Showing papers in "Review of Polarography in 1968"

A Theoretical Study on the Diffusion Current at the Stationary Electrodes of Circular and Narrow Band Types

Abstract: The oxygen diffusion current on the platinum electrode was studied theoretically in both circular and band type electrodes. For the oxygen current of the circular electrode the following equation was obtained; I=4naFDC0, where a is the radius of the electrode, D the diffusion coefficient and C0 the oxygen concentration in the bulk solution. The current of the band electrode was independent of the thickness of the band, as shown by the following; I=nΠlFDC2/Ψ, where l is the length of the band and Ψ is the parameter showing the extension of the boundary of the diffusion region; outside the boundary the concentration is maintained at a constant C0. The theoretical equation of the oxygen current of the circular electrode agreed well qualitatively with the experimental result. In the case of the band electrode, the oxygen diffusion current was found to be independent of the thickness; this was approximately ascertained by the experimental data.

Polarographic Adsorption Wave

Abstract: The present review deals with the so-called anomalous wave which is caused by the adsorption of depolarizing species or its electrolysis product. Adsorption current of such a type was first observed 1941 by Brdicka and Knobloch in the reduction of riboflavin and 1942 by Brdicka in the reduction of methylene blue. The physicochemical elucidation of adsorption wave was mainly given by Brdicka on the basis of Langmuir adsorption isotherm. It is clarified that in the reduction wave a prewave appears if the reduction product is easily adsorbable on the dropping mercury electrode surface, since the adsorbed reduction product is more stabilized than the unadsorbed product. On the contrary, if the oxidized form is adsorbable, the adsorbed oxidant is more stabilized than the unadsorbed oxidant, a postwave appears in the reversible reduction process. The most important property of the adsorption pre- or postwave consists in the fact that its wave height increases with concentration lower than the critical concentration corresponding to the adsorption saturation. When the concentration is higher than the critical one, the current intensity of the adsorption prewave remains constant, and the second step called the main wave appears. The sum of main and adsorption wave heights is proportional to the depolarizer concentration. There have been found some criteria for distinguishing whether a given polarographic wave is a normal diffusion current wave or an adsorption wave. It is emphasized that in the experimental work one should pay attention to the following properties of polarographic wave. These are:(1) dependence of wave height on concentration, (2) dependence of wave height on the mercury head h, (3) current-time curve during one drop-life, (4) measurement of the electrocapillary curve, (5) dependence of wave height on temperature. As to the analytical application it is shown that the depolarizers which give an ad-sorption wave can well be determined when the a.c. or high-frequency polarographic method is applied. Since the adsorption provokes a prominent increase in the capacitative component of the electrode surface resulting in an enhanced signal in the a.c. or high-frequency polarogram, the microdetermination of the depolarizers exhibiting adsorption wave, such as methylene blue, riboflavin, diethyldithiocarbamate and others, can comfortably carried out.

Polarographic Reduction of 5-Hydroxyquinoline in Dimethylformamide

Abstract: The electrode reaction mechanisms of quinoline and its derivatives at the dropping mercury electrode in non-aqueous solvent have been studied by Fujinaga et al. They found that, in dimethylformamide, quinoline gives two one-electron reduction waves corresponding to successive reductions of the pyridine ring, and 6-chloroquinoline gives three waves: the first two-electron reductoin wave corresponds to dechlorination reaction and the successive two one-electron waves the reductions of the pyridine ring. On the other hand, the polarographic behavior of 8-hydroxyquinoline was very intricate, as if a hydrogen bridging between the oxygen and nitrogen atoms complicates the electrode reaction by its stabilizing action on resonance. In this respect, the present paper deals with the investigation on 5-hydroxyquinoline free from such a hydrogen bridging.

Polarographic and Voltammetric Studies of Diphenylthiocarbazone and its Chelates with Mercury and Copper in the Ternary Solvent Mixture

Abstract: The solvent extraction method has widely been applied to the analysis of trace elements, particularly in their colorimetric determination. However, in order to apply the extraction method in electroanalytical chemistry, the organic layer containing the extracted sample species has to be evaporated and the residue redissolved in water, or else the sample species must be back-extracted from the organic layer into a suitable aqueous solution.In order to avoid these time-consuming procedures, the present author, with Fujinaga et al., has proposed the use of a ternary solvent mixture, consisting of chloroform, methylcellosolve and water, for the polarographic analysis and has investigated the polarographic behavior of diethyldithiocarbamate in this solvent mixture.The solution for polarographic analysis can be prepared simply by mixing chloroform, into which the sample species has been extracted, with appropriate amounts of methylcellosolve, water and supporting electrolyte. This homogeneous ternary solvent mixture has a dielectric constant high enough to ensure the dissociation of supporting electrolytes.A new cathodic stripping method, depending on the adsorption and subsequent reduction of adsorbed mercuric diethyldithiocarbamate, has been developed in this solvent for the determination of trace amount of mercury.In the preceding report, the polarography of diphenylthiocarbazide in an aqueous solution of 50% methylcellosolve was studied in order to use diphenylthiocarbazide or diphenylthiocarbazone as an organic reagent for the inorganic polarographic analysis. When the concentration of diphenylthiocarbazide was larger than approximately 0.1 mM, two anodic waves could be observed in d.c. polarogram and two corresponding peaks in a.c. polarogram. Since the oxidation of thiol on a platinum electrode occurs at a potential of about + 1.0 V, the electrochemical process of diphenylthiocarbazide at the dropping mercury electrode was confirmed as follows, in acidic media: 2H4Dz-Hg_??_Hg(HDz)2+6H++6e (1)The wave at the more negative potential was an adsorption pre-wave corresponding to the formation of the mercuric complex, Hg(HDz)2, adsorbed on the electrode surface. And in alkaline solution:H4Dz+2OH-_??_H2O+2e (2)H4Dz+Hg+4OH-_??_HgDz+4H2O+4e (3) in which H4Dz, H2Dz and Dz represent diphenylthiocarbazide, diphenylthiocarbazone and diphenylthiocarbodiazone, respectively.As a result of the adsorption of HgDz on the surface of dropping mercury electrode, a minimum was observed at the plateau of the oxidation wave according to equation(2). The total wave was due to the oxidation reaction of equation (3). A similar adsorption wave has been reported for gluthathione, thioglycolic acid, diethyldithiocarbamate, and 2, 3-dimercaptopropanol.In the present paper, the polarographic and voltammetric behaviors of diphenylthiocarbazone and its complexes were investigated in the ternary solvent mixture containing 0.2 M sodium acetate, 0.1 M acetic acid and 0.1 M potassium chloride as the supporting electrolytes.

Polarographic and Chronopotentiometric Behaviours of Uranyl Ion in Fused Potassium Thiocyanate

Abstract: Using dropping mercury electrode, the electrochemical behaviours of uranyl ion dissolved in fused potassium thiocyanate at 185°C were investigated through ordinary polarography and chronopotentiometry. Reproducible polarogram and chronopotentiogram, both consisted of two reduction waves for uranyl ion were obtained. Limitting current for the first step of the polarogram was proportional to the uranyl ion concentration and the square root of the mercury height at constant depolarizes concentration, but not for the second. No linear relationship was obtained in the log [il(id - i)] vs. E plots for both the first and second waves. The i0τ11/2-values for the first step of chronopotentiogram at constant uranyl ion concentration were independent of the current density at the polarized electrode and proportional to the bulk concentration. A plot of log {(τ11/2 - t1/2)/t1/2} vs. E yields a straight line for the first step and the average reciprocal slope (0.0993) is in good agreement with the theoretical value (0.091) in the case of n = 1 at 185°C. The ratio of τ2/τ1 was calculated for two successive reduction steps to be about 3. According to these observations it is clearly concluded that electroreduction procces of uranyl ion is diffusion-controlled and proceeds to two steps as follows ;UO22+ + e = UO2+ (1)UO2++ e = UO2 (2) The diffusion constant of uranyl ion in this melt at 185°C was calculated from Sand's equation to be 6.56 × 10-7 cm2/sec, which is about one-third compared with that for lead ion in the same melt at the same temperature.