Dropping mercury electrode diffusion limited current

Considering now the dropping mercury electrode, the limiting current is on the order of 3.0 (xA/mM, for a drop time of 1.0 s, corres-ponding to a surface area of A = 0.02 cm and to n = 1. The flux of the impurity reaching the surface, in units of mole per second per square centimeter, is equal to the diffusion-limited current divided by the charge per mole, nF. Assuming, as before, an impurity concentration of 1.0 p.M, this leads to a flux of... 

Electrochemical reduction of iridium solutions in the presence azodye (acid chrome dark blue [ACDB]) on slowly dropping mercury electrode is accompanied by occurrence of additional peaks on background acetic-ammonium buffer solutions except for waves of reduction azodye. Potentials of these peaks are displaced to cathode region of the potential compared to the respective peaks of reduction of the azodye. The nature of reduction current in iridium solutions in the presence ACDB is diffusive with considerable adsorptive limitations. The method of voltamiuetric determination of iridium with ACDB has been developed (C 1-2 x 10 mol/L). 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

The derivation of the equation for the diffusion-limited current at the dropping mercury electrode differs from that of the other electrodes described above owing to its cyclic operation. Mercury flowing down through a fine capillary forms a drop at the bottom end of the capillary. This drop, the electrode, increases in size until it falls by the force of gravity the electrode is then renewed by formation of another drop. There are thus virtually no problems of electrode poisoning. The mode of... 

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. 

Although measurements of the diffusion-limited current in dc polarography and variations of this technique provide a variety of means to measure the concentration of reducible species in solution, it is also possible to use the dropping mercury electrode to obtain kinetic... 

Diffusion current is the limiting current observed in polarography when the current is limited only by the rate of diffusion to the dropping mercury electrode surface. 

Fig. 11. Plots of the diffusion-controlled currents at the dropping mercury electrodes vs. time. Id the instantaneous current (full line), according to Eq. (23) the maximum current (according to Eq. 24) the mean diffusion-limited current (dot-dashed line, according to Eq. (25) Irec the recorded current (dashed curve) (absolute values of currents are considered).

The invention of polarography by Heyrovsky in 1920 s as a new experimental method followed by detail theoretical treatment in 1930 s which led to quantitative interpretation of the polarographic current-potential transients marked a milestone in electrochemistry (8,9). This technique permitted the electrochemist to interpret mass transfer electrode processes in terms of diffusion or convection it was subsequently applied in studies of reaction kinetics. Although the technique was limited to dropping mercury electrodes, its impact on electrochemistry and development of novel electroanalytical techniques was so tremendous that the inventor was awarded a Nobel Prize in Chemistry in 1959. 

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