Mercury Drop Growth

Metrohm and BAS have also introduced improved DME models capable of operating in the SMDE mode. The Metrohm electrode (Fig. 14.6b) has a needle valve and small-bore capillary. Much of it is pneumatically controlled. The BAS version (Fig. 14.6c) is called the controlled growth mercury electrode (CGME). It is based on the work of Kowalski, Osteryoung, and coworkers [30]. Its features include a low-resistance electrical contact to the mercury thread in the capillary via a stainless steel tube and a fast response valve. The fast valve has allowed unique experiments to be performed where precise control of mercury drop growth during the experiment is desirable [31-33]. The BAS (Fig. 14.7), EG G Princeton Applied Research (Fig. 14.8), and Metrohm (Fig. 14.9) electrodes offer this easy and reproducible drop renewal in fully equipped cell stands. 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

Barker and Jenkins45 attempted to solve the problem by application of the polarising current in a series of pulses one pulse of approximately 0.05 second duration being applied during the growth of a mercury drop, and at a fixed point near the end of the life of the drop. Two different procedures may, however, be employed (a) pulses of increasing amplitude may be superimposed upon a constant d.c. potential, or (b) pulses of constant amplitude may be applied to a steadily increasing d,c. potential. 

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). 

Several mercury electrodes combine the features of the DME and HMDE. In particular, one employs a narrow-bore capillary that produces DMEs with drop lives of 50-70 s (14). Another involves a controlled-growth mercury drop (15). For this purpose, a fast-response valve offers a wide range of drop sizes and a slowly (step-by-step) growing drop. 

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. 

Lead in water may he analyzed very precisely at low concentrations hy anodic stripping voltametry using an electrochemical analyzer static or controlled growth mercury drop electrodes, reference calomel or silver-silver chloride electrodes and silica or TEE cells. Copper, silver, gold, and certain organic compounds may interfere in the test. (APHA, AWWA and WEE. 1998. Standard Methods for the Examination of Water and Wastewater, 20 ed. Washington, D.C. American Public Health Association.)... 

R. A. Osteryoung and coworkers [5] have performed chronocoulometric measurements of surface charge density at a controlled-growth mercury electrode. After initial formation and equilibration of the mercury drop, it was expanded by further addition of mercury and the charge corresponding to the new area was directly measured. The obtained value... 

The first hydrodynamic electrode to be invented was the dropping mercury electrode [1]. It has a cyclic operation and can thus be considered only as quasi-steady-state its hydrodynamic character derives from drop growth. The principal advantage of a dropping electrode is that a fresh electrode surface is constantly exposed to the solution however, there are few electrode materials available and mathematical solution of the mass transport to the drop surface is complicated by the fact that the surface is expanding. 

Actually, experimental results indicate that the constant in Eq. (3.9) is too small by a factor of V7/3. We now realize that this V7/3 quantity is not empirical but is the appropriate contribution due to the growth of the mercury drop into the solution away from the capillary orifice. Thus, the correct diffusion current expression for a dropping-mercury electrode is... 

Controlled growth mercury drop electrode - static mercury drop electrode (SMDE)... 

Figure 33. Effect of increasing proton activity, a through e, on the polarogram of anthracene. For the sake of clarity the five polarograms have been shifted horizontally. Note also that the oscillations caused by the growth and fall of the mercury drops are not shown. (From Ref. 76).

In polarography, not enough time is available for the diffusion layer to reach its stationary thickness. Instead, the current per unit electrode area decreases with the square root of time, the signature time-dependence for diffusion. On the other hand, the area of the growing drop expands, proportional to the two-thirds power of drop age r (i.e., time elapsed since the previous mercury drop fell off). These two counteracting effects, diffusion currents per area proportional to r 1/2, and area growth as t2/3, combine to yield polarographic current-time curves with a time dependence of t-1/2 X t2/3 = t1/6, as expressed in the Ilkovid equation. 

Computers are also used to control timing of different phases of experiments (e.g., deaeration of solutions, stirring, and growth of a mercury drop) and are well-suited to the performance of a series of experiments without operator attention. 

The DME is virtually spherical its volume can be calculated from the rate of flow of mercury m (mg/sec), the time t (sec) measured from the beginning of drop growth, and the density of mercury. This gives the radius of the drop, from which the surface area, A in mm, is... 

For diffusion-controlled processes the faradaic or electrolysis current grows during the drop lifetime I = ktb (Fig. 1.5d). This growth is the resultant of two opposing processes. The first is the increase in the size of the growing mercury drop causing an increase in the current. The second is the depletion of the electroactive species from the solution and growth of a depletion zone around the electrode, due to electrolysis. The latter produces a decrease in the current Fig. 3.2a shows this decrease in current for an electrode of fixed size. 

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