Polarograph stationary electrode

The phrase pulse voltammetry encompasses a sizable suite of methods whose practice has changed substantially since the first edition appeared. The methods originated in a classical polarographic context and were based on the desire to suppress the charging current arising from continuous expansion of the mercury drop at the DME. Since 1980, practice has departed from the DME, because the SMDE has become the dominant electrode for practical polarographic work and because the use of these methods at stationary electrodes has become more common (35). 

Square wave voltammetry is normally carried out at a stationary electrode such as an HMDE, and involves the waveform and measurement scheme shown in Figure 7.3.13. As in other forms of pulse voltammetry, the electrode is taken through a series of measurement cycles however there is no renewal of the diffusion layer between cycles. In contrast to NPV, RPV, and DPV, square wave voltammetry has no true polarographic mode. The waveform can be viewed as a special case of that used for DPV (Figure 7.3.9), in which the preelectrolysis period and the pulse are of equal duration, and the pulse is opposite from the scan direction. However, the interpretation of results is facilitated by considering the waveform as consisting of a staircase scan, each tread of which is superimposed by a symmetrical double pulse, one in the forward direction and one in the... 

The voltammetric method, developed at ORNL for determination the [U(1V)]/[U(111)] ratio in the MSBR fuel salt [47], is based on measuring of the difference between the redox potential of the melt Feq and 1/2, the voltammetric equivalent of the standard redox potential of the U(1V)AJ(111) couple, at [U(IV)] S> [U(Ill)]. In conditions of linear voltammetry, at a stationary electrode and a reversible charge transfer of the melt-soluble oxidized and reduced forms of uranium, is approximately equal to the polarographic half-wave potential 1/2 and corresponds to the potential in the voltammogram, at which the current accounts for 85.2% of the peak current. The relationship between eq, 1/2 and [U(lV)]/[U(in)] ratio is given by the Nernst equation ... 

Molten carbamide is known to be a good solvent for the salts of many metals [1] and was used as a supporting electrolyte in polarographic investigations both at a dropping mercury electrode and at a stationary... 

Anionic and Cationic Carbonyls. The polarographic behaviour of Et N-[Fe(CO)3NO] at dropping mercury and stationary platinum electrodes has been studied. Two anodic waves and one cathodic wave were observed and the following reactions were suggested ... 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

Ebel et al. have used a microliter vessel in the voltammetry and polarographic determination of small sample volumes of chlorpromazine [166]. The concentration of cells in glass or PTFE was described for use with a dropping-mercury electrode (sample volume 180 pL), a rotating disc electrode (sample volume 1 mL), or a stationary vitreous-carbon electrode (sample volume 80 pL). Chlorpromazine was determined using oxidative voltammetry at a 3 mm vitreous-carbon or a rotating electrode. 

Eletrochemical detection has been used for the detection of synthetic dyes. Fogg et al. (226) described a method for the qualitative and quantitative determination of several synthetic dyes using polarographic detection. The system was a stationary mercury drop electrode operated in the differential-pulse mode. Ashkenazi et al. (131) used fast-scan square-wave voltammetry for the polarographic detection of five synthetic dyes. The voltametric mode was observed to be much faster than the differential-pulse method. Another advantage is that the experimental measurement produces, in addition to the peak current, the redox potential of the dye, which can serve to identify the analyte further. 

Polarographic maximum — A peak-shaped current signal that is caused by enhanced streaming of the solution, or mercury and solution, in measurements with a - dropping mercury electrode (DME) and sometimes also on - stationary mercury drop electrodes (SMDE). Two different types are to be distinguished (a) Polaro-... 

In polarography, not enough time is available for the diffusion layer to reach its stationary thickness. Instead, the current per unit electrode area decreases with the square root of time, the signature time-dependence for diffusion. On the other hand, the area of the growing drop expands, proportional to the two-thirds power of drop age r (i.e., time elapsed since the previous mercury drop fell off). These two counteracting effects, diffusion currents per area proportional to r 1/2, and area growth as t2/3, combine to yield polarographic current-time curves with a time dependence of t-1/2 X t2/3 = t1/6, as expressed in the Ilkovid equation. 

Macchi (35) used a cathode-ray polarograph with a slow dropping mercury electrode so that it was, in effect, stationary during a 60-second plating time. He was able to determine zinc, in sea water directly with a sensitivity of 1 /xg./liter. 

Comparison of polarographic curves at the dme with the current-potential curves obtained at a stirred mercury pool is Important with regard to the identification of products of electrolysis. In most cases, the number, the shape, and the half-wave potentials of the waves are Identical, e.g., as in the behavior of p-diacetylbenzene referred to by Zuman. However, cases have been reported in which differences are observed and in some there is no resemblance at all between the results obtained at a dropping mercury electrode and at a stationary mercury pool. This may arise because extents of adsorption of the reactant are different, since studies at the dme are usually conducted with solutions of low concentration, 10 -10 M, while preparative experiments at a pool electrode are conducted at much higher concentrations. 

There are several modifications of this technique. In ac polarography, which employs a dropping mercury working electrode, 4 is changed step-wise (one step per drop lifetime) and the diffusion layer is completely renewed after every drop fall. In linear sweep ac voltammetry, the working electrode is stationary, and dc is a linear function of time. However, when the sweep rate is slow, the polarographic and voltammetric responses are quite similar, and we will neglect the difference between those two modifications. For more details, one should consult Chapter 10 in reference (1) and the review articles cited therein. 

McBride and Evans [10] developed a rapid voltametric method for the estimation of antioxidants and tocopherols in oils and fats. The sample solutions were prepared by dissolving the oil or lard sample in an appropriate solvent, e.g., in most cases 0.12 M sulfuric acid in ethanol - benzene (2 1). The solutions were analysed with use of a linearly varying potential and a stationary, planar vitreous-carbon electrode, with a standard calomel electrode and a platinum-wire counter-electrode. Separate peaks were obtained for a-, y- and 8-tocopherol the peak for the P-isomer, was superimposed on that for the y-tocopherol. BHA (>10 ppm) can be determined in vegetable oil under the same conditions, provided that 8-tocopherol is absent. Kohler and co-workers [11] described a polarographic method for the determination of 4-4 thiobis(BHT) in food. The antioxidant was first nitrated, preferably with fuming nitric acid - concentrated sulfuric acid (1 1) at 20 °C for 1 hour. The polarography was carried out on the resulting solution after dilution and addition of urea and sodium acetate buffer solution. The Ey for the nitrated compound was -0.54 V versus the... 

---