Analysis and Application of Voltammetry - Report Example

Analysis and application of Voltammetry References Aabraha, Taame, and Assefa Sergawie. "Assessment of Some Selected Beverages and Fresh Edible Vegetables as Nutritional Source of Vitamin C (Ascorbic Acid) by Cyclic and Square Wave Voltammetry." Alghamdi, Ahmad H. "Determination of zinc by square-wave adsorptive stripping voltammetry using alizarin as a chelating agent." Journal of Saudi Chemical Society 14.1 (2010): 1-7. Alghamdi, Ali F. "Voltammetric Analysis of Montelukast Sodium in Commercial Tablet and Biological Samples Using the Hanging Mercury Drop Electrode." Portugaliae Electrochimica Acta 32.1 (2014): 51-64. Dogan-Topal, Burcu, Sibel A. Ozkan, and Bengi Uslu. "The analytical applications of square wave voltammetry on pharmaceutical analysis." The Open Chemical and Biomedical Methods Journal 3 (2010): 56-73. Farghaly, O. A., R. S. Abdel Hameed, and Abd-Alhakeem H. Abu-Nawwas. "Electrochemical Analysis Techniques: A Review on Recent Pharmaceutical Applications." International Journal of Pharmaceutical Sciences Review & Research 25.2 (2014). Ignas, Tonle K, and Ngameni Emmanuel. Voltammetric Analysis of Pesticides. INTECH Open Access Publisher, 2011. Print. Kalcher, Kurt. "Chemically modified carbon paste electrodes in voltammetric analysis." Electroanalysis 2.6 (1990): 419-433. Kalvoda, Robert, and Miloslav Kopanica. "Adsorptive stripping voltammetry in trace analysis." Pure and applied chemistry 61.1 (1989): 97-112. Buffle J, Tercier-Waeber M.-L. Voltammetric environmental trace metal analysis and speciation: from laboratory to in situ measurements. Trends in Analytical Chemistry, Vol. 24, No. 3, 2005 Thomas, F. G., and Günter Henze. Introduction to Voltammetric Analysis: Theory and Practice. Collingwood, Vic.: CSIRO Pub., 2001. Print. Gulaboski Rubin and Pereira Carlos M. “Electroanalytical Techniques and Instrumentation in Food Analysis”. Handbook of Food Analysis Instruments. Boca Raton: CRC Press, 2008. Print. Analysis and application of Voltammetry Abstract Electrochemical analytical techniques have progressively become effective means in the analysis of organic and inorganic substances. These methods offer high sensitivity, precision and accuracy that are desired in any analytical testing method. In conjunction with this property they are relatively time saving and don’t require any tedious preparation of samples prior to analysis. Electroanalytical techniques have been utilized in the analysis of both organic and inorganic compounds. The compounds of interest are expected to possess electrochemically reducible or oxidizable groups. Pharmaceutical and environmental applications of electroanalytical techniques have become of interest in the improvements in electrodes of high selectivity and sensitivity. 1.0. Introduction In the pharmaceutical, industrial and metal industries, electrochemical analytical techniques have steadily taken precedence in the course of time over previously used conventional analyzes methods. The exhibited high selectivity, reduction in solvent and sample consumption, speed of the analysis, low operation costs and rapid scan rates are some of the factors that propel electrochemical techniques into becoming methods of choice (Farghaly, Hameed, and Abu-Nwas 3288). The discovery of voltammetry in 1922 by Joroslav Heyrovsky (Gulaboski and Pereira 380) led to the development of voltammetry. Electrochemistry has thus provided tools that are efficient in detecting low concentrations of substances while enabling wider linear relationships between the measurement of the current signal and the concentration of the analyte of interest. These methods have also allowed the simultaneous determination of numerous analytes that is advantageous over other contemporary techniques that require implementation of separations techniques to allow for selective and direct analysis of analytes. Contrary to other methods of analysis that are localized to laboratories, most voltammetric methods are portable hence offering the analyst the convenience of flexibility (in situ), avoids sample handling problems, losses by absorption, and sample degradation due to transport from site to the laboratory (Ignas, Tonle and Ngameni 19). With the increase in their applications in environmental monitoring, foodstuffs, pharmaceuticals, and other analysis, electrochemical techniques are progressively becoming inevitable techniques both in the chemical and biochemical research (Gulaboski and Pereira 379). Voltammetry techniques have been applied in oxidation-reduction studies to determine reaction mechanisms, studying kinetics and thermodynamics of electrons and in transfer processes. They have also been found useful in studies of adsorption and crystallization at electrode surfaces. Their high sensitivity, rapid analysis times, and simultaneous determination property (Baffle and Waeber 172) have promoted polarography and voltammetry techniques as principal methods in most food analysis. 2.0. The working principle of voltammetry Voltammetric experiments occur in electrochemical cells whose assembly consists of a working electrode (WE), a reference electrode and an auxiliary electrode. The reaction of interest commonly occurs at the working electrode. According to the specifications of voltammetric techniques, it is a perequisite that one electrode must always operate at constant potential. This electrode is the reference electrode which in lieu of its unique characteristic is designed to possess a constant (reversible) potential (Gulaboski and Pereira 380). A counter reactin that is opposite to that which is taking place at the working electrode occurs at the auxiliary electrode whose purpose is to balance the charge in the system. The prescence of the electroactive species in the electrochemical working electrode causes the applied potential to provoke a change in the concentration of the electroactive species that is being monitored. The change in concentration is achieved through the reduction or oxidation of the electroactive species at the electrode surface. The change in concentration is accompanied by the mass transport toward the electrode with a consecutive flow in current through the electrodes that are directly proportional to the concentration of the analyte. Therefore, this current flowing between the electrodes is considered to be dependent on the concentration of the analyte. Voltammetry techniques are broadly based on the recording of the current that is flowing between the working electrode and the auxiliary electrode due to the oxidation-reduction processes of the test element as a function of potential energy imposed on the working electrode and is expressed with respect to that of the reference electrode (Buffle and Waeber 172). The quantitative voltammetry determination of organic and inorganic compounds is based on this deduction. There exists variations in voltammetry techniques all of which are drawn from the same electrochemical theory as discussed by Gulaboski and Pereira (381). Stripping voltammetry techniques, cyclic voltammetry and pulse voltammetry with their variants are some of the techniques in practice presently. Anodic stripping voltammetry (ASV) is a modern application of voltammetry. It is used for the analysis of metal ions and organic compounds at trace levels. Electro-deposition occurs at the mercury electrode, and the analyte is reduced to form an amalgam at the negative cathodic potential. The application of a positive anodic potential reoxidises and strips out the amalgamated analyte from the electrode and produces a quantifiable peak current that is proportional to the concentration of the analyte. This peak potential is, therefore, used to identify the analyte. Cathodic stripping voltammetry (CSV) has been applied to both organic and inorganic compounds that form insoluble Salts with the electrode materials. The application of anodic potential to the working electrode causes a positive deposition of analyte and successively stripping in a negative-going potential (Farghaly 3289). The sensitivity of cathodic stripping voltammetry is sensitive to the amount that is plated within any period. Also, the density of the insoluble salt film that is formed at the electrode and the dissociation rate of the insoluble mercury compounds during stripping. Adsorption stripping voltammetry is analogous to cathodic stripping voltammetry and anodic stripping voltammetry. However, in adsorption stripping voltammetry there exists a pre-concentration step. AdSV has applications in the trace analysis of organic and inorganic analytes. Potentiometric stripping analysis is also another stripping voltammetry technique. Cyclic voltammetry has extensive applications in the study of redox reactions and to obtain the dynamics of the chemical reaction. However, it is rarely used in a quantitative determination of the analyte. Pulse voltammetry is also another branch of voltammetry. In this form normal pulse voltammetry, differential pulse and square wave voltammetry are conventional techniques. The latter method is fast and when compared to the differential pulse voltammetry for both reversible and irreversible incidences it has been observed to produce 4 and 3.3 times higher currents respectively. 3.0. Types of electrodes Typical electrode system have comprised of a dropping mercury electrode (DME) as the working electrode, a coiled platinum wire as the auxiliary electrode and a saturated calomel electrode as the reference electrode. Sometimes a bubbler has been fixed in the assembly to facilitate the aeration of the solution (Tamraka and Pitre 843). 3.1. Mercury electrode Hanging dropping mercury electrode (HDME) and mercury film electrode (MFE) provide essential conditions stripping voltammetry. The electrode is required to be stationary, enable favorable redox behavior of analyte, a little background amount, and reproducible area. 3.2. Solid electrodes Compared to their predecessors, solid electrodes have an extended anodic potential. Among the solid electrodes, Gold, platinum, epoxy bonded graphite electrodes, carbon fiber, carbon paste, and glass carbon electrode are commonly used as working electrodes. The heterogeneity of the surfaces of solid electrodes is presumed to cause deviations from expected behavior. A carbon fiber electrode has also been used in differential pulse stripping voltammetry. 3.3. Chemically modified electrodes Modifications are made on electrodes to improve their inherent properties. Reagents can be on the electrodes surface or within its matrix. Sensors and biosensors and potentiometry are some essential applications of voltammetry. Electroanalytical techniques exhibit versatility, selectivity, and high specificity for qualitative and quantitative analysis of trace metals with added advantage of being less tedious, economical and less time-consuming. Voltammetry has broad applications in organic and inorganic substances. The analysis of metals is one of its primary uses. 4.0. Applications of Voltammetry techniques 4.1. Trace metal analysis Environmentally significant trace elements are Pb, Cd, As, Hg, Cr, Al, Cu, Zn, Ni, Se, and Bi. Voltammetry techniques have been used extensively in the monitoring of these trace elements. Previously total metal concentrations measurements required sample acidification, UV irradiation, physical and chemical pretreatments of samples. Voltammetry has been used in the determination of complexion properties such s stability constants and total ligand concentrations of natural occurring complexants (Buffle and Weiber 175). Accuracy convenience and precision are some of the qualities of a technique that a researcher finds appealing. Tamraka and Pitre (843) determined metal ions contained in unused engine oil samples using a direct current polarography (DCP), differential pulse polarography (DPP) and differential pulse anodic stripping voltammetry (DPASV). Engine oils and lubricant quality and characteristics are improved with additives that comprise of organic, organometallic formulations, trace metals as organometals. Since the concentrations of these metals are low there is a need for determination of this metals at trace levels to ensure effective blending and quality of products. From the study significant amounts of Cu, Pb, Cd, Zn, Co, Fe and Cu were found in the samples. A statistical comparison with Atomic absorption spectroscopy showed that there were high accuracy and precision in the application of these methods to determine heavy metals. DCP, Differential Pulse Polarography, and Differential Pulse Anodic Stripping Voltammetry have been used for the determination of trace metals in petroleum products. The study revealed that the voltammetry provided simultaneous quantitative and qualitative multi-element analysis with a 99% recovery of metals ions with high accuracy and precision. However, ASV has a low limit of determination in heavy metal ions trace analysis (Kalvoda and Kopaniza 98). 4.2. Analysis of Pharmaceuticals An understanding of the physical and chemical processes in the developed procedure is crucial for effective application of electrochemical analytical methods. In pharmaceutical quality control, separation techniques are the most widely used since they are excellent in dealing with complex samples and tracing the products of drug metabolism. Electroanalytical techniques have provided simple sample handling, speed of analysis, high sensitivity, comparable and better accuracy, cheaper instrumentation, low cost of chemicals used and a limited use of environmentally unfriendly organic solvents. However, it important to note that the techniques have been effective in single physiologically potent components (Zuman 98). Square wave cathodic adsorptive stripping voltammetry has had applications as a validated, highly sensitive and accurate method for determination of nitrofuranion. Cyclic and square wave voltammetry is extensively used to analyze the adsorptive behavior of trimetadizine hydrochloride using glassy carbon electrode. Zopidone has been quantitatively determined by adsorptive stripping voltammetry using a glassy carbon electrode. Cyclic linear sweep voltammetry are used to investigate electrochemical oxidation of carvedilol. DPAdSV and SWAdSV have been used to determine the second oxidation step of carvedilol Constraints have been found in the applications of voltammetry. The component under investigation must present electroactive properties and under suitable conditions undergo reduction or oxidation, catalyze the redox process or get converted to a species that undergoes reduction-oxidation in the analyzed solution. Voltammetry methods need professionalism in the form of trained personnel with an understanding of the underlying principles and applications. This knowledge will enable them propose optimum conditions for analysis. They should also be able to identify faults recognize their origins and present suitable remedies. The implementation of square wave voltammetry in the analysis of pharmaceuticals between1997-2010 has been done on 95 drugs with variable therapeutic properties. The variations of the working electrode have been employed (Dongan-Topai et al. 68). Variants of the working electrode for pharmaceutical analysis are being developed. The variants are either native or modified glassy carbon electrodes. HMDE is commonly utilized. The design of the working electrode, its fabrication and characteristics of the materials are critical factors that significantly control the performance of the direct and stripping voltammetry techniques. 4.3. Food analysis Square wave adsorption stripping voltammetry has been shown to be useful in ultra-trace quantitative analysis. It is faster than differential pulse voltammetry and with high sensitivity. It is highly suited for the determination of ultra-trace amounts of inorganic ions through adsorbed complexes (Alghamdi 14). In adsorption stripping voltammetry, the metal ion is adsorbed on the working electrode by a non-electrolytic process prior to the voltammetric scan. Analytes are determined as complexes with varying organic ligands adsorbed on the electrode surface. The sensitivity of SW-AdSV is strongly dependent on the pH and the nature of the supporting buffer which facilitates formation and stability of the complex, adsorption process and electrochemical reduction process. Square wave voltammetry has shown better results in ascorbic acid quantification than cyclic voltammetry. Cyclic voltammetry and square wave voltammetry has also been used in the qualitative characterization and quantitative determination of vitamin C (Aabraha and Assefa 40). Conventional volumetric methods that utilize oxidant solutions have been used for determination of vitamin C both kin pharmaceutical and industrial application (vitamin C containing beverages, ascorbic acid). However, volumetric analysis has shortcomings of not being able to determine total ascorbic acid and a lack of specificity. Fluorimetric analysis methods have been more selective but associated with pH control and high equipment quest. HPLC has been a more competent method for determination of vitamin C with high specificity ad selectivity. 4.4. Analysis of Pesticides Voltammetric techniques have been efficient in studies in aqueous media or at solid state of electro active species. Fluorometry, capillary electrophoresis, spectrophotometry, mass spectroscopy, and chromatography have been conventional methods for analysis of pesticides. The determination of pesticides in water and soils is complicated by the high number of chemicals present in the matrix, specificity in reactivity and several active ingredients in the pesticides. Cyclic voltammetry and pulse voltammetry techniques such as differential pulse polarography, differential pulse voltammetry, and square wave voltammetry are some of the electroanalytical techniques commonly applied to fertilizers. Electrodes have frequently been modified so as increase efficiency in amperometric sensors. These modifications enable the coupling of individual and specific properties to promote synergy. The chemically modified electrodes are used in the detection of pesticides. Examples of modified electrodes are the carbon paste electrodes (Kalcher 419), immobilized enzyme electrodes, carbon nanotubes modified electrodes, and clay film modified electrodes (Ignas, Tonle and Ngameni 472). Future trends in the development of pesticides control voltammetric techniques are geared towards the development of miniaturized and disposable voltammetric sensors. It will also focus on new electrode configurations that provide high selectivity and sensitivity to pesticides. The development of new electrode material will also play a vital role in applications of voltammetric techniques for pesticide analysis. 5.0. Conclusion From the review of voltammetric techniques and their applications in analysis, it is evident that their use in conventional analysis is unprecedented. With the modification of electrodes, they have found broad applications in areas of analysis that previously seemed improbable. As with any analytical method, precision and accuracy have been the theme and couple with this property, electrochemical analysis and voltammetry in particular with the added advantage of reducing cost and being less time-consuming. Their potential for qualitative and quantitative multi-elemental analysis is also a factor in favor of them. It is also evident that electroanalytical methods offer high specificity and selectivity that are comparable to the widely accepted conventional techniques. Read More