Molecular-selective electrode

Terminology related to electroanalytical chemistry are chronoamperometry, voltammetry, coulometry, amperometric titrimetry, coulometric titrimetry, conductivity, con-ductimetry and high frequency titrimetry, electrometric titrimetry, electrogravimetry, electrodeposition, anodic stripping voltammetry (ASV), cathodic stripping voltammetry (CSV), polarography, differentia] pulse polarography (DPP), ion-selective electrode (ISE), ion-specific electrode (ISE), molecular selective electrode, potentiometry, potentio-metric titrimetry, and chronopotentiometric titrimetry. 

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... 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Liquid Membrane Ion-Selective Electrodes Response Mechanisms Studied by Optical Second Harmonic Generation and Photoswitchable lonophores as a Molecular Probe... 

The purpose of this chapter is to describe these experimental approaches for understanding the molecular mechanism of the membrane potentials for ionophore-incorpo-rated liquid membrane ion-selective electrodes. 

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. 

Figure 3. Components of an ion-selective electrode chemical sensor (left) and photographs of electrode body (right) showing electrode barrel with silver-silver chloride electrode, and screw-on electrode tip with end-clip for attaching the PVC membrane containing immobilised molecular receptors that will selectively bind specific target species.

K. Chiba, K. Tsunoda, H. Haraguchi, K. Fuwa, Determination of fluorine in urine and blood serum by aluminum monofluoride molecular absorption spectrometry and with a fluoride ion selective electrode. Anal. Chem. 52 (1980) 1582-1585. 

Gupta and D Arc [373] have prepared the Cd(II) ion-selective electrode based on cyanocopolymer using 8-hydroxyquinoline as an electroactive ionophore. The effect of concentration of ion exchanger, plasticizer, and molecular weight of cyanocopolymers on selectivity and sensitivity of Cd(II) ion-selective electrode was also investigated [374]. 

Development of Polymer Membrane Anion-Selective Electrodes Based on Molecular Recognition Principles... 

The incorporation of vitamin B12 derivatives into plasticized poly(vinyl chloride) membranes has resulted in the development of several ion-selective electrodes (ISEs). The response of the electrodes has been related to principles of molecular recognition chemistry. In addition, ISEs have been prepared by electropolymerization of a cobalt porphyrin. These electrodes have selectivity properties that are controlled by both the intrinsic selectivity of the metalloporphyrin and the characteristics of the polymer film (e.g., pore size). 

In order to develop selective electrodes, it is necessary to introduce specific interactions between the ionophore and the anion of interest. This can be achieved by designing an ion carrier whose structure is complementary to the anion. This type of design can be based on molecular recognition principles, such as the ones that involve complementarity of shape and charge distribution between the ion and the ionophore. 

In this paper, we report the development of ISEs that have been designed by using molecular recognition principles. Specific examples include the development of polymer membrane anion-selective electrodes based on hydrophobic vitamin B12 derivatives and a cobalt porphyrin. The selectivity patterns observed with these electrodes can be related to differences in the structure of the various ionophores, and to properties of the polymer film. 

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

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