Potentiometry Potentiostat

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

The reader familiar with controlled-potential methodology will have no trouble understanding a controlled-current apparatus. Figure 6.18 illustrates classical approaches to two- and three-electrode constant-current chronopotentiometric experiments (see Chap. 4). The simplicity of these circuits was for many years an attractive feature of chronopotentiometry. Improvements in potentiostats have been largely responsible for a decline in the popularity of chronopot in recent years. Nevertheless, constant-current experiments are even more important with respect to coulometric titrations and stripping potentiometry (Chap. 24). 

In general, electroanalytical detection principles can be divided into three potentiometry, amperometry, and conductometry (or impedometry). Potentiostats used for electrochemical biosensors are mostly equipped with amperometric and... 

Figure 3. (a) Potentiostatic multi-pulse potentiometry for the determination of 0.1, 0.5 and 5 mM NADH in buffer solution with a redox-polymer modified FET, (b) calibration curve for a measurement of NADH in 0.1M phosphate buffer, pH 7.5. 

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... 

What sort of instrumentation would be needed for electrochemical experiments A potentiometry experiment requires little more than a pH meter. A potentiostat or galvanostat can be used for the controlling potential or current in an experiment. In a coulometric procedure, a device to integrate the current (i.e., a coulometer) would also be needed. A hydrodynamic voltammetry [e.g., a rotating disk electrode (RDE)] experiment would require an electrode rotor (to spin the electrode at a precisely known rotation speed), and the rotating ring-disk or RRDE refinement (see below) would necessitate the use of a bipotentiostat so that the disk and ring potentials can be independently controlled. An ac impedance measurement involves the use of a sine-wave oscillator and... 

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