Molecular-selective electrode systems

A General Principles 659 23B Reference Electrodes 660 23C Metallic Indicator Electrodes 662 23D Membrane Indicator Electrodes 664 23E Ion-Selective Field-Effect Transistors 675 23F Molecular-Selective Electrode Systems 677 23G Instruments for Measuring Cell Potentials 684 23H Direct Potentiometric Measurements 686 231 Potentiometric Titrations 691 Questions and Problems 692... 

Thermodynamics describes the behaviour of systems in terms of quantities and functions of state, but cannot express these quantities in terms of model concepts and assumptions on the structure of the system, inter-molecular forces, etc. This is also true of the activity coefficients thermodynamics defines these quantities and gives their dependence on the temperature, pressure and composition, but cannot interpret them from the point of view of intermolecular interactions. Every theoretical expression of the activity coefficients as a function of the composition of the solution is necessarily based on extrathermodynamic, mainly statistical concepts. This approach makes it possible to elaborate quantitatively the theory of individual activity coefficients. Their values are of paramount importance, for example, for operational definition of the pH and its potentiometric determination (Section 3.3.2), for potentiometric measurement with ion-selective electrodes (Section 6.3), in general for all the systems where liquid junctions appear (Section 2.5.3), etc. 

The challenge in this field is to control both the physical architecture and chemical reactivity of the film so as to promote selected electron transfer reactions while inhibiting others. With polymer-modified electrodes (PMEs), the electrode is conferred with the molecular selectivity and specificity that is lacking at a conventional pristine electrode. For example, poly(4-vinyl)pyridine and poly(N-vinyl)imi-dazole can be functionalized with osmium and ruthenium polypyridyl complexes. These synthetic macromolecules act as useful model systems for... 

This chapter is arranged as follows Sect. 3.2 presents brief comments to the LJP between two different molecular solvents Sect 3.3 is devoted to the most advanced area, the LJP between two different electrolyte solutions in a certain solvent (known also as diffusion potential) and includes some comments cOTiceming the temperamre-induced UP in these systems Sect 3.4 cmitains brief notes about UPs in solvent-free systems, namely melts and room temperature irmic liquids (RTCLs) the crmcluding Sect. 3 A presents various specific cases of UPs and outlines the links with electroanalysis, especially ion-selective electrodes, the functirmal properties of which are stTOTigly affected by diffusion potentials. This specific case should be considered separately because the membrane/solutimi interfaces (as well as the bormdaries of immiscible liquids) keep the exact geometry of boimdaries. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

Mechanistic smdies are needed on a select nnmber of electrochemical reactions, particularly those involving oxygen. These smdies are far from routine and reqnire advances in knowledge of molecular interactions at electrode surfaces in the presence of an electrolyte. Recent achievements in surface science under ultrahigh vacuum conditions snggest that a comparable effort in electrochemical systems would be equally fmitful. 

Chapters 4 and 5 are devoted to molecular and biomolecular catalysis of electrochemical reactions. As discussed earlier, molecular electrochemistry deals with transforming molecules by electrochemical means. With molecular catalysis of electrochemical reactions, we address the converse aspect of molecular electrochemistry how to use molecules to produce better electrochemistry. It is first important to distinguish redox catalysis from chemical catalysis. In the first case, the catalytic effect stems from the three-dimensional dispersion of the mediator (catalyst), which merely shuttles the electrons between the electrode and the reactant. In chemical catalysis, there is a more intimate interaction between the active form of the catalyst and the reactant. The differences between the two types of catalysis are illustrated by examples of homogeneous systems in which not only the rapidity of the catalytic process, but also the selectivity problems, are discussed. 

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