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International Journal of Modern Analytical and Separation Sciences, 2013, 2(2): 61-70 International Journal of Modern Analytical and Separation Science ISSN: 2167-7778 Florida, USA Journal homepage: www.ModernScientificPress.com/Journals/IJMAS.aspx Article Recent Advances in the Application of Polarographic Techniques in Chemical Analysis U. Lawal and E. E. Etim * Department of Chemical Sciences, Federal University Wukari, Nigeria * Author to whom correspondence should be addressed; E-Mail: [email protected]. Article history: Received 30 January 2013, Received in revised form 8 June 2013, Accepted 18 June 2013, Published 6 July 2013. Abstract: Created in 1922 by the 1959 Nobel prize winner in chemistry, Jaroslav Heyrovsky, polarography a simple study of solutions by the means of electrolysis rose to become one of the five most used methods of chemical analysis in the post world war one, finding variety of uses even in the development of the atomic bomb ‘the Manhattan Project’. The application of polarography has spread to almost all branches of chemistry. The use of the brilliantly shining and pure liquid metal, mercury, an unlikely candidate for an electrode material ushered in a new era in electrochemistry during the last century. At a time when the solid electrode materials were dogged by the problems of irreproducibility due to surface heterogeneity and impurity, mercury, which is largely free from such malaise, became a new benchmark to study many fundamental electrochemical processes quite accurately and with great precision. This paper aims at reviewing the principle, types and recent advances in the application of polarography. Keywords: chemical analysis; polarography; electrolysis; metal; mercury. 1. Introduction Polarography no doubt is one of the most advanced and enhanced methods used in chemical analysis, and its increasing accuracy and compatibility make it a competitive tool in instrumentation of Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 62 analysis. A majority of the chemical elements can be identified by polarographic analysis, and the method is applicable to the analysis of alloys and to various inorganic compounds. Polarography is also used to identify numerous types of organic compounds and to study chemical equilibria and rates of reactions in solutions. The impact of this technique in electrochemistry was extended far beyond the routine electroanalysis. With the discovery of this technique the face of electrochemistry, which was mostly referred to in the context of ionic equilibria, Debye-Huckel theory and solution electrochemistry in general, started changing [1]. Polarography extended its influence in the study of such diverse phenomena as electrocatalysis, electrochemical energy sources, batteries, fuel cells, biological processes such as ion transport across membranes, solar cells, etc. The technique had a major impact in the study of the mechanism of electrode reactions and consequent theoretical developments of electrode processes such as studies of electrolytic diffusion processes and other transport phenomena. 2. Principle of Polarography The classical polarography method is based on the measurement of current as a function of applied voltage where a current-voltage plot is referred to as a polarogram. A simple schematic of a classical polarographic setup is shown in Fig. 1. The DME will continuously release mercury drops that range in diameter from 0.1 to 1 mm in a preset frequency. In case of analyses that involve metal ions, metal ions are reduced at the negatively charged mercury drops due to diffusion of positive ions. Mn+ + ne = M (Hg) D C s our ce Vari able res is tanc e V _ D ME H g pool + Figure 1. A simple schematic of a classical polarographic setup Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 63 From the measurements of the current-potential curves resulting from electron transfer processes at the surface of a DME, the identity and concentration of the reactant substance can be determined. One of the features of polarography is that solutions as dilute as 10 -8 M can be analyzed and sample volumes as small as 0.05 mL can be manipulated. Thus, the position of a wave in a polarogram along the potential axis gives the identity of the substance while the magnitude of the limiting current gives picture of the concentration variation of this material [2]. A very pure mercury patch can be obtained through distillation under vacuum (99.99 % pure). Each drop represents a fresh electrode with a new exposed surface. The reproducibility of geometry of each drop with the laps of time is another advantage of the DME over other electrodes. The large activation overpotential for hydrogen gas evolution makes this electrode valuable for the study of cathodic processes. One of the most important drawbacks of the Hg as electrode is its ease of oxidation. Thus, Hg undergoes anodic dissolution at +0.25 V vs. SCE and is oxidized to insoluble Hg2Cl2 in presence of chloride ions at zero V vs. SCE so it cannot be used for anodic oxidation above +0.25 V vs. SCE. Also it is important to mention that mercury vapors are very poisonous besides Hg itself is considered to be one of the major pollutants of the environment [2-4]. i av i i d i r E Figure 2. Typical polarogram The slowly increasing current at the foot of the wave is known as the residual current. This current is non faradaic in nature. The diffusion current id is, as shown, the distance between the limiting diffusion plateau and the residual current. The potential at the midpoint of the wave, where the current is exactly half its limiting value, is known as the half-wave potential E1/2 and its quantity is characteristic of a particular species under fixed experimental conditions [5,6]. Thus E1/2 value serves as finger-print for the species undergoing redox. Furthermore, the limiting current is usually Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 64 proportional to concentration of the species being reduced or oxidized and thus serves for quantitative analyses. Over the diffusion current plateau region the DME is behaving as a completely polarized electrode since it adopts any applied potential to it with no change in current flow. It is interesting to note that polarization always results from slow stage in the overall electrode process. The slow stage here is the diffusion process which occurs very much slower than the electron transfer. This type of polarization is known as concentration polarization and the DME is said to be concentration polarized. At potentials corresponding to the rising part of the wave the electrode is depolarized since here the current flow is strongly dependent on the applied potential. At this stage it is important to emphasize that in any electrolysis two types of processes are encountered [7,8]: (1) mass transfer process which brings the electroactive material to the electrode surface, and (2) electrochemical process which involves the act of electron transfer between the surface of electrode and the electroactive species. The mass transfer is usually achieved through: (a) migration, (b) diffusion, and (c) convection. Migration is an electric field effect and depends upon the charge on the species, concentration and mobility in a field of force. Diffusion depends upon differences in concentration between species at the surface of the electrode and in the bulk of solution. Finally convection arises from any mechanical or thermal disturbance in the solution. For a redox process to occur it is essential that electrons pass between the electrode and the species in solution. However, by no means electron transfer in its crude definition acts alone, thus adsorption, rearrangement of electronic configuration within the species to give a suitable form for the electron exchange is a normal observable sequence [9]. After the electron exchange a primary product is formed which re-undergoes an electronic rearrangement, desorption and may suffer further side reactions to form the final product. These electrolytic processes may be reversible or irreversible in nature depending on the activation energy values. 3. Types of Polarography Various types of polarography are evaluated according to their sensitivity (the minimum concentration that can be determined) and resolution (the permissible ratio of concentrations of the supporting component to the component being determined) and depend on the shape and rate of change of the polarizing voltage. 3.1. Direct–Current Polarography In direct-current (classical) polarography, which is based on the dependence of Ie on the slowly varying polarizing Edir, Ie is proportional to the number of electrons n that take part in the reaction. The sensitivity in determining reversibly reactive substances is 10 -5 mole per liter (M), and resolution is about 10. In alternating-current polarography, based on the dependence on Edir of the alternating Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 65 current Ialt that arises upon superimposition of various forms of a voltage Ealt ((low-amplitude rectangular, trapezoidal, and sinusoidal), Ialt is proportional to n2. The high sensitivity of alternatingcurrent polarography (10-7M) results from the possibility of separating the effective signal Ialt from Ic, and its high resolution (up to several thousand) results from the bell shape of the polarogram (the ordinate rapidly tends toward zero upon a deviation of Edir from peak potential) and by the possibility of determining reversibly reactive substances in the presence of components with irreversible reactivity (sensitivity in determining the latter is low) [7-9]. 3.2. High-Frequency Polarography High-frequency polarography involves the superimposition of Edir and a high-frequency E modulated by a low-frequency E. In this case Imf, the component of the current for the modulated frequency, depends on Edir and is proportional to n3. The difference in variation between Imf and Ic upon application of a high frequency is used to separate the effective signal Imf from Ic [5,6]. Highfrequency polarography makes it possible to determine the rate constant of fast reactions. 3.3. Pulse Polarography Pulse polarography is based on the measurement of the current Ip, which arises upon application of a 0.04-sec voltage pulse at the moment when the surface of the mercury drop is maximal. The current Ip is separated from Ic by measuring Ip at the moment of damping of Ic. Pulse polarography has a sensitivity of 1–5 × 10-8 M and resolution ~5 × 103 [3-5]. 3.4. Oscillographic Polarography Oscillographic polarography is based on measurement of the dependence of I, on the rapidly varying Edir (0.1–100 volts per sec). The polarograms produced in oscillographic polarography, which are recorded by means of a cathode-ray tube, have a distinct maximum. In this type of polarography Ie is proportional to n2/3, sensitivity is 10-6 M, and resolution is ~400. In addition to the DME, stationary mercury and solid electrodes are also used in polarography [4,5]. A distinction is made between direct and inversion polarography, depending on the nature of the current being measured. In inversion polarography, the accumulation method is used to increase sensitivity (up to 10-9 M) and resolution (5 × 105 and higher). In this case electrodes with a constant surface are used: at limiting current potentials or upon formation of an insoluble compound, the substance being analyzed accumulates on the electrode surface (the pre-electrolysis stage), and the accumulated solid compound is subsequently dissolved upon a change in Edir. Electrodes made of mercury, graphite, and noble metals are used. Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 66 4. Advances in the Application of Polarography The majority of the chemical elements can be identified by polarographic analysis, and the method is also applicable to the analysis of alloys and various inorganic compounds. In addition, polarography is useful in identifying numerous types of organic compounds and in the study of chemical equilibria and the rates of reactions in solutions. The measurement of polarographic current provides a simple method for the estimation of the rate of several fast electrode reactions. The technique has contributed substantially to the study of adsorption of several surface active molecules on a mercury surface. 4.1. Analyses of Pharmaceuticals A large number of pharmaceuticals can be reduced or oxidized in the available potential range and their waves can be used in their determination. It seems that often the therapeutical activity is paralleled by electrochemical reactivity. Some drugs can be determined using polarography directly, without separation, in some physiological liquids, for example in blood, lymph and saliva. Numerous examples of such applications have been reported early. In some cases, nevertheless, simple separation procedures, such as extraction, have to be used. In numerous cases of this type the uses of polarography would result in procedures which are as selective, but faster and less expensive to use than applications of the most widely used chromatographic methods. On the other hand, in analyses of complex mixtures like urine or in the following of metabolic products, the use of separation techniques is definitely preferable [12]. The situation is different, when the content of the drug is to be determined in matrices, in which the drug is the only electroactive species. Such situation is often faced in analyses of tablets, solutions like eye drops or those used for injections, but also in analyses of some creams and ointments. In such cases the solution to be analyzed should contain the electroactive species at concentrations varying between 1 × 10–5 and 1 × 10–3 M. In such situations, DC polarography is the preferable technique of choice. The method is sufficiently sensitive and measurements of limiting currents are less affected than those of peak currents, used in other electroanalytical procedures, by the presence of electroinactive components of the sample. Presence of components in the sample, like that of starches, of some polymers or long-chain saturated compounds, has little effect on limiting currents, but can affect peak currents, as obtained in differential pulse or square wave variants. Use of chromatographic methods for analyses of such simple matrices resembles using a canon on a fly. Use of polarographic methods for analyses of such simple matrices yields results often much faster, with a better accuracy and without using organic solvents. But, in addition to the lack of trained Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 67 supervising personnel, mentioned above, there is another, administrative hurdle preventing a wider use of polarographic methods in the determination of drugs. Pharmaceutical companies will use, whenever possible, officially approved methods of analysis. In the past, some polarographic analytical procedures were listed in numerous pharmacopoeias [12]. It should be a goal of electroanalytical chemists around the world to have them listed again. The lower costs, faster results, and the possibility for quickly detecting mishandlings by technicians, are powerful arguments. 4.2. Basic Studies Polarography proved to be useful for determination of oxidation states of metals, both in the ionic form and in complexes. Both the equilibria of labile complexes and properties of substitution inert complexes as well as characteristics of metalorganic compounds may be investigated by DC polarography (DCP). Structure-reactivity relationships for such species can be established [12]. In investigations of organic compounds, polarography enables distinguishing the degree of the role of conjugation, effects of ring size, substituent effects in linear free energy relationships and other structure reactivity relationships. Recently, the comparison of polarographic half-wave potentials indicated limited conjugation in compounds containing the >C=N–N=C< grouping, as opposed to the extensive conjugation for those containing groupings –N=C–C=N–. 4.3. Rapidly Established Equilibria Polarography is also a useful tool in investigations of solution chemistry, in particular of equilibria and kinetics. Thus for rapidly established equilibria, both acid-base and of formation of complexes, involving reducible heavy metal ions and a variety of ligands, the equilibrium constants can be determined based on shifts of potentials, using graphs in which these measured quantities are plotted as a function of pH or of a logarithm of ligand concentration [12]. In some instances this enables determination of values of equilibrium constants of reactions involving organic compounds that would be difficult to obtain by other techniques. Similarly, from the shifts of half-wave potentials of the reduction of metal ions, it is possible to obtain information about stability constants of formation of some labile complexes. Polarographic investigations also enabled detection of some unusual species formed in acid-base equilibria, for example of the diprotonated forms of hydrazones and oximes. Whereas DC polarography is well suited for investigation of chemical equilibria preceding the electron transfer (systems denoted as CE) and for identification of relatively stable intermediates in consecutive electroreductions, for investigation of faster chemical reactions following the electron transfer (EC systems) and for studies of properties of short-lived intermediates, cyclic voltammetry is the technique of choice. Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 68 4.4. Slowly Established Equilibria At the other extreme, when investigated equilibria are established slowly, as compared to the rate of the electrode process, the equilibrium constants can be determined based on measurements of limiting currents. The rates of dissociation of OH, SH or NH3 + type acids are too fast to be followed in this way, but some equilibria involving cleavage of the C–H bond are established relatively slowly and can be investigated based on the dependence of the limiting current of the conjugate acid on pH. This approach was possible in establishing the acid-base properties of 3-thianaphthenone, ethyl benzoylacetate, ethyl benzoylbenzoates,ω-cyanoacetophenone and of 1-phenyl-1,3-butanedione. Another group of processes, where the equilibria are not completely shifted in the favor of the products and are established relatively slowly, are some nucleophilic additions to carbonyl groups. Thus the formation of a separate wave of an imine at more positive potentials than that of the reduction of the carbonyl group, made it possible to determine equilibrium constants of additions of amines to aldehydes and ketones. The intermediate of such reactions is a carbinolamine, the presence of which was demonstrated and properties followed in the reactions of benzaldehyde and terephthalaldehyde with hydrazine. 4.5. Kinetics of Fast Reactions Finally, there are some equilibria which are established neither extremely rapidly nor extremely slowly. The rates of their establishments are comparable with the rate of the electrode process. In these cases, where the rate of the establishment of a chemical equilibrium, that takes place before the electron transfer, is comparable with the rate of the transfer of the electron, the rate of the re-establishment of the chemical equilibrium perturbed by electrolysis controls the limiting current. When the equilibrium is perturbed by removal of the electroactive component by reduction or oxidation, the re-establishment of such equilibrium takes place [12]. The rate constant of the establishment of the equilibria can be obtained, provided that the value of the equilibrium constant is known, or obtained by another technique, such as potentiometry or spectrophotometry. For such reactions taking place within the reaction layer in the vicinity of the dropping mercury electrode (a layer which is much narrower than the diffusion layer), it is possible to obtain rate constants of the order of 104 to 1010 L mol–1 s–1. In those cases, where the same reaction was followed by relaxation techniques, good agreement was found. The condition must be fulfilled that the reaction studied electrochemically takes place as a homogeneous process in the solution in the vicinity of the DME. The currents controlled by the rate of a chemical reaction are called kinetic and are observed particularly for compounds undergoing chemical reactions belonging to two large groups: some acidbase equilibria and some hydration-dehydration processes. Investigations of acid-base equilibria by Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 69 means of DC polarography are possible, because the reductions of conjugate acids invariably take place at potentials more positive than those of the conjugate base. On i-E curves of such reducible acids, obtained at varying pH-values, it is possible to observe waves of the acid form decreasing with increasing pH and the waves of the corresponding base increasing at more negative potentials with the total limiting current remaining constant [11,12]. When the wave of the acid form becomes smaller than about 25% of the diffusion limiting current, it becomes controlled purely by the rate of protonation of the base and a kinetic current result. The value of the pH, at which the reduction wave of the acid reaches 50% of the diffusion current, is denoted pK’, which is larger than pKa of the acid obtained for example by spectrophotometry. From values of pK’ and p- Ka it is possible to calculate the rate constant of the reaction: base + H+ = acid. In this way the rate constants of protonations of anions of some α-ketoacids or maleic and fumaric acids for example, have been determined. 4.6. Bulk Reactions Polarography can also be used as an analytical tool in the study of kinetics of reactions, where τ1/2 > 0.5 min, often as a complementary method to spectrophotometry. For reactions with half-lifes τ1/2 between about 0.5 min and 5.0 min it is possible to record the current continuously as a function of time at a constant potential, corresponding usually to the limiting current. For reactions with τ1/2 > 5 min the segments of the i-E curve, where changes of waves occur with time, are recorded after chosen time intervals. On such curves the limited currents at individual times are plotted as a function of time or as a function of a log t. Such approach is possible when dealing with simple zero, first or second order reactions [11,12]. For the treatment of higher order or more complex reactions solutions can be found in treatises on reaction kinetics. The rate constants obtained from these plots at varying compositions of the reaction mixture can be used in interpretation of the mechanism involved. 5. Conclusions Despite the pass of its glorious days in the 1950s and 1960s, polarography still enjoy variety of application in some aspects of chemistry with full relevance like in analysis of pharmaceuticals containing a single components, which is both physiologically and electrochemically active, yielding a reduction or oxidation wave and in determining an electroactive species in heterogeneous suspensions. Polarography is also a useful technique in physical organic chemistry, and can be used for investigation of equilibria and reaction kinetics. In some cases polarography can offer quantitative information about equilibrium and rate constants, where use of other techniques would be difficult or impossible. Copyright © 2013 by Modern Scientific Press Company, Florida, USA Int. J. Modern Anal. Sep. Sci. 2013, 2(2): 61-70 70 References [1] Lakshminarayanan, V. Polarography. Resonance 2004, 28: 51-61. [2] Adams, R. N. Electrochemistry at Solid Electrodes. Marcel Dekker Inc., NY, 1969. [3] Fry, A. J. Synthetic Organic Electrochemistry. Harper and Row, NY, 1972. [4] Heinze, J. Cyclic voltammetry-electrochemlcal spectroscopy. Angewandte 1984, 23: 831. [5] Grow, D. R.; Westwood, J. V. Polarography. Methuen Co. Ltd, London, 1968. [6] Grow, D. R. Principles and Applications of Electrochemistry, 4th ed. Blackie Academic & Professional, NY, 1994. [7] Reinmuth, W. H. Theory of stationary electrode polarography. Anal. Chem. 1961, 33: 1793-1794. [8] Nicholson, R. S.; Irving. S. Theory of stationary electrode polarography: Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems. Anal. Chem. 1964, 36: 706-723. [9] Skoog, D. A.; West, D. M.; Holler, J. F. Fundamentals of Analytical Chemistry, 7th ed. Harcourt Brace College Publishers, 1995. [10] Heyrovský, J.; Kuta, J. Osnovy Poliarografii (Translated from Czech), Moscow, 1965. [11] Kriukova, T. A.; Siniakova, S. I.; Arefeva. T. V. Poliarograficheskii Analiz. Moscow, 1959. [12] Zuman, P. What can DC polarography offer today. Acta Chim. Slov. 2009, 56: 18-25. Copyright © 2013 by Modern Scientific Press Company, Florida, USA