Voltammetric techniques

A number of voltammetric experiments are routinely used in quantitative and qualitative analyses. Several of these methods are briefly described in this section. 

Determination of limiting current and halfwave potential in linear scan hydrodynamic voltammetry. 

A form of voltammetry using a dropping mercury electrode or a static mercury drop electrode. 

Potential-excitation signal and voltammogram for normal polarography. 

Polarography is used extensively for the analysis of metal ions and inorganic anions, such as lOg and NOg. Organic compounds containing easily reducible or oxidizable functional groups also can be studied polarographically. Functional groups that have been used include carbonyls, carboxylic acids, and carbon-carbon double bonds. 

In single- and double-step chronoamperometries, one or two constant potential pulses are applied and the signal corresponds to the variation of the 

Linear sweep and cyclic voltammetries are probably the most widely used techniques given that they enable simple and direct qualitative analysis of the electrochemical system. However, caution must be taken since the program applied with modern digital equipment does not correspond to a linear ramp but rather to a staircase. 

---

In voltammetry a time-dependent potential is applied to an electrochemical cell, and the current flowing through the cell is measured as a function of that potential. A plot of current as a function of applied potential is called a voltammogram and is the electrochemical equivalent of a spectrum in spectroscopy, providing quantitative and qualitative information about the species involved in the oxidation or reduction reaction.The earliest voltammetric technique to be introduced was polarography, which was developed by Jaroslav Heyrovsky... 

Stripping Voltammetry One of the most important quantitative voltammetric techniques is stripping voltammetry, which is composed of three related techniques anodic, cathodic, and adsorptive stripping voltammetry. Since anodic strip-... 

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. 

In the previous section we saw how voltammetry can be used to determine the concentration of an analyte. Voltammetry also can be used to obtain additional information, including verifying electrochemical reversibility, determining the number of electrons transferred in a redox reaction, and determining equilibrium constants for coupled chemical reactions. Our discussion of these applications is limited to the use of voltammetric techniques that give limiting currents, although other voltammetric techniques also can be used to obtain the same information. 

Conditions are controlled, such that equation 13.20 is valid. The reaction is monitored by following the rate of change in the concentration of dissolved O2 using an appropriate voltammetric technique. 

Which voltammetric technique can be used for estimating the surface coverage of an adsorbed molecule How ... 

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

The properties of anodic layers of HgS formed on mercury in sulfide solutions have been investigated in comparison with anodic sulfide layers of cadmium and bismuth. Also, the electrochemistry of mercury electrodes in aqueous selenite solutions has been studied (see Sect. 3.2.1). The problem with the presence of several cathodic stripping peaks for HgSe in acidic Se(IV) solutions has been addressed using various voltammetric techniques at a hanging-mercury-drop electrode [119]. 

Modern sensitive chromatographic and voltammetric techniques now make it possible to estimate the release of unlabelled endogenous transmitter from slices of brain tissue (commonly the hippocampus and striatum) or spinal cord (Fig. 4.4). However, whatever analytical method is used, the thickness of the slice is paramount. It is important to maintain the balance between preserving the integrity of the tissue (the thicker the slice, the better) against maintaining tissue viability by perfusion with oxygenated aCSF (the thinner the slice, the better). 

At the beginning of Section 3.3 a distinction was made between voltammetric techniques with non-stationary and stationary electrodes the first group consists of voltammetry at the dme or polarography, already treated, and voltammetry at hydrodynamic electrodes, a later subject in this section however, we shall now first consider voltammetry at stationary electrodes, where it is of significance to state whether and when the analyte is stirred. 

The way in which automation of electroanalysis can be achieved depends very much on the specific requirements of the application. In order to illustrate this we have selected a number of typical examples. However, in doing so, we did not consider normal automation inherent to the nature of the analytical method, e.g., automatic scanning of the voltammetric curve in polarography and other voltammetric techniques, in addition to many additional refinements within these methods such as those treated already in Chapter 3 therefore, the selection of the examples in this chapter cannot be other than arbitrary, where the borderline between the common and the uncommon in the future certainly will shift towards the former. 

The Model 384B (see Fig. 5.10) offers nine voltammetric techniques square-wave voltammetry, differential-pulse polarography (DPP), normal-pulse polar-ography (NPP), sampled DC polarography, square-wave stripping voltammetry, differential pulse stripping, DC stripping, linear sweep voltammetry (LSV) and cyclic staircase voltammetry. 

Donat and Bruland [217] determined low levels of nickel and cobalt in seawater by a voltammetric technique, and the nioxime complexes of the two elements were concentrated on a hanging mercury drop electrode. The current resulting from the reduction of Co (II) and Ni (II) was measured by differential pulse cathodic stripping voltammetry. Detection limits are 6 pM (cobalt) and 0.45 nM (nickel). 

Achterberg, Van den Berg, and others [786] used a voltammetric technique to take continuous real-time measurements of nickel, copper, and zinc in the Irish Sea. 

Pinheiro, J. P., Mota, A. M., Simoes Gonsalves, M. L. S. and van Leeuwen, H. P. (1998). The pH effect in the diffusion coefficient of humic matter influence in speciation studies using voltammetric techniques, Coll. Surf. A, 137, 165-170. 

This is a dynamic electrochemical technique, which can be used to study electron transfer reactions with solid electrodes. A voltammo-gram is the electrical current response that is due to applied excitation potential. Chapter 18b describes the origin of the current in steady-state voltammetry, chronoamperometry, cyclic voltammetry, and square wave voltammetry and other pulse voltammetric techniques. 

The characteristic shape of i-E curve depends on the nature of the redox couple in the condensed phase, its thermodynamics, kinetics, mass transfer, and on the voltage-time profile (E—t). In this section we will discuss various voltammetric techniques and their applications in modern chemistry. 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

Analytical applications of selected reactions studied by voltammetric techniques... 

The following section describes various voltammetric techniques and applications. In most cases we assume a reversible fast reaction and diffusion as the rate-controlling step. 

The realization that current sampling on a step pulse can increase the detection sensitivity by increasing the faradaic/charging ratio is the basis for the development of various pulse voltammetric (or polarographic) techniques. Also, the pulses can be applied when it is necessary and can reduce the effect of diffusion on the analyte. Figure 18b. 11 shows the waveform and response for three commonly used pulse voltammetric techniques normal pulse voltammetry (NPY), differential pulse voltammetry (DPV), and square-wave voltammetry (SWV). 

---