Dropping mercury electrode differential pulse voltammetry

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

Batley [28] examined the techniques available for the in situ electrodeposition of lead and cadmium in estuary water. These included anodic stripping voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen and in situ electrodeposition on mercury coated graphite tubes. Batley [28] found that in situ deposition of lead and cadmium on a mercury coated tube was the more versatile technique. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals were then determined by electrothermal atomic absorption spectrometry, Hasle and Abdullah [29] used differential pulse anodic stripping voltammetry in speciation studies on dissolved copper, lead, and cadmium in coastal sea water. 

The electrochemical behavior of the components of a commercial plant growth stimulator (Sviton) was studied. This included determination of o-nitrophenol, p-nitrophenol, 2-methoxy-5-nitrophenol and 2,4-dinitrophenol by differential pulse voltammetry at a hanging mercury drop electrode. The optimum conditions were established for their quantitation over the 1 x 10 7 to 1 x 10-5 M range516. 

The adsorption behavior of the psychotropic drug flunitrazepam (256) at the hanging mercury drop electrode was studied by staircase voltammetry and by adsorptive stripping differential pulse voltammetry. 256 can be determined down to nanomolar levels by using adsorptive preconcentration prior to the differential pulse voltammetry scan. The method was applied to determination of 256 in human urine530. 

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. 

NPV), differential pulse voltammetry (DPV) and SWV, and polarography methods (use of a mercury drop electrode) like normal (normal and pulse polarography (NPP)) and DPP, although sometimes these polarographic versus voltammetric terms are used interchangeably. These step methods do not typically use return scans and therefore often do not provide information about reversibility of the redox process and can sometimes give data that, unknown to the researcher, are characteristic of decomposition products. 

Differential pulsed voltammetry (DPV) is a technique in which potential pulses of fixed but small amplitudes are superimposed periodically on a linear voltage ramp. The most commonly used working electrode is the SMDE and one pulse is applied for each drop. The mV pulse is applied near the end of the life of the mercury drop. Current is measured once before the pulse and after the pulse. The difference between the currents is plotted against potential (Figure 5.8). The resultant peak-shaped current-voltage signal, which... 

Lead azide, Pb(N3)2, can be analysed by voltammetry, oxidizing azide ions at a carbon paste electrode and reducing lead ions at a dropping mercury electrode at pH 4.6 in an acetate buffer solution. Lead azide is poorly soluble in a pure aqueous solution, but can be dissolved when acetate ions are present. In Fig.l7 a differential pulse voltammogram for the determination of azide at a carbon paste electrode is shown. Some results from the analysis of lead azide are given in Table 7. 

Reichart, E. and Obendorf, D. (1998) Determination of naringin in grapefhiit juice by cathodic stripping differential pulse voltammetry at the hanging mercury drop electrode. Ana/. Chim. Acta, 360,179-187. 

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