Voltammetric techniques amperometry

Ampcromctry The final voltammetric technique to be considered is amperome-try, in which a constant potential is applied to the working electrode, and current is measured as a function of time. Since the potential is not scanned, amperometry does not lead to a voltammogram. 

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

This system is governed via AutoAnalysis software that allows the implementation of a number of polarographic and voltammetric techniques ranging in complexity from amperometry with three electrodes to anodic or cathodic differential pulse stripping voltammetry. 

As it can provide some of the most basic electrochemical information related to the reactivity of the selected analyte (peak potential and peak current) most instruments that perform amperometry can also perform some of the most basic voltammetric techniques. These techniques determine the current as a function of the potential applied to the WE (in a conventional three-electrode cell) and can be performed with relatively simple instrumentation [105,106]. As different signals can be combined in the input ports of the instrument, multiple variations of the technique have been developed including cyclic voltammetry, linear sweep voltammetry, linear sweep stripping voltammetry, stripping voltammetry [107, 108], fast-scan cyclic voltammetry [109], square-wave voltammetry [110],and sinusoidal voltammetry [111]. 

As mentioned in the introduction of the amperometry techniques, the voltammetry with periodical renewal of the diffusion layer is particularly effective in monitoring a process differently involving an electroactive species, e.g., in the already mentioned amperometric titrations, in the determination of the stability of a species, etc. In particular cases, also simple chronoamperometry, i.e., at a fixed, suitably chosen potential, may be effective to this purpose. Noteworthy, it will be clear in the following that the much more widely diffused linear potential scan and cyclic voltammetric techniques are not always suitable to substitute for voltammetry with periodical renewal of the diffusion layer to the purpose of monitoring electroactive species during their transformation. Voltammetry with periodical renewal of the diffusion layer, as well as the voltammetry at rotating disk electrode, only allows the estimation of the concentrations of both partners of a redox couple, on the basis of the ratio between the anodic and cathodic limiting currents. 

Flow injection analysis (FIA) (Ruzicka and Hansen), since 1975 In continuous flow, stopped flow or with merging zones (FIA scanning or intermittent pumping) Adapted voltammetric electrodes Membranes for Partial dialysis Membrane amperometry (Clark) Differential techniques (Donnan) Computerization, including microprocessors Special measuring requirements in plant control (to avoid voltage leakage, etc., Section 5.5)... 

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

Amperometric Sensors A number of voltammetric systems are produced commercially for the determination of specific species of interest in industry and research. These are usually based on measuring the limiting current at a constant applied potential and relating the measured current to concentration. This technique is often called amperometry. Amperometric devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors. Two of these devices are described here. 

Classically amperometry has been concerned with the maintenance of a fixed potential between two electrodes. More recently, pulsed techniques have come to the fore some details of these are included in Table 8.3. The applied potential at which the current measurement is made is usually selected to correspond to the mass transport limited portion of the corresponding voltammetric scan. Theoretically, in a quiescent solution, this is electrochemical suicide. The current ii obtained at conventional electrodes under such circumstances gradually decays to zero according to the Cottrell equation (4) ... 

Another electrochemical technique used for the determination of antioxidant capacity is amperometry (Milardovic et al., 2006 Intarakamhang and Schulte, 2012). The current utilized in this technique is selected after a voltammetric analysis. Milardovic et al. (2006) determined the antioxidant activity of eight samples of different types of tea, wine, and other beverages. The amperometric method is based on the electrochemical reduction of DPPH at a glassy-carbon electrode. Initially, the voltammetric study of different standard antioxidants (caffeic acid and Trolox) was performed to obtain their respective reduction potentials. The potential selected for the analysis was 140 mV, to avoid possible interference from electrochemical processes of caffeic acid and Trolox. The concentration of DPPH significantly decreased after the antioxidant addition. The results were expressed as Trolox equivalents. 

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