Dynamic potentiometry

The diversity of interfacial electrochemical methods is evident from the partial family tree shown in Figure 11.1. At the first level, interfacial electrochemical methods are divided into static methods and dynamic methods. In static methods no current passes between the electrodes, and the concentrations of species in the electrochemical cell remain unchanged, or static. Potentiometry, in which the potential of an electrochemical cell is measured under static conditions, is one of the most important quantitative electrochemical methods, and is discussed in detail in Section IIB. 

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

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. 

Figure 18.6—Example of a measurement by direct potentiometry. The slope for the chloride selective electrode has almost an ideal value. The dynamic range for most selective electrodes spans 4 to 6 orders of magnitude, depending on the ion.

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

The different methods - potentiometry conductometry turbidimetry vis-cometry " calorimetry , kinetira, sedimentation , dynamic flow birefringence, light scattering , high resolution H-NMR spectrometry , chromatography, spectroscopy , electron microscopy , and others have been used for the investigation of the formation and composition of polymer-polymer complex. ... 

Interest in Zr carbonates in part originates from their presence in nuclear waste.495 Equilibrium constants for the formation of [M(C03)4]4 have been measured,496 and the coordination mode and kinetic behavior have been determined by dynamic NMR and Raman polarization studies.495 The interactions of carbonate ions with Zr has been investigated by potentiometry, NMR spectroscopy,497 and electrospray mass spectrometry.498 Although four-coordination modes are possible, the data suggested a bidentate binding mode for the carbonates in [Zr(CC)3)4]4. ... 

In recent years, it has become possible to measure more selective properties on-line by incorporating instruments such as potentiometry (Figure 9.6), IR and GC into the process stream. These provide a dynamic rather than historic measure of what is going on with the process. The data are fed back to controllers this permits better regulation of the process... 

Concluding Remarks on Potentiometry. Due to the unique characteristics of the liquid-liquid interface system, factors in addition to the concentration of analyte ion must be considered in potentiometric studies. These factors include the nature and concentration of the supporting electrolytes and the relative volume of the phases in contact. Numerical solutions of the theoretical relationship derived by Hung (15) are useful to predict the effect of such factors as volume of the phases and concentration of added ions. Experimental results with an oxacyanine dye in a water-nitrobenzene system show a linear response in the 10 3-10 5-mol/L concentration range, which corresponds to a 120-mV dynamic range of these dyes for use as potential sensors. This response agrees with measurements on biological... 

Simple electroanalytical techniques fall into three groups potentiometry, conductivity, and voltammetric/amperometric techniques. This volume is mostly concerned with the third type of method, often referred to under the general heading dynamic electrochemistry. Given the importance of potentiometric methods of analysis, these will be introduced and discussed in this chapter. As mentioned in Sect. 1.1.3, there is renewed... 

Importantly, and unlike potentiometry, voltammetric methods are dynamic and give information on kinetics, that is, rates of electron transfer and coupled (EC) reactions the latter include those in which electron transfer drives a reaction such as ion/proton transfer, or is gated , that is, the case in which the electron-transfer event is controlled by a preceding chemical process. Redox reactions can be quantified in both the potential and time domains, and these may be separated and resolved for example, steady-state catalytic studies of adsorbed enzymes reveal how catalytic electron transport varies as a function of potential, which can be important if the rate is sensitive to the oxidation state of a particular site in the molecule [1]. 

The electrochemical techniques can be divided into two major groups static (i = 0) and dynamic (i + 0) (6). Potentiometry is a static method, and it measures the rest potential vs. time the most common applications in potentiometry are the use of ion-selective electrodes and pH meters. The dynamic methods comprise mostly all the other electrochemical techniques (3). Table 1.1 lists the most commonly used methods among this group. 

The potentiostatic multi-pulse potentiometry described here allows the dynamic measurement of potentials. The advantages of this method are the short time required for the analysis and the low noise of the signal. The "ancestor" of this technique, enzyme chronopotentiometry [7 ], posed problems of reproducibility when it was applied to the immobilized redox polymer. The excellent reproducibility of our method is clearly shown in fig. 3b. These techniques were the fundamental developments to conceive redox-FETs for the first time. After immobilization of NAD -dependent dehydrogenases covalently on the surface of the transducer the enzymatically produced NADH would be catalytically oxidized in situ by the polymeric mediator. To this very compact combination the substrate and NAD+ as cosubstrate have to be applied externally. The coimmobilization of the coenzyme NAD+ would lead to reagentless sensors. This is a subject of forthcoming investigations... 

Carvalho S, Delgado R, Felix V (2010) Evalutition of the binding ability of a macrobicyclic receptor for anions by potentiometry and moleculeu dynamics simulations in solution. Tetrahedron 66 8714-8721... 

Different electrochemical techniques depend on whether they are bulk methods as in conductometry or interfacial methods. The latter may be static as in potentiometry or dynamic. Dynamic methods are classified on the basis of the current used. Conductometry uses a constant current but controlled current methods include voltametry, amperometry and coulometry. 

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