Mercury cathode, dropping, 452,

Controlled-potential separation of many metals can be effected with the aid of the mercury cathode. This is because the optimum control potential and the most favourable solution conditions for a given separation can be deduced from polarograms recorded with the dropping mercury electrode see Chapter 16. 

It is necessary to consider the factors which affect the limiting current with a dropping mercury cathode. 

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 reductant differs from the oxidant merely by n electrons, and together they form an oxidation-reduction system. Consider the reversible reduction of an oxidant to a reductant at a dropping mercury cathode. The electrode potential is given by ... 

There is a rich literature on preparative electrochemical reductions of halosilanes. Dessy et al. reported that Ph3SiH was formed by the electrochemical reduction of Ph3SiCl in 1,2 dimethoxyethane (DME) using a dropping mercury cathode at - 3.1 V vs. AgClOJAg [80]. Although the ultraviolet spectra of the solutions after electrolysis were identical with the spectra of an authentic sample of Ph3SiH, the product was not isolated. 

The cathodic deposition of arsenic at the dropping mercury cathode has been studied7 and appears to be complex both in acid and in alkaline solutions the polarisation curves do not show reversible shifts. From acid solutions the deposition of antimony or bismuth proceeds reversibly. 

Rasierowska (1+5) has applied alternate-current oscillopolarographic method to pharmaceuticals containing saccharin and sodium saccharin. Both forms a well developed oscillopolarographic curve in MH2SOI1 in air with mercury anode and dropping-mercury cathode, and the results are comparable to those obtained by UV spectrometry. 

J. Heyrovsky bases his explanation of the mechanism of hydrogen deposition on a polarographic study of hydrogen overvoltage at the dropping mercury cathode he assumes it to he the result of three successive reactions ... 

IlkoviC, D., and G. Semerano Polarographie Studies with the Dropping Mercury Cathode. Part XXV. Increased Sensitivity of Microanalytical Estimations by a Compensation of Current. Collect Czechoslov. Chem. Comm. 4, 176 (1932). 

Fig. 5. Cell for preparative reduction (K, mercury cathode A, carbon anode R, reference eleotrode D, dropping mercury electrode N, inlet for nitrogen). From Lund.54...

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. 

The conditions at a dropping mercury cathode are clearly different from those at a stationary electrode, and the limiting current, or diffusion... 

The Half-Wave Potential.—In the preceding description of the analytical applications of the dropping mercury cathode it has been supposed that the nature of the reducible substance has been determined and that the position of the corresponding wave on the current-potential curv c is known. If the substance has not been previously identified, however, it is possible to do so by means of the polarographic curve. The reducible material is characterized by its half-wave potential this is the potential... 

The most reliable data are from studies of hydrogen evolution on mercury cathodes in acid solutions. This reaction has been studied most extensively over the years. The use of a renewable surface (a dropping mercury electrode, in which a new surface is formed every few seconds), our ability to purify the electrode by distillation, the long range of overpotentials over which the Tafel equation is applicable and the relatively simple mechanism of the reaction in this system all combine to give high credence to the conclusion that p = 0.5. This value has been used in almost all mechanistic studies in electrode kinetics and has led to consistent interpretations of the experimental behavior. It... 

Coulometric determinations of metals with a mercury cathode have been described by Lingane. From a tartrate solution, copper, bismuth, lead, and cadmium were successively removed by applying the appropriate cathode potential, which was selected to correspond to a region of diffusion-controlled current determined from current-voltage curves with a dropping mercury electrode. With a silver anode, iodide, bromide, and chloride can be deposited quantitatively as the silver salt. By controlling the anode potential, Lingane and Small determined iodide in the presence of bromide or chloride. The separation of bromide and chloride, however, was not successful because solid solutions were formed (Section 9-4). 

O ) polarography uses a dropping mercury cathode and the current is measured 00 the rotating-disc method uses a spinning platinum disc as the cathode and the current in the steady state situation at a series of rotation speeds is measured Hi) the potentiostatic method uses fixed electrodes and the fall of current with time is measured. 

The anode is nonpolarizable (i.e., its potential is independent of the current flowing through the cell) and is typically a pool of mercury at the bottom of the ceU. The dropping mercury cathode comprises a capillary tube of about 0.03 mm bore connected to a mercury reservoir and held with its tip below the surface of the electrolyte. The mercury head, of the order of 50 cm, is adjusted to give a drop every 3 sec or so. A platinum wire is immersed in the mercury reservoir and the circuit is... 

Heyrovsky, J. (1922) Electrolysis with a dropping mercury cathode. Chemicke Listy, 16, 255. 

Heyrovsky, J. and Shikata, M. (1925) Researches with the dropping mercury cathode. Part II. The Polarograph, Recueil des Travaux Chimiques des Pays-Eas, 44, 495 98. 

Figure 1. Distribution of fermium as a function of applied voltage between mercury in a dropping mercury cathode and 0.1M tetramethyl ammonium perchlorate at pH = 2.4. The slope of the logarithmically transformed line indicates the number of electrons exchanged in the electrolysis reaction (24).

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