Voltammetric analysis

Accuracy The accuracy of a voltammetric analysis often is limited by the ability to correct for residual currents, particularly those due to charging. For analytes at the parts-per-million level, accuracies of+1-3% are easily obtained. As expected, a decrease in accuracy is experienced when analyzing samples with significantly smaller concentrations of analyte. 

The determination of such compounds was measured by their effect on the oxidation signal of tire guairine peak of calf tlrymus DNA immobilised on tire electrode surface and investigated by chronopotentiometric or voltammetric analysis. Applicability to river and wastewater samples is demonstrated. 

Ni, Y.N., Bai, J.L., and Jin, L., Simultaneous adsorptive voltammetric analysis of mixed colorants by multivariate calibration approach. Anal. Chim. Acta, 329, 65, 1996. 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

The great attraction of SV lies in the effect of pre-concentration of the analyte at the electrode with, as a consequence for the stripping current, a very high ratio of faraday current to charging and impurity currents it is this high ratio which has made SV the most sensitive voltammetric analysis method to date. 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

Garcia-Monco Carra et al. [405] have discussed the use of a hybrid mercury film electrode for the voltammetric analysis of lead (and copper) in acidified seawater. 

The standard potentials of practically all oxidation and reduction reactions, especially those common in the environment and soil, are known or can easily be determined. Because of the specificity and relative ease of conducting voltammetric measurements, they might seem well suited to soil analysis. There is only one major flaw in the determination of soil constituents by voltammetric analysis and that is that in any soil or soil extract, there is a vast array of different oxidation-reduction reactions possible, and separating them is difficult. Also, it is not possible to start an investigation with the assumption or knowledge that all of the species of interest will be either oxidized or reduced. 

Given the one electron nature and chemical reversibility of the oxidation, let us pass to the cyclic voltammetric analysis of the process, Table 1. 

The Glucose Sensor a Worked Example of Voltammetric Analysis... 

Wang and Freiha have designed a carbon-paste electrode, based on a piston principle that enables an outer layer of paste to be extruded and removed, and used this in investigating possible extractive accumulation and further characteristics of the pre-concentration step in voltammetric analysis [163]. Chlorpromazine was used as a model compound. 

Kamal etal. [188] have described adsorptive stripping voltammetric analysis of 2,3,6-tri(2 -pirydyl)-l,3,5-triazine at mercury electrode, based on preliminary adsorption and accumulation of the determined compound. 

Ulakhovich NA, Medyantseva EP, Budnikov G K (1993) Carbon paste electrodes as chemical sensors in voltammetric analysis. Zh Anal Khim 48 980-998. 

Also known as vitreous carbon, glassy carbon has been the subject of intense electroanalytical research in the past 10 years. It is impermeable to liquids and gases, and thus porosity is not an issue, as it is with polycrystalline graphite. It is easily mounted, polishable, and compatible with all common solvents. These properties have led to widespread use in mechanistic electrochemistry, LC detection, and voltammetric analysis. 

The introduction of ultramicroelectrodes in the field of voltammetric analysis offers access to cyclic voltammetry experiments that are impossible with conventionally sized macroelectrodes. In addition to analyses in small volumes or at microscopic locations, microelectrodes allow measurements in resistive media and make it possible to perform high scan rate voltammetry [9,10]. 

W. Yantasee, L.A. Deibler, G.E. Fryxell, C. Timchalk and Y. Lin, Screen-printed electrodes modified with functionalized mesoporous silica for voltammetric analysis of toxic metal ions, Electrochem. Commun., 7 (2005) 1170-1176. 

Microfluidic chip devices are also shown to be attractive platforms for performing microscale voltammetric analysis and for integrating voltammetric procedures (linear-sweep, square-wave and adsorptive-stripping voltammetry) with on-chip chemical reactions and fluid manipulations [97]. 

Since the pencil leads have a composite structure with mixed graphite and insulator [10,11], PGE has been used as an excellent electrode material for voltammetric analysis of DNA and it has also been used as supporting material in the development of novel genosensors based on nanomaterials. 

The electrode/electrolyte interface is intimately involved in many important processes, including voltammetric analysis, electrodeposition, metal corrosion, battery... 

Quasi-reference electrodes can be employed in situations where the high reproducibility of potential is not necessary, such as in many voltammetric analysis experiments. Mercury pools (referred to above) or silver wires in aqueous halide media are examples. Platinum wires can also be used. The advantage of wires, apart from their small size, is in reducing the uncompensated resistance in resistive media, relative to conventional reference electrodes. 

The larger part of this book has been devoted to studies related to electrochemical measurements away from equilibrium. As demonstrated, these permit the determination of kinetic and thermodynamic parameters of the electrode processes, whereas measurements at equilibrium furnish only thermodynamic data. So, whilst potentiometric analysis is a powerful tool in the determination of activities or concentrations, the specificity arising from the electrode material, amperometric or voltammetric analysis permits other parameters besides these to be obtined. 

---