Analytical pulse polarography

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... 

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

In conclusion, synthetic dyes can be determined in solid foods and in nonalcoholic beverages and from their concentrated formulas by spectrometric methods or by several separation techniques such as TEC, HPLC, HPLC coupled with diode array or UV-Vis spectrometry, MECK, MEECK, voltammetry, and CE. ° Many analytical approaches have been used for simultaneous determinations of synthetic food additives thin layer chromatography, " " derivative spectrophotometry, adsorptive voltammetry, differential pulse polarography, and flow-through sensors for the specific determination of Sunset Yellow and its Sudan 1 subsidiary in food, " but they are generally suitable only for analyzing few-component mixtures. 

Classical polarography is not optimised for analytical sensitivity. This inefficiency has been remedied in pulse polarography. Instead of applying a steadily increasing voltage on the mercury droplet, in pulse... 

The following analytical techniques seem to be adequate for the concentrations under consideration copper and nickel by Freon extraction and FAA cold vapour atomic absorption spectrometry, cobalt by Chelex extraction and differential pulse polarography, mercury by cold vapour atomic absorption absorptiometry, lead by isotope dilution plus clean room manipulation and mass spectrometry. These techniques may be used to detect changes in the above elements for storage tests Cu at 8 nmol/kg, Ni at 5 nmol/kg, Co at 0.5 nmol/kg, Hg at 0.1 nmol/kg, and Pb at 0.7 nmol/kg. 

Instead of applying a steadily increasing voltage on the mercury droplet, the voltage can be pulsed. The advantage of this approach is increased sensitivity and better distinction between analytes with close half-wave potentials (i.e. which differ by only a few tens of mV). Pulse polarography has two major modes of operation, which are described in the following sections. 

Although normal pulse polarography was developed mainly for analytical purposes, it is a valuable and simple method to study kinetics of not-too-fast electrode reactions. As the other controlled potential techniques, it has the advantage of being applicable to systems where only one of the redox components is present initially. The technique is closely related to d.c. polarography [11] and the expressions discussed in this section are directly applicable to the case of d.c. polarography performed with the static mercury drop electrode (SMDE) if the correction for the spherical shape of this electrode is negligible [21, 22]. 

Steady-state and pseudo-steady state techniques (d.c. polarography, pulse and differential pulse polarography, a.c. polarography) are especially suitable for analytical purposes, i.e. the determination of the composition of a sample and of concentrations of single species in such a sample. It is less commonly recognized that these techniques are also indispensable for the determination of less interesting properties of the components... 

Generally, sensitivity in the analytical sense is greater if the technique employed is faster, i.e. the electrolysis time is shorter, or the frequency of a periodic electrolysis is higher. Resolution of half-wave potentials, and thus accuracy of standard potentials and stability constants, is better if a derivative technique such as differential pulse polarography, a.c. polar-ography, and, preferably, the second derivative technique second-harmonic a.c. polarography, is employed. 

The Dimensionless Parameter is a mathematical method to solve linear differential equations. It has been used in Electrochemistry in the resolution of Fick s second law differential equation. This method is based on the use of functional series in dimensionless variables—which are related both to the form of the differential equation and to its boundary conditions—to transform a partial differential equation into a series of total differential equations in terms of only one independent dimensionless variable. This method was extensively used by Koutecky and later by other authors [1-9], and has proven to be the most powerful to obtain explicit analytical solutions. In this appendix, this method will be applied to the study of a charge transfer reaction at spherical electrodes when the diffusion coefficients of both species are not equal. In this situation, the use of this procedure will lead us to a series of homogeneous total differential equations depending on the variable, v given in Eq. (A.l). In other more complex cases, this method leads to nonhomogeneous total differential equations (for example, the case of a reversible process in Normal Pulse Polarography at the DME or the solutions of several electrochemical processes in double pulse techniques). In these last situations, explicit analytical solutions have also been obtained, although they will not be treated here for the sake of simplicity. 

A differential pulse CSV method for the determination of traces of butyltin species in water was compared with two other voltammetric methods, namely differential pulse polarography and ASV (Schwartz et at., 1995). The butyltin species were accumulated on the mercury drop electrode as their tropolone complexes. Detection limits were 5 mg 1 1 for tributyltin (TBT), 0.5 mgl-1 for dibutyltin (DBT) and 0.5 mgl-1 for monobutyltin (MBT). These detection limits were better than the corresponding values obtained in the other analytical methods. 

Pulse polarography has enjoyed increasing popularity because of the availability of commercial instruments. Furthermore, there is general agreement that the differential form of the method provides the greatest analytical sensitivity in the electrochemical field. Review articles provide more detail of the method and its applications.12 13,17-20... 

In a comparison of chemical analytical methods to bioassays, results obtained using methods utilizing derivatives (DNP and 7 - hydroxyquinoline) combined with colorimetric or fluorimetric detection were not specific for acrolein and consistently did not correlate with those obtained from bioassays. Certain direct methods of detection (nuclear magnetic resonance (NMR), fluorescence, and differential pulse polarography) gave the best correlation to the bioassay results (see Table 6-1). 

Polarography (discovered by Jaroslav Heyrovsky in 1922) is a technique in which the potential between a dropping mercury electrode and a reference electrode is slowly increased at a rate of about 50 200 mV min while the resultant current (carried through an auxihary electrode) is monitored the reduction of metal ions at the mercury cathode gives a diffusion current proportional to the concentration of the metal ions. The method is especially valuable for the determination of transition metals such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, W, V, and Pt, and less than 1 cm of analyte solution may be used. The detection hmit is usually about 5 X 10 M, but with certain modifications in the basic technique, such as pulse polarography, differential pulse polarography, and square-wave voltammetry, lower limits down to 10 M can be achieved. 

Henry, F.T., Kirch, T.O. and Thorpe, T.M., Speciation of trace level arsenic (III) and arsenic (V) by differential pulse polarography. Abstract No. 180, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1978. 

One should not be misled to think that polarography for analytical purposes is always conducted at long times. Sensitivity can be enhanced by using pulse polarography, with measuring times of the order of milliseconds. As long as interference with other reactions does not occur, this will be the better way to do the experiment. 

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