Solid Membrane Electrodes

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... 

Ion-selective electrodes (ISEs) can be designed using various ion-selective elements, such as glass membranes (for pH electrodes), solid membranes (for ions such as F , Cl , Br , I, SCN , or S ), liquid ion-exchange membranes (for ions such as N03, Cl , BF4, or K+), or gas-sensitive electrodes on the basis of pH electrodes, containing a hydrophobic, gas-permeable membrane (for acidic or basic gases such as NH3 or CO2). An example of the latter is depicted in Fig. 3 [40-42]. 

Ion-selective membranes derive their permselective properties from either ion exchange, solubility or complexation phenomena. Current ion-selective electrodes contain membranes which consist of glass, solid or liquid phases. 

Materials similar to Nation containing immobilized —COO- or —NR3+ groups on a perfluorinated skeleton were also synthesized. These are available in the form of solid membranes or solutions in organic solvents the former can readily be used as solid electrolytes in the so called solid polymer electrolyte (SPE) cells, the latter are suitable for preparing ion-exchange polymeric films on electrodes simply by evaporating the polymer solution in a suitable solvent. 

Ion-selective electrodes are membrane systems used as potentiometric sensors for various ions. In contrast to ion-exchanger membranes, they contain a compact (homogeneous or heterogeneous) membrane with either fixed (solid or glassy) or mobile (liquid) ion-exchanger sites. 

A.K. Jain, R.P. Singh, and C. Bala, Solid membranes of copper hexacyanoferrate(III) as thallium(I)-sensitive electrode. Anal. Lett. 15, 1557-1563 (1982). 

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. 

Barium B3CI2/B3CO3 BaS04 (s) 0 0 0 Low S04 level High running cost toxicity and solid waste affect electrode and membrane... 

In solid-state electrodes the membrane is a solid disc of a relatively insoluble, crystalline material which shows a high specificity for a particular ion. The membrane permits movement of ions within the lattice structure of the crystal and those ions which disrupt the lattice structure the least are the most mobile. These usually have the smallest charge and diameter. Hence, only those ions that are very similar to the internal mobile ions can gain access to the membrane from the outside, a feature that gives crystal membranes their high specificity. When the electrode is immersed in the sample solution, an equilibrium is established between the mobile ions in the crystal and similar ions in the solution and the resulting potential created across the membrane can be measured in the usual manner. 

Macroelectrodes with solid membranes contain homogeneous [142] or heterogeneous [25] membranes. The construction of an ISE of this type with an internal reference electrode is shown in fig. 4.1. For good functioning of an ISE it is necessary that the membrane be completely sealed in the electrode body, with no cracks leading to short-circuiting between the external and internal solutions. Cements based on Teflon, PVC or epoxy resin are used (170). 

The membrane surface may become passivated by some solution components that are strongly adsorbed. This effect is often encountered in measurements on biological fluids containing proteins. These adsorption effects can sometimes be prevented by selecting a suitable compoation of the sample and standard solutions for example by adding trypsin and triethanolamine to dissolve proteins [108]. Passive electrodes can sometimes be reactivated by soaking in suitable solutions (for example pepsin in 0.1M HCl [68]) and in more serious cases the membrane must be replaced or a solid membrane be repolished. 

ISEs are well suited for flow measurements because the instrumentation and signal handling are simple, the measurement is almost independent of the liquid flow-rate, the linear dynamic range is broad, the temperature dependence is not very pronounced and the measurement is selective (the selectivity is, however, a drawback in applications to chromatography). The experimental conditions are readily adjusted and often only consist of ionic strength and pH maintenance. ISEs with solid membranes usually exhibit better performance than liquid membrane electrodes and gas probes, because their response is faster and they are mechanically stronger. The most difficult problem is passivation of the electrodes in some media, for example, biological fluids or surface and waste waters. 

For more than the last half century experiments have been carried out to obtain electrodes with solid membranes containing inorganic insoluble compounds which the authors expect (consciously or not) to have ion-exchange properties (for a review, see [57]). These substances, for example insoluble cyanoferrates(II), phosphomolybdates, calcium fluoride, etc., are not, however, promising materials and probably only generate problems for the editors of specialized journals. 

The solid-membrane ISE has certain disadvantages for the determination of chloride inside cells and thus ion-selective microelectrodes containing ion-exchanger Corning No. 477315 (based on a nitroxylene mixture) are used [223]. Reviews of intracellular applications of this electrode can be found in [23, 78, 86, 211,217]. 

The determination of iodide with ion-selective electrodes is possible with commercial sensors often based on ion conducting Ag2S—Agl solid membranes [57]. A PVC membrane-based sensor employing a silver complex with thiourea derivatives has been reported by El Aamrani et al. [202]. Interference from thiocyanate and bromide was investigated and a limit of detection in the nanomolar range was determined. A study assessing the performance... 

They are classified by membrane material into glass membrane electrodes, crystalline (or solid-state) membrane electrodes, and liquid membrane electrodes. Liquid membrane electrodes are further classified into liquid ion-exchange membrane electrodes and neutral carrier-based liquid membrane electrodes. Some examples are shown in Fig. 5.36 and Table 5.3. If the membrane is sensitive to ion i of charge Z and the activities of i in the sample and internal solutions are equal to (i) and a2(i), respectively, the membrane potential, m, which is developed across the membrane, is... 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

Conventional non-aqueous pH titrations are useful in detecting and determining acidic and basic impurities. On the other hand, the ion-prove method proposed by Coetzee et al. [9] is convenient in characterizing trace amounts of reactive impurities. The principle of the method was described in Section 6.3.5. In Fig. 10.4, the method is applied to reactive impurities in commercial acetonitrile products. The prove-ion and the ISE were Cd2+ and a Cd2+-selective electrode (CdS-Ag2S solid membrane), respectively. The solid line TR is the theoretical relation expected when the ISE responds to Cd2+ in the Nernstian way. The total concentrations of impurities, which were reactive with Cd2+, were estimated to be 4x10 5 M, 8xlO-6M,... 

The following table lists the most commonly used solid membrane electrodes, their applications, and major interferences.1 Often the membrane is composed of a salt (listed first) and a matrix (listed second). Thus, a AgCl-Ag2S electrode involves the finely divided AgCl in a Ag2S matrix. The salt should be more soluble than the matrix, but insoluble enough so that its equilibrium solubility gives a lower anion (Cl-) activity than that of the sample solution. 

In contrast to solid-membrane electrodes, liquid-membrane electrodes can extract counterions from the solution-phase into the membrane phase. Selectivity is provided by the charged nature of the membrane carriers and arises from the competitive degree of extractability of various counterions. Totally liquid systems can be employed but are impractical. Instead, a porous support or an inert polymer support is used in most commercial electrodes. 

Figure 5.41 Selective-ion electrodes (a) glass membrane (b) liquid ion exchange (c) homogeneous solid membrane (d) heterogeneous solid membrane (e) solid membrane without reference electrode (/) gas-permeable membrane 1, sensing electrode 2, electrolyte, 2(e) ohmic contact, 2(f) gas-permeable membrane 3, membrane sur-port 4, reference electrode, 4(f) outer electrode body, 5(b) liquid ion exchanger 5(f) electrode body 6(b) reference electrode body, 6(f) electrolyte 7, liquid junction.

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. 

Considerable work has been devoted to the development of solid membranes that are selective primarily to anions. The solid-state membrane can be made of single crystals, polycrystalline pellets, or mixed crystals. The resulting solid-state membrane electrodes have found use in a great number of analytical applications. 

There are three basic types of selective electrode those based on glass membranes, on inorganic salt solid membranes, and on ion exchange. Other more complex electrodes are sensitive to dissolved gases and enzymes. These various types are now described. 

In this type of selective electrode, the membrane is an ionic solid which must have a small solubility product in order to avoid dissolution of the membrane and to ensure a response that is stable with time. Conduction through the membrane is principally ionic and is due to point defects in the crystal lattice, relying on the fact that no crystal is perfect. 

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