Thin mercury films

The mercury film electrode has a higher surface-to-volume ratio than the hanging mercury drop electrode and consequently offers a more efficient preconcentration and higher sensitivity (equations 3-22 through 3-25). hi addition, the total exhaustion of thin mercury films results in sharper peaks and hence unproved peak resolution in multicomponent analysis (Figure 3-14). 

The above conclusions clearly indicate that the use of very thin mercury films gives maximal sensitivity as it enables fast sweep rates to be combined with excellent peak resolution. 

Stripping voltammetry or stripping analysis has a special place in electrochemistry because of its extensive application in trace metal analysis. Stripping voltammetry (SV) is a two-step process as shown schematically in Fig. 18b. 12. In the first step, the metal ion is reduced to metal on a mercury electrode (thin mercury film on glassy carbon or a HMDE) as amalgam. 

After tq is passed, the second step starts by scanning the potential from Ed to a potential when all the deposited metals are re-oxidized (the reverse of reaction 25). The oxidation current recorded as a function of potential is the anodic stripping voltammogram (ASV). A typical ASY of three metals (Cd, Pb, and Cu) deposited on a mercury film electrode is shown in Fig. 18b.12b. The sensitivity of ASY can be improved by increasing the deposition time and by using the pulse technique to record the oxidation current. ASV in Fig. 18b. 12b was obtained by using the square wave voltammetry. In most cases a simple linear or step ramp is sufficient to measure sub-ppm level of metals in aqueous solution. The peak current of a linear scan ASV performed on a thin mercury film electrode is given by... 

Reactions of Amalgam-Forming Metals on Thin Mercury Film Electrodes... 

Figure 2.22 shows SWV responses of electrochemically reversible reaction on stationary planar electrodes covered with a thin mercury film ... 

The working electrodes found to be useful at room temperature can also be used at low temperature. There are no special constraints. Platinum is probably the most widely used, simply because it is the most common electrode material for room-temperature work. A mercury electrode can also be employed, either as a hanging mercury drop electrode (HMDE) [23], a thin mercury film on a solid support, or an amalgam [26]. The HMDE was reported to extend the range of usable potentials to somewhat more negative values than found with platinum [23]. Of course, below -39°C, the HMDE is actually a solid electrode however, no detectable change in its voltammetric behavior is noted at the phase transition. 

For reversible stripping reactions, the applied potential controls the concentration at the mercury-solution interface (according to the Nernst equation). Because of the rapid depletion of all the metal from thin mercury films, the stripping behavior at these electrodes follows a thin-layer behavior. The peak current for the linear scan operation at thin mercury film electrodes is thus given by... 

Anodic stripping voltammetry (ASV) is the oldest, and still the most widely used version of stripping analysis [3]. The technique is applicable to metal ions that can be readily deposited at the working electrode, and particularly for those metals that dissolve in mercury. In this case, the metals are being preconcentrated by electrodeposition into a small-volume mercury electrode (a thin mercury film or a hanging mercury drop). The preconcentration is done by cathodic deposition at a controlled potential and time. The deposition potential is usually ca. 0.3 Y more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach the mercury electrode by diffusion and convection, where they are reduced and concentrated as amalgams ... 

Fig. 32.6. (a) Optimisation of the thin mercury film in terms of f mm using d.c. adsorptive stripping voltammetry of 5.0 x 10-8M aminopterin in pH 5 acetate buffer. Mercury salt solution = 1.0 x 10 3M in 0.5M HC1 tm — 60s Eacc — 0.0V, tacc. = 60s film was stripped anodically at lOOmV/s. (b) Optimisation of the thin mercury film in terms of film using d.c. adsorptive stripping voltammetry of aminopterin. Emm — -0.8 V. Rest of conditions as in (a), (c) Optimisation of the thin mercury film in terms of film versus a.c. stripping voltammetry of aminopterin 5 x 10 9M. Rest of the conditions as above, (d) Two anodic stripping peaks of the thin mercury film under optimum deposition conditions tmm = 90s Emm = -0.8V scan rate = lOOmV/s. 

Braun and Metzger [52] showed that trace amounts of nickel obtained from natural environmental samples could be determined voltammetrically as nickel dimethyl glyoximate following adsorptive enrichment onto a rotating glassy carbon electrode, on which a thin mercury film has been deposited electrolytically. See Sect. 7.34.1. 

The stoichiometric composition of Pyr-mercury was determined 8,9) by using thin mercury films, plated onto a platinum disk, as the cathodes. Under conditions where the amount of mercury constituting the cathode limited the amount of Pyr-mercury that could be formed, it was found that the composition is 1 TAA+/5 Hg. Similarly, it was found that the product of (CH3)4N+ consists of (CH3)4N+/5 Hg. In a recent work Bard and co-workers 10) used exhaustive electrolysis of (C4H9)4N+ at mercury in acetonitrile and suggested the stoichiometry (C4H9 )4N+/4 Hg. 

Note DC = direct current DP = differential pulse SW = square wave DME = dropping mercury electrode SMDE = static mercury drop electrode HMDE = hanging mercury drop electrode TMFE = thin mercury film electrode. 

In anodic stripping voltammetry the mercury film and the metal ion to be determined are often co-deposited (called in situ mercury deposition). The thin mercury film has characteristics similar to a thin-layer cell, described in Section 9.10. Additionally, it can be easily used in hydrodynamic systems29. 

Thin-film electrode — An electrode covered with a thin film of a given substance. The purpose of placing a thin film on the electrode surface is to obtain desired electrode properties. Many different substances have been used to prepare film electrodes they include among others mercury (see - thin mercury film electrodes) gold, boron-doped diamond (see - boron-doped diamond electrode), conductive polymers (see - polymer-modified electrode), and alkanethiols. The film thickness can vary from several micrometers (mercury) to monomolecular layers (thiols). In some cases (e.g., for - spectroelectrochemistry purposes) very thin layers of either gold or tin oxide are vapor-deposited onto glass plates. Thin film electrodes are often called - surface-modified electrodes. 

Anodic stripping voltammetry (ASV) with the tubular mercury graphite electrode (TMGE) possesses adequate sensitivity and precision under repeated use to characterize zinc in San Diego Bay water. The TMGE, made by electrolysis of a mercuric nitrate solution to form a thin mercury film inside a graphite tube, is described elsewhere (I). 

DPASV equipped with the Thin Mercury Film Electrode (TMFE) plated onto a Rotating Glassy Carbon Disc Electrode (RGCDE), is one of the most sensitive and powerful techniques at present available for the determination of ultra-trace metals in real time samples (59, 60). Cadmium, Cu, Pb and Zn are the most frequently determined metals at subnanomolar or even picomolar concentration in sea water, without any preconcentration step (17, 61, 62), but other elements can also be determined (63-66). Detection limits lower than 0.5 ng/1 are normally reached (17, 60). 

Both metallic and membrane electrodes have been used to detect end points in potentiometric titrations involving complex formation. Mercury electrodes are useful for EDTA titrations of cations that form complexes that are less stable than HgY- . See Section 21D-1 for the half-reactions involved and Equation 21-5 for the Nernst expression describing the behavior of the electrode. Hanging mercury drop and thin mercury film electrodes appropriate for EDTA titrations are available from a number of manufacturers. As always, whenever mercury is used in experiments like these, we must take every precaution to avoid spilling it, and it must be stored in a well-ventilated hood or a special cabinet to remove the toxic vapors of the liquid metal. Before working with mercury, be sure to read its Materials Safety Data Sheet (MSDS), and follow all appropriate safety procedures. 

Stripping methods encompass a variety of electrochemical procedures that include a bulk electrolysis preconcentration step followed by a voltammetric step. In all these procedures, the analyte is first deposited into a small volume of mercury, usually from a stirred solution. A hanging mercury drop or a thin mercury film is most often used. After an accurately measured deposition time, the electrolysis is dis-... 

Although pulse techniques were developed specifically for the DME, they can be employed analytically with other kinds of electrodes. As important examples, one can cite differential pulse anodic stripping at a hanging mercury drop or at a thin mercury film on a rotating substrate. See Section 11.8 for details. 

Figure 3.18. Linear-sweep anodic-stripping voltammogram, 2 ppm Pb and 1 ppm Cu in 0.1 MHNO3. Conditions 5-minplating time at —1.10 V 15-sec rest time a thin-mercury-film electrode on glassy carbon and a voltage scan rate of 1 V/min for the strippir step.

---