Polarography

Polarography is an electrochemical technique which makes use of current-voltage curves under conditions of concentration polarization of an indicator 

S being the surface area of the electrode and /um the limiting diffusion-controlled current. 

It is not easy to exploit the above equation directly as a basis for quantitative analysis, despite the simple proportionality between current and concentration. This is because reproducibility - a prime requirement in analysis is obtainable only with great difficulty with solid electrodes due to contamination by the products of electrolysis. 

Polarography is based on the use of a dropping mercury electrode which consists of mercury in the form of small droplets issuing from the end of a fine-bore capillary. Despite the obvious slight practical complication of working with a dynamic rather than a static electrode, mercury in this form shows a number of distinct advantages over almost all other forms  

The above advantages largely outweigh a few minor disadvantages, the most important of which are  

Classical DC polarography uses a linear potential ramp (i.e., a linearly increasing voltage). It is, in fact, one subdivision of a broader class of electrochemical methods called voltammetry. Voltanunetric methods measure current as a function of applied potential where the WE is polarized. This polarization is usually accomplished by using microelectrodes as WEs electrode 

Another example of an amperometric method is the method of polarography, which was widely used for analytical purposes some decades ago and which is described in detail in the following section. 

Charge for formation of an EDL corresponding to the set potential E must constantly be supplied to the DME because of the continuous renewal and growth of the surface. The charge, ( di present on each side of the EDL depends on the potential 

The problem of convective diffusion toward the growing drop was solved in 1934 by Dionyz Ilkovic under certain simplifying assumptions. For reversible reactions (in the absence of activation polarization), the averaged cnrrent at the DME can be represented as 

The first version of a polarographic technique was put forward in 1922 by the Czech scientist Jaroslav Heyrovsky. Classsical polarography is the measurement of quasisteady-state polarization curves with linear potential scans applied to the DME sufficiently slowly (v between 1 and 20mV/s), so that within the lifetime, of an individual drop, the potential would not change by more than 3 to 5 mV. With special instruments (polarographs), one can record the resulting 7 vs. E curves (polaro-grams) automatically. 

In the classical version one uses a two-electrode cell with DME and a mercury AE (the pool) at the bottom of the cell (see Fig. 23.2). The latter, which has a large surface area, is practically not polarized. The current at the DME is low and causes no marked ohmic potential drop in the solution and no marked polarization of the AE. Hence, to change the DME potential, it will suffice to vary the external voltage applied to the cell. During the measurements, 7 vs. % rather than 7 vs. E curves are recorded. 

Electrochemical measurement of surfactant concentration by polarography and ion-selective electrodes is the subject of a recent review (1). While direct measurement does not promise to have practical application except perhaps for HPLC detection, it is briefly described here for the sake of completeness. 

Surfactants are commonly added to polarographic cells to suppress the overpotential associated with dissolved oxygen or other species. This phenomenon is periodically proposed as a general quantitative method for determination of surfactants in environmental samples (2-4). The approach is not useful for determining which surfactant is present. In natural waters, naturally occurring materials like humic acids are the predominant surface active materials detected by polarographic methods (5). 

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Ethoxylated nonylphenols (24) and ethoxylated alcohols (25) can be determined with some selectivity by alternating current polarography, measuring the amount of surfactant adsorbed at the mercury surface by the effect on the double-layer capacitance. 

Because of the lack of specificity, polarographic techniques are only useful for analysis of real world samples if they are coupled with separation procedures. For example, one team has demonstrated that the BIAS procedure for trace analysis of nonionics can be improved by using an electrochemical procedure for surfactant determination after first precipitating and isolating the potassium iodobismuthate-nonionic complex from the sample (18,26). They prevent interference of hydrocarbons by washing the precipitate with isooctane (27). 

This relationship holds for any electrochemical process that involves semiinfinite linear diffusion and is the basis for a variety of electrochemical methods (e.g., polarography, voltammetry, and controlled-potential electrolysis). Equation (3.6) is the basic relationship used for solid-electrode voltammetry with a preset initial potential on a plateau region of the current-voltage curve. Its application requires that the electrode configuration be such that semiinfinite linear diffusion is the controlling condition for the mass-transfer process. 

When this equation is solved for r and the latter is substituted into the equation for the area of a sphere, an expression for the area of the dropping-mercury electrode drop as a function of the experimental parameters is obtained 

This then can be substituted into Eq. (3.6) to give a calculated diffusion current for the dropping-mercury electrode  

Actually, experimental results indicate that the constant in Eq. (3.9) is too small by a factor of V7/3. We now realize that this V7/3 quantity is not empirical but is the appropriate contribution due to the growth of the mercury drop into the solution away from the capillary orifice. Thus, the correct diffusion current expression for a dropping-mercury electrode is 

It is noteworthy that much of the polarographic literature is tabulated on the basis of Eq. (3.12). In spite of this almost all modem recording polarographs use potentiometric stripchart recorders, which have time constants such that die maximum of the polarographic oscillations is the instantaneous current. Furthermore, the mode of response of a potentiometric recorder is such that 

Plesch and collaborators have recently introduced this technique for the study of carbenium and oxonium ions and of Br nsted acid dissociation equilibria ° The ultimate aim of this work is of course to gain a better understanding of the mechanism of initiation in cationic polymerisation. For the moment however it is difficult to assess the real potential of polarography in that context and we feel that, althou it certainly possesses a hi sensitivity, the conditions required for meaningful measurements are still too distant from those employed in an actual polymerisation, in particular the high ionic strength of the medium. Its use is therefore still doubtful and one can only hope that more research will bridge the gap. 

The molecular-wei t distribution of a polymer can sometimes offer interesting indications of the mechanisms by which its chains have been generated and built up. Of the various methods available for determining the DP distribution, gel permeation 

To derive the expression for the current response, one must account for the variation of the drop area with time  

FIGURE 3-2 Polarograms for 1 M hydrochloric acid (Curve A) and 4 x 10 4 M Cd+2 in 1 M hydrochloric acid B. id represents the limiting current is the half-wave potential. 

To determine die diffusion current, it is necessary to subtract the residual current. This can be achieved by extrapolating the residual current prior to the wave or by recording die response of the deaerated supporting electrolyte (blank) solution. Addition of a standard or a calibration curve are often used for quantitation. Polarograms to be compared for this purpose must be recorded in the same way. 

The potential at which the current is one-half of its limiting value is called the half-wave potential, El/1. The half-wave potential (for electrochemically reversible couples) is related to the formal potential, E°, of the electroactive species according to 

TABLE 3-1 Functional Groups Reducible at the Dropping Mercury Electrode 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Some analytical applications of chronoamperometry (e.g., in vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed time intervals. Some popular test strips for blood glucose (discussed in Chapter 6) involve potential-step measurements of an enzymatically liberated product (in connection with a preceding incubation reaction). Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first one). 

The potential-step experiment can also be used to record the charge-time dependence. This is accomplished by integrating the current resulting from the potential step and adding corrections for the charge due to the double-layer charging (0) and reaction of the adsorbed species (Qi)  

Such a charge measurement procedure, known as chronocoulometry, is particularly useful for measuring the quantity of adsorbed reactants (because of the ability to separate the charges produced by the adsorbed and solution species). A plot of the charge (0 versus /12, known as an Anson plot, yields an intercept at t = 0 that corresponds to the sum of 0 and Qt (Fig. 3.2). The former can be estimated by subtracting the intercept obtained in an identical experiment carried out in the blank solution. 

Polarography is a subclass of voltammetry in which the working electrode is dropping mercury. Because of the special properties of this electrode, particularly its renewable surface and wide cathodic potential range (see Chapters 3-5 for details), polarography has been widely used for the determination of 

The electron transfer is a main stage in the chemical reaction mechanism, photosynthesis, catalysis, transfer energy, etc. 

By increasing the medium pH the first half-wave potential (E 1/2) value of all investigated compounds is displaced in the negative region (Table 3.41). Besides, for TV-substituted nitroazoles a considerably greater displacement is observed, as in alkaline media these compounds form anions. This allows the determination in alkaline media the presence of both TV-substituted and TV-unsubstituted forms of nitroazoles. The authors also utilized the E1/2/pH dependence for identification of the earlier-stated nitroazole forms (Table 3.41) [903, 904, 909, 910], 

As seen from Table 3.41, the half-wave potentials of 3(5)-nitropyrazole and l-methyl-3-nitropyrazole are practically identical at all pH values. In the authors opinion, this may be due to the fact that 3(5)-nitropyrazole contains, mainly, 3-nitro tautomer [904], A similar regularity is observed for 4(5)-nitroimidazole and 1-methyl-4-nitroimidazole [903], The E1/2 values of l-methyl-3-nitropyrazole lie in more a negative region than those of l-methyl-5-nitropyrazole. Probably it is related to the fact that nitro group in 1-methyl-3-nitropyrazole is located near electron-donative pyridine nitrogen atom (N-2). This is also the case for imidazoles l-methyl-4-nitro- and 1 -methyl-5-nitroimidazole. 

S atranidazole (1 -methylsulfonyl-3 -(1 -methyl-5-nitro-2-imidazolyl)-2-imidazo-lidinone) is reduced more easily at pH 7 and is more sensitive to pH than other nitroa-zoles (Table 3.42). Based on the comparison of the reduction potentials of satranidazole (at pH 7) with the potentials of 5-nitro- and 2-nitroimidazole derivatives and on the 

In polymerization reactions by methyl methacrylate [915] a relationship between the reduction potentials of l-methyl-3-nitropyrazole, l-methyl-5-nitropyrazole, and l,2-dimethyl-5-nitropyrazole and their inhibited properties has been established. 

Polarographic analysis has been used mainly in the field of combustion for the analysis of peroxides and aldehydes. The technique has been developed for this purpose by Minkolf et The limiting diffusion currents are proportional to 

The polarographic reduction of TcOj at a dropping mercury electrode was studied in several supporting electrolytes. 

Pertechnetate in 4 M IICI was found to undergo reduction to Tc(lV). A double wave was observed corresponding to a one- and a two-electron transfer. The corre- 

For direct current polarographic determination of TcOj in the concentration range 0.1-1.1 ppm in fission product solutions, a phosphate buffer of pH 7 was recommended. 12 of the wave used was -0.68 V V4- SCE. Neither rhenium nor ruthenium nor other fission products interfered. However, [AsPh4]Cl, present in certain fission product solutions, must be separated out [97], A rapid method was developed for the determination of Tc in fission product mixtures. It consists of a selective reduction of Tc04 at a dropping mercury electrode at -1.55 V v.f SCE in a medium of I M 

ij will have units of amperes (A) when Z) is in cm s, 7wisings, /isin seconds and C is in mol cm. Tills expression represents the current at the end of 

Each fresh drop exposes a new Hg surface to the solution. The resulting behavior is more reproducible than that with a solid surface, because the liquid drop surface does not become contaminated in the way solid electrodes can be contaminated. Organic contaminants or adsorbants must undergo reequilibration with each new drop and are less likely to interfere. 

As mentioned earlier, there is a high overpotential for H ion reduction at mercury. This means that it is possible to analyze many of the metal ions whose standard reduction potentials are more negative than that of the H2/H ion couple. It is easier, too, to reduce most metals to their mercury amalgam than to a solid deposit. Conversely, however, mercury is easily oxidizable, which severely restricts the use of the DME for the study of oxidation processes. 

Solid electrodes have surface irregularities because of their crystalline nature. Liquid mercury provides a smooth, reproducible surface that does not depend on any pretreatment (polishing or etching) or on substrate inhomogeneity (epitaxy, grain boundaries, imperfections, etc). 

In electroanalysis, diffusion currents are quite small ( 100 /rA), which means that the aqueous solution IR drop between the reference electrode and the DME can be neglected in all but the most accurate work. Electrolytes prepared with organic solvents, however, may have fairly large resistances, and in some instances IR corrections must be made. 

As each new drop commences to grow and expand in radius, the resulting current is influenced by two important factors. The first is the depletion by electrolysis of the electroactive substance at the mercury drop surface. This gives rise to a diffusion layer in which the concentration of the reactant at the surface is reduced. As one travels radially outward from the drop surface, the concentration increases and reaches that of the bulk homogeneous concentration. The second factor is the outward growth of the drop itself, which tends to counteract the formation of a diffusion layer. The net current waveform for a single drop is illustrated in Fig. 15.27. 

In polarographic analysis, current-voltage curves are recorded as they are formed on a polarizable micro-electrode when the diffusion of the ions in 

A dropping Hg-electrode usually serves as polarizable micro-electrode. Either an Hg bottom electrode, a calomel electrode or another electrode of the second type is used as non-polarizable reference electrode. 

A variable voltage is applied to both electrodes in the solution to be 

If ions ( depolarizers ) which can be oxidized or reduced in a specific voltage interval are present in the solution to be analyzed, the current also changes with changes in voltage. The change in current is dependent on 

The curve is divided into three sections. In field A, the voltage applied is not sufficient to reduce the depolarizer. The low current nevertheless flowing is referred to as residual or basic current. The charging of the Hermholtz double layer at the mercury/solution interface is above all responsible for the generation of this current. This layer acts as a condenser with constantly increasing capacity. 

Analyte solution Dropping-mercury working electrode 

Polarography was invented in 1922 by Jaroslav Heyrovsky. who received the Nobel Prize in 1959. 

Boron-doped diamond has one of the widest available potential ranges and is chemically inert. [From. Cva ka et al., Anai Chem. 2003, 75, 2678. Courtesy G. M. Swain, Michigan State University.) 

Single atomic layers of graphite, called graphene, make electrodes with a working range similar to that of boron-doped diamond. 

i2 is characteristic of a particular analyte in a particular medium. Analytes can be distinguished from one another by their half-wave potentials. 

Instrumentation for this technique is discussed in Chapter 1 and Appendix 1. 

Residual amounts of styrene and acrylonitrile monomers usually remain in manufactured batches of styrene-acrylonitrile copolymers and acrylonitrile-butadiene-styrene terpolymers (ABS), As these copolymers have a potential use in the food packaging field, it is necessary to ensure that the content of both of these monomers in the finished copolymers is below a stipulated level. In a polarographic procedure [9, 10] for determining acrylonitrile (down to 2 ppm) and styrene (down to 20 ppm) monomers in styrene-acrylonitrile copolymer, the sample is dissolved in 0.2 M tetramethylammonium iodide in dimethyl formamide base electrolyte and polarographed at start potentials of -1.7 V and -2.0 V, respectively, for the two monomers. Excellent results are obtained by this procedure. Table 5.3 shows the results obtained for determinations of acrylonitrile monomer in some copolymers by the polarographic procedure. 

Betso and McLean [11] have described a differential pulse polarographic method for the determination of acrylamide and acrylic acid in polyacrylamide. A measurement of the acrylamide electrochemical reduction peak current is used to quantify the acrylamide concentration. The differential pulse polarographic technique also yields a well-defined acrylamide reduction peak at 2.0 V versus SCA (reduction potential), suitable for qualitatively detecting the presence of acrylamide. The procedure involves extraction of the acrylamide monomer from the polyacrylamide, treatment of the extracted solution on mixed resin to remove interfering cationic and anionic species, and polarographic reduction in an 80/20 v/v) methanol/water solvent with tetra-n-butylammonium hydroxide as the supporting electrolyte. The detection limit of acrylamide monomer by this technique is less than 1 ppm. 

Reprinted from T.R. Crompton and D. Buckley, Analyst, 1965, 90, 76, with permission from the Royal Society of Chemistry [10]  

In the great majority of cases polarographic techniques are of little 

Sara has used polarography to determine tin in a dichromic acid medium, 

Solutions of stannous (+2) tin are common in any inorganic analytical laboratory. Stannous chloride, for exaniple, is a very effective reducing agent for a large number of species. Stannous tin may be prepared quite easily, and the reduction of Sn to Sn with iron or nickel metal is a usual preliminary step in the most common analytical method for quantitative determination of tin volumetric titration with a standard iodine solution. 

Unfortunately, stannous tin solutions are quite unstable and, if left in 

Since the Sn — Sn potential is only—0.15 v. tin is easily oxidized to its highest oxidation state and most radiochemical tin procedures deal almost-exclusively with stannic tin. With a single oxidation state predoininating, one would expect the chemistry of tin to be relatively simple. Tin salts may be fairly easily volatilized, however, and stannic tin in aqueous solution has a notable tendency to hydrolyze at the slightest provocation. This behavior, plus the fact that there are very few chemical separation steps which are at all selective for tin, makes tin one of the more difficult elements to obtain radiochemically pure. A more detailed description of the chemical behavior of stannic tin will be presented in subsequent sections. 

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Ilkovic equation The relation between diffusion current, ij, and the concentration c in polarography which in its simplest form is... 

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... 

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 hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

Potential-excitation signals and voltammograms for (a) normal pulse polarography, (b) differential pulse polarography, (c) staircase polarography, and (d) square-wave polarography. See text for an explanation of the symbols. Current is sampled at the time intervals indicated by the solid circles ( ). 

When either pulse polarography or anodic stripping voltammetry can be used, the selection is often based on the analyte s expected concentration and the desired... 

The concentration of As(III) in water can be determined by differential pulse polarography in 1 M HCl. The initial potential is set to -0.1 V versus the SCE, and is scanned toward more negative potentials at a rate of 5 mV/s. Reduction of As(III) to As(0) occurs at a potential of approximately —0.44 V versus the SCE. The peak currents, corrected for the residual current, for a set of standard solutions are shown in the following table. 

Peak currents in differential pulse polarography are a linear function of the concentration of analyte thus... 

Differential pulse polarography and stripping voltammetry have been applied to the analysis of trace metals in airborne particulates, incinerator fly ash, rocks. 

Miscellaneous Samples Besides environmental and clinical samples, differential pulse polarography and stripping voltammetry have been used for the analysis of trace metals in other samples, including food, steels and other alloys, gasoline, gunpowder residues, and pharmaceuticals. Voltammetry is also an important tool for... 

Garda-Armada, P. Losada, J. de Vicente-Perez, S. Cation Analysis Scheme by Differential Pulse Polarography, /. 

The amount of sulfur in aromatic monomers can be determined by differential pulse polarography. Standard solutions are prepared for analysis by dissolving 1.000 mb of the purified monomer in 25.00 mb of an electrolytic solvent, adding a known amount of S, deaerating, and measuring the peak current. The following results were obtained for a set of calibration standards... 

Zinc can be used as an internal standard in the analysis of thallium by differential pulse polarography. A standard... 

Differential pulse polarography is used to determine the concentrations of lead, thallium, and indium in a mixture. ... 

The following data were collected for the reduction of Pb + by normal pulse polarography... 

The following sources provide additional information on polarography and pulse polarography. 

Analysis of Trace or Minor Components. Minor or trace components may have a significant impact on quaHty of fats and oils (94). Metals, for example, can cataly2e the oxidative degradation of unsaturated oils which results in off-flavors, odors, and polymeri2ation. A large number of techniques such as wet chemical analysis, atomic absorption, atomic emission, and polarography are available for analysis of metals. Heavy metals, iron, copper, nickel, and chromium are elements that have received the most attention. Phosphoms may also be detectable and is a measure of phosphoHpids and phosphoms-containing acids or salts. 

A review pubHshed ia 1984 (79) discusses some of the methods employed for the determination of phenytoia ia biological fluids, including thermal methods, spectrophotometry, luminescence techniques, polarography, immunoassay, and chromatographic methods. More recent and sophisticated approaches iaclude positive and negative ion mass spectrometry (80), combiaed gas chromatography—mass spectrometry (81), and ftir immunoassay (82). 

Finally, the techniques of nmr, infrared spectroscopy, and thin-layer chromatography also can be used to assay maleic anhydride (172). The individual anhydrides may be analyzed by gas chromatography (173,174). The isomeric acids can be determined by polarography (175), thermal analysis (176), paper and thin-layer chromatographies (177), and nonaqueous titrations with an alkaU (178). Maleic and fumaric acids may be separated by both gel filtration (179) and ion-exchange techniques (180). 

Formylbenzoic acid and -toluic acid can be deterrnined by high performance hquid chromatography. In some cases, polarography is used for 3-formylbenzoic acid and esterification gas chromatography for the y -toluic acid content. 

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

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