A buffer solution (sodium nitrate-l,2-diamino-cyclohexane-N,N,N. AT-tetra-acetic acid-acetic acid pH 5.5) was then added to adjust the total ionic strength and the fluoride ions were determined by means of a solid membrane fluoride-selective electrode (Orion, model 94-09). Results were in close agreement with and more reproducible than those obtained after distillation [72]. Omission of the extraction led to lower results. Four determinations can be made in one hour. 

Fig. 18a.6. Photograph of a commercial La3F selective electrode with expanded schematic view of the solid-state sensing membrane.

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