Membrane ISE

The SHG technique is therefore a valuable probe for insight into the surface chemistry of liquid membrane ISEs. 

The above SHG studies exhibited several most important facts in the response mechanisms occurring at the surfaces of ionophore incorporated liquid membrane ISEs. 

Neither the usual membrane ISEs nor the gas-sensing electrodes, in which their internal indicator electrode functions as a zero-current potentiometric half-cell, are under consideration here. 

There are two general types of liquid-membrane ISEs, namely one which involves liquid-phase ion exchange, with the response being selective to the anion or cation under scrutiny (generally polyvalent ions), while the other type involves... 

In both types of liquid-membrane ISEs, the membrane acts as an inunis-cible phase boundary between the aqueous and non-aqueous solutions inside the ISE (see the schematic diagram presented in Figure 3.13). In order to minimize mixing, the liquid membrane is held in place by an inert, porous material such as a rigid glass frit or a flexible synthetic polymer - the choice will depend on the manufacturer rather than on experimental considerations. 

One of the most important examples of a liquid-membrane ISE is the calcium-selective electrode. The salt utilized as the ion exchanger is the calcium salt of dodecylphosphoric acid, e.g. dissolved in di-( -acetylphenyl) phosphonate. The sensitivity of the electrode depends on the solubility of the ion exchanger in the test solution. The electrode response is generally nemstian down to a concentration of 10 mol dm . In the preferred pH range of 5.5-11, the selectivity of... 

Figure 3.13 Schematic diagram of a liquid-membrane ISE. The silver wire at the top acts as one of the electrodes, and so an additional electrode will be required to complete the cell.

Fig. 5.6. A flow-through electrode system for liquid-membrane ISEs [111] 1 - reference electrode 2 - hole through which the sample solution flows 3 - liquid ion-exchanger reservoir, 4 - triangular piece of a frit soaked with the liquid ion-exchanger.

Fig. 7.3. Correlation between the selectivity coefficient and the ratio of stability constants in water for a liquid membrane ISE based on valinomycin dissolved in nitrobenzene. (After Morf [151].)...

JSEs with liquid membranes Table 7.2. Solvents used in liquid-membrane ISEs. 

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 previous chapters dealt with ISE systems at zero current, i.e. at equilibrium or steady-state. The properties of the interface between two immiscible electrolyte solutions (ITIES), described in sections 2.4 and 2.5, will now be used to describe a dynamic method based on the passage of electrical current across ITIES. Voltammetry at ITIES (for a survey see [3, 8, 9, 10, 11, 12,18]) is an inverse analogue of potentiometry with liquid-membrane ISEs and thus forms a suitable conclusion to this book. 

POLYMERIC MEMBRANE ISEs WITH LIQUID INNER CONTACT... 

Mixtures of alkylsulphate surfactants 10 PVC membrane ISEs for anionic surfactants [100]... 

Polymeric membrane ISEs with solid inner contact 43 Polyphenols 257, 362 oxidase 362 Polypyrrole 76 pattern 928... 

Optical second harmonic generation (SHG), which stems from the conversion of two photons of frequency to to a single photon of frequency 2o>, is an inherently surface-sensitive technique.169 Whereas no optical second harmonic wave is generated in the centrosymmetric bulk of a liquid, molecules participating in the asymmetry of the interface between two liquids (noncentrosymmetric environment) contribute to SHG. Since the square root of SHG signal intensity, V/(2m), is proportional to the number N (per unit area), the molecular orientation (7) and the second order nonlinear polarizability a<2) of the SHG active species at the interface of the two liquids, the SHG technique provides a valuable tool to investigate the surface of liquid membrane ISEs. 

Among the anion carriers, calixpyrrole receptors have found applications as components in anion-selective membrane electrodes. The potentiometric selectivity for membranes ISEs based on calix[4]pyrrole, 1 towards a range of anions, namely fluoride, chloride, bromide and dihydrogen phosphate was found to be pH-dependent. 

Most polymer membrane ISEs are prepared by dissolving an ionophore in a polyvinylchloride (PVC) membrane. A large variety of plasticisers are used to increase the dielectric constant of the PVC and improve its hydrophilicity. Some membranes have complexes of the ions to be sensed to increase membrane conductivity, such as potassium tetraphenylborate in K -selective membranes. There is an extensive literature on the arcane arts of polymer membranes for electrodes with dissolved ionophores and a good review of this is given by Professor Ronald Armstrong in Section 3.7 of Gabor Harsanyi s book. Polymer films in sensor applications [14]. 

The phenomena observed in living cells have much in common with those in artificial polymer membrane ISEs. In membrane electrodes, ionophores allow the net movement of ions in a membrane only down their electrochemical gradients. The equilibrium state is reached when the electrochemical gradient becomes zero and the cell potential reaches its final equilibrium value net transport no longer occurs. 

For the construction of ISMEs, solid membranes and liquid membranes are proposed. The construction of solid membrane ISEs is based on the insertion of electroactive material (ion pair complex), or only of a counter ion into a polyvinyl chloride (PVC) matrix. The PVC type as well as the plasticizer used for the membrane construction affect the reliability of the electrode s response. Thomas proposed some rules for solid membranes based on a PVC matrix construction [3-5]. The procedure most often used for solid membrane construction is known as the Moody and Thomas procedure [5], as this assures the best reliability of solid membrane electrodes. 

Polymer membrane ISEs are employed for monitoring pH and for measuring electrolytes, including Na% CL, Ca % Li, Mg h and CO (for total CO2 measurements). They are the predominant class of potentiometric electrodes used in modern clinical analysis instruments. 

A typical formulation of a PVC membrane ISE used in clinical instrumentation is ... 

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