In ion-selective membranes

FIGURE 4.1 Structures of common ionophores used in ion-selective membranes. 

M.M. Shultz, O.K. Stefanova, S.S. Mokrov, and K.N. Mikhelson, Potentiometric estimation of the stability constants of ion-ionophore complexes in ion-selective membranes by the sandwich membrane method theory, advantages, and limitations. Anal. Chem. 74, 510-517 (2002). 

Ammann, D. Bissig, R. Gueggi, M. Pretsch, E. Simon, W. Borowitz. I.J. Weiss. L. Preparation of neutral ionophores for alkali and alkaline earth metal cations and their application in ion selective membrane electrodes. Helv. Chim. Acta 1975. 58, 1535-1548. 

The present chapter describes the issues associated with the use of PVC in ion-selective membrane electrodes. There are also examples of application of different structures of polymer electrodes, along with a description of their optimum properties and applications. The chapter focuses in particular on the electrodes for determination of nonsteroidal anti-inflammatory drugs. 

The preparation of an ion-selective electrode for salicylate is described. The electrode incorporates an ion-pair of crystal violet and salicylate in a PVC matrix as the ion-selective membrane. Its use for the determination of acetylsalicylic acid in aspirin tablets is described. A similar experiment is described by Creager, S. E. Lawrence, K. D. Tibbets, C. R. in An Easily Constructed Salicylate-Ion-Selective Electrode for Use in the Instructional Laboratory, /. Chem. Educ. 1995, 72, 274-276. 

Individual polyethers exhibit varying specificities for cations. Some polyethers have found appHcation as components in ion-selective electrodes for use in clinical medicine or in laboratory studies involving transport studies or measurement of transmembrane electrical potential (4). The methyl ester of monensin [28636-21 -7] i2ls been incorporated into a membrane sHde assembly used for the assay of semm sodium (see Biosensors) (5). Studies directed toward the design of a lithium selective electrode resulted in the synthesis of a derivative of monensin lactone that is highly specific for lithium (6). 

Of interest is the manner in which cavities of the appropriate size are introduced into ion-selective membranes. These membranes typically consist of highly plasticized poly(vinyl chloride) (see Membrane technology). Plasticizers (qv) are organic solvents such as phthalates, sebacates, trimelLitates, and organic phosphates of various kinds, and cavities may simply be the excluded volumes maintained by these solvent molecules themselves. More often, however, neutral carrier molecules (20) are added to the membrane. These molecules are shaped like donuts and have holes that have the same sizes as the ions of interest, eg, valinomycin [2001-95-8] C H QN O g, and nonactin [6833-84-7] have wrap around stmctures like methyl monensin... 

Ruthenium, iridium and osmium Baths based on the complex anion (NRu2Clg(H20)2) are best for ruthenium electrodeposition. Being strongly acid, however, they attack the Ni-Fe or Co-Fe-V alloys used in reed switches. Reacting the complex with oxalic acid gives a solution from which ruthenium can be deposited at neutral pH. To maintain stability, it is necessary to operate the bath with an ion-selective membrane between the electrodes . 

The general theoretical treatment of ion-selective membranes assumes a homogeneous membrane phase and thermodynamic equilibrium at the phase boundaries. Obvious deviations from a Nemstian behavior are explained by an additional diffusion potential inside the membrane. However, allowing stationary state conditions in which the thermodynamic equilibrium is not established some hitherto difficult to explain facts (e.g., super-Nemstian slope, dependence of the selectivity of ion-transport upon the availability of co-ions, etc.) can be understood more easily. 

Ion-selective bulk membranes are the electro-active component of ion-selective electrodes. They differ from biological membranes in many aspects, the most marked being their thickness which is normally more then 105 times greater, therefore electroneutrality exists in the interior. A further difference is given by the fact that ion-selective membranes are homogeneous and symmetric with respect to their functioning. However, because of certain similarities with biomembranes (e.g., ion-selectivity order, etc.) the more easily to handle ion-selective membranes were studied extensively also by many physiologists and biochemists as model membranes. For this reason research in the field of bio-membranes, and developments in the field of ion-selective electrodes have been of mutual benefit. 

The most popular theoretical description of the potentiometric behavior of ion-selective membranes makes use of the three-segmented membrane model introduced by Sollner53), Teorell 30,54), and Meyer and Sievers 31-5S). In this model the two phase boundaries and the interior of the membrane are treated separately. Here, the... 

Of fundamental importance in understanding the electrochemistry of ion-selective membranes and also of biomembranes is the research in the field of voltammetry at ITIES mainly pioneered by Koryta and coworkers 99 101 . Koryta also demonstrated convincingly that a treatment like corroding metal electrodes is possible 102). For the latter, the description in the form of an Evans-diagram is most appropriate Fig. 4 shows schematically some mixed potentials, which are likely to arise at cation-selective membranes if interfering ions disturb an ideal Nernstian behavior82. Here, the vertical axis describes the galvani potential differences (absolute po-... 

The explicit mathematical treatment for such stationary-state situations at certain ion-selective membranes was performed by Iljuschenko and Mirkin 106). As the publication is in Russian and in a not widely distributed journal, their work will be cited in the appendix. The authors obtain an equation (s. (34) on page 28) similar to the one developed by Eisenman et al. 6) for glass membranes using the three-segment potential approach. However, the mobilities used in the stationary-state treatment are those which describe the ion migration in an electric field through a diffusion layer at the phase boundary. A diffusion process through the entire membrane with constant ion mobilities does not have to be assumed. The non-Nernstian behavior of extremely thin layers (i.e., ISFET) can therefore also be described, as well as the role of an electron transfer at solid-state membranes. 

Otherwise it has been shown that the accumulation of electrolytes by many cells runs at the expense of cellular energy and is in no sense an equilibrium condition 113) and that the use of equilibrium thermodynamic equations (e.g., the Nemst-equation) is not allowed in systems with appreciable leaks which indicate a kinetic steady-state 114). In addition, a superposition of partial current-voltage curves was used to explain the excitability of biological membranes112 . In interdisciplinary research the adaptation of a successful theory developed in a neighboring discipline may be beneficial, thus an attempt will be made here, to use the mixed potential model for ion-selective membranes also in the context of biomembrane surfaces. 

For an interfering redox reaction at an ion-selective membrane, the overpotential t B can be easily determined experimentally. It is the potential difference between the ion-selective membrane and an inert redox electrode in the same solution containing the measured ion and an interfering redox system. 

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