Dropping mercury polarograph

The second meeting was also held under the auspices of the American Chemical Society, in Toronto in June 1988.63 The subject was the general one of electrochemistry only a proportion dealt with historical aspects of instrumentation. One paper provided an overview of electrochemical instrumentation64 and others considered the specific topics of the pH meter,65 the glass electrode,66 and the dropping mercury polarograph of Jaroslav Heyrovsky.67... 

The basic apparatus for polarographic analysis is depicted in Fig. 16.1. The dropping mercury electrode is here shown as the cathode (its most common function) it is sometimes referred to as the working or micro-electrode. The... 

Polarographic maxima. Current-voltage curves obtained with the dropping mercury cathode frequently exhibit pronounced maxima, which are reproducible and which can be usually eliminated by the addition of certain appropriate maximum suppressors . These maxima vary in shape from sharp peaks to rounded humps, which gradually decrease to the normal diffusion-current curve as the applied voltage is increased. A typical example is shown in Fig. 16.3. Curve A is that for copper ions in 0.1 M potassium hydrogencitrate solution, and curve B is the same polarogram in the presence of 0.005 per cent acid fuchsine solution. 

The polarographic determination of metal ions such as Al3 + which are readily hydrolysed can present problems in aqueous solution, but these can often be overcome by the use of non-aqueous solvents. Typical non-aqueous solvents, with appropriate supporting electrolytes shown in parentheses, include acetic acid (CH3C02Na), acetonitrile (LiC104), dimethylformamide (tetrabutyl-ammonium perchlorate), methanol (KCN or KOH), and pyridine (tetraethyl-ammonium perchlorate), In these media a platinum micro-electrode is employed in place of the dropping mercury electrode. 

As a result of polarographic investigations using e.g. a dropping mercury electrode electrochemical rate constants at the half wave potential Ey2 are reported ... 

Convective diffusion to a growing sphere. In the polarographic method (see Section 5.5) a dropping mercury electrode is most often used. Transport to this electrode has the character of convective diffusion, which, however, does not proceed under steady-state conditions. Convection results from growth of the electrode, producing radial motion of the solution towards the electrode surface. It will be assumed that the thickness of the diffusion layer formed around the spherical surface is much smaller than the radius of the sphere (the drop is approximated as an ideal spherical surface). The spherical surface can then be replaced by a planar surface... 

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

Mohamed [63] investigated the complexation behavior of amodiaquine and primaquine with Cu(II) by a polarographic method. The reduction process at dropping mercury electrode in aqueous medium is reversible and diffusion controlled, giving well-defined peaks. The cathodic shift in the peak potential (Ep) with increasing ligand concentrations and the trend of the plot of EVl versus log Cx indicate complex formation, probably more than one complex species. The composition and stability constants of the simple complexes formed were determined. The logarithmic stability constants are log Bi = 3.56 log B2 = 3.38, and log B3 = 3.32 [Cu(II)-primaquine at 25 °C]. 

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... 

Bond et al. [791 ] studied strategies for trace metal determination in seawater by ASV using a computerised multi-time domain measurement method. A microcomputer-based system allowed the reliability of the determination of trace amounts of metals to be estimated. Peak height, width, and potential were measured as a function of time and concentration to construct the database. Measurements were made with a potentiostat polarographic analyser connected to the microcomputer and a hanging drop mercury electrode. The presence of surfactants, which presented a matrix problem, was detected via time domain dependent results and nonlinearity of the calibration. A decision to pretreat the samples could then be made. In the presence of surfactants, neither a direct calibration mode nor a linear standard addition method yielded precise data. Alternative ways to eliminate the interferences based either on theoretical considerations or destruction of the matrix needed to be considered. 

In classical polarographic techniques, a dropping mercury electrode is used. This is a complex device in which continuously produced small droplets of mercury are used as the active electrode in order to prevent poisoning of the electrode and to provide constant conditions throughout the analysis. For many applications, specifically designed electrodes are available which are simpler to use. 

The existence of various oxidation states of technetium indicates the possibility of using polarography for its quantitative determination. Polarographic reduction of the pertechnetate ion at a dropping mercury electrode has been studied in different supporting electrolytes . 

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 ... 

Figure 1.7. Current versus time profile for an electrochemical reaction under polarographic conditions at a dropping mercury electrode, drop time 3 s.

Shetty and Fernando investigated the polarographic behavior of Ni(ethyl-dtp)2 at the dropping mercury electrode in ethanol and ethanol-water media. The nickel ion was catalytically reduced in the presence of small quantities of ethyl-dtp at more positive potentials than in the absence of ethyl-dtp. In ethanol a single wave that is almost completely controlled by diffusion was obtained whereas in ethanol-water mixtures, in which the water content was less than 40% by volume, two waves were obtained. The first wave is the... 

Powdered chlorpromazine hydrochloride (50 mg) was dissolved in 50 mL of water, a portion of the solution treated with 0.5N KCl and 0.2% gelatin solution, and diluted. Nitrogen was passed through the solution before polarography was performed out at 20 C. The polarographic scan initiated at -1.2V, and used an internal calomel compression electrode and a dropping mercury electrode. The method was used for the determination of chlorpromazine in injectable solutions and tablets [153]. 

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. 

Many polarographic studies of the reduction of cobalt(II) to form an amalgam at a dropping mercury electrode have been reported, but most of the work has focused on systems involving complexes with ligands other than water [1, 2]. In one of the few investigations of the behavior of Co(H20)6 + (in aqueous 0.1 M potassium nitrate) [3], the following information was deduced ... 

In a classic study, Hume and KolthofF[13] obtained polarographic evidence that, in a 1 M aqueous solution of potassium cyanide, Co(H20)(CN)s is irreversibly reduced at a dropping mercury electrode to a cobalt(I) species, the composition of which was not elucidated. furthermore, the cobalt(I) complex was reported to undergo neither oxidation nor reduction. In addition, the cobalt(III) complex, Co(H20)(CN)5 , was seen to be reducible at the dropping mercury electrode, whereas Co(CN)6 is not electroactive. In earlier work [14], cobalt(II) cyanide complexes were reduced electrolytically to cobalt(I) cyanide species. 

Polarographic reductions of (21) are easier than those of ketones (138) and (139). The size of the bridging ring influences both the absorption of the ketone on the dropping mercury electrode (139 > 138) and the reduction potential (70MI52200). 

In a polarographic experiment, a potential difference E is applied across the cell consisting of the dropping-mercury electrode and a nonpolarizable interface (e.g., a calomel electrode). In response to this potential difference, a current density i flows across the drop/solution interface. As each drop grows and falls, however, the surface area of the drop also grows, and then becomes effectively zero when the drop falls. Thus, the instantaneous current (current density times surface area) shows fluctuations, but the mean current is a unique function of the potential difference across the drop/solution interface, and therefore of that across the cell. 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

Fig. 5.8 Schematic diagram of polarographic (or voltammetric) circuits for two-electrode (a) and three-electrode (b) systems. WE(DME) indicator or working electrode (dropping mercury electrode in the case of polarography) RE reference electrode CE counter electrode DC voltage (V) DC voltage source Current (/) current measuring device.

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