Mercury accumulation drop electrode

Here Ee is the standard potential of the reaction against the reference electrode used to measure the potential of the dropping electrode, and the potential E refers to the average value during the life of a mercury drop. Before the commencement of the polarographic wave only a small residual current flows, and the concentration of any electro-active substance must be the same at the electrode interface as in the bulk of the solution. As soon as the decomposition potential is exceeded, some of the reducible substance (oxidant) at the interface is reduced, and must be replenished from the body of the solution by means of diffusion. The reduction product (reductant) does not accumulate at the interface, but diffuses away from it into the solution or into the electrode material. If the applied potential is increased to a value at which all the oxidant reaching the interface is reduced, only the newly formed reductant will be present the current then flowing will be the diffusion current. The current / at any point... 

To learn that one of the most important areas of coulometry for the electroanalyst is stripping , in which analyte is allowed to accumulate on the surface of, e.g. a hanging mercury-drop electrode ( pre-concentration ), and then electro-oxidized ( stripped ). 

Figure 5.6 Schematic representation of stripping when using a cathodically biased hanging mercury-drop electrode (HMDE) suspended in a solution of reducible ions (a) the sequence of processes occurring during the stripping process (b) the accumulation of copper at the surface of the HMDE (the increasing surface concentration of Cu is indicated by the intensity of colour).

Hanging mercury-drop electrode (HMDE) A commonly employed working electrode in polarography. The HMDE is often preferred to the experimentally simpler dropping mercury electrode (DME) because resultant polarograms do not have a sawtoothed appearance and because accumulation (for stripping purposes) is readily achieved at its static surface. 

Lobacz et al. [52] have described partial adsorption ofTl+-cryptand (2,2,2) complex on mercury electrode. From voltocoulom-etry, cyclic voltammetry, and chrono-coulometry, it has been deduced that electroreduction of this complex proceeds via two parallel pathways from the solution and from the adsorbed states, which are energetically close. Also, Damaskin and coworkers [53] have studied adsorption of the complexes of alkali metal cations with cryptand (2,2,2) using differential capacity measurements and a stationary drop electrode. It has been found that these complexes exhibit strong adsorption properties. Novotny etal. [54] have studied interfacial activity and adsorptive accumulation of U02 " "-cupferron and UO2 - chloranilic acid complexes on mercury electrodes at various potentials in 0.1 M acetate buffer of pH 4.6 and 0.1 M NaCl04, respectively. 

Fraga etal. [71] have proposed a new stripping voltammetric method for the determination of titanium and Co(II) [72] based on the adsorptive accumulation of its hydroxynaphthol blue complex on a static mercury drop electrode. 

On one hand, mercury-film electrodes give increased resolution when compared to the hanging mercury drop electrode (HMDE). On the other hand, BCFMEs have the inherent characteristics of an ERD. Hence, it would be extremely desirable to combine all properties. Furthermore, this combination may provide additional advantages such as easy handling, low cost, and other well-known analytical advantages associated with the use of UMEs itself, for example, the elimination of convective hydrodynamics along the accumulation step in stripping voltammetric techniques. 

A differential pulse CSV method for the determination of traces of butyltin species in water was compared with two other voltammetric methods, namely differential pulse polarography and ASV (Schwartz et at., 1995). The butyltin species were accumulated on the mercury drop electrode as their tropolone complexes. Detection limits were 5 mg 1 1 for tributyltin (TBT), 0.5 mgl-1 for dibutyltin (DBT) and 0.5 mgl-1 for monobutyltin (MBT). These detection limits were better than the corresponding values obtained in the other analytical methods. 

Adsorption stripping methods are quite similar to the anodic and cathodic stripping methods we have just considered. Here, a small electrode, most commonly a hanging mercury drop electrode, is immersed in a stirred solution of the analyte for several minutes. Deposition of the analyte then occurs by physical adsorption on the electrode surface rather than by electrolytic deposition. After sufficient analyte has accumulated, the stirring is discontinued, and the deposited material is determined by linear-scan or pulsed voltammetric measurements. Quantitative information is based on calibration with standard solutions that are treated in the same way as samples. 

We have recently demonstrated similar findings for several antigen-antibody reactions at the hanging mercury drop electrode (HMDE) using the technique of adsorptive stripping voltammetry (AdSV). These have included the reaction of human serum albumin (HSA) with anti-HSA, mouse IgG with anti-mouse IgG and hCG with two anti-/ -hCG antibodies. In all cases, the current observed following accumulation of the antigen (at the HMDE) decreased on addition of the specific antibody. This is illustrated for the case of mouse IgG with anti-mouse IgG in Fig.l. 

As the present study shows, submicromolar levels of timolol can be measured using this technique, controlling their accumulation on a hanging mercury drop electrode and optimizing the instrumental parameters. As a specific application, timolol was determined in aqueous humor of rabbits and human serum. 

The study by cyclic voltammetry confirms the spontaneous accumulation of timolol on the hanging mercury drop electrode after the solution was stirred for 60 seconds. Fig.l shows the cyclic voltammogram of a solution of 5x10 M of timolol at pH = 4.0. A big, well-defined cathodic peak can be seen at —1.03 V, which in succesive scans decreases rapidly assuring a fast desorption from the electrode surface. Voltammogram B shows the same pattern, but without accumulation. The resulting peak is much smaller than that obtained by accumulation. 

Table II.7.8 Trace elements determined by adsorptive accumulation of solochrome violet RS complex on a hanging mercury drop electrode...

The electrode reaction is rarely as simple as described above. In many cases the product is either insoluble, or partly adsorbed at the electrode surface. Besides, the reactants of many reactions are also surface active. Furthermore, the electrode reaction can be either preceded or followed by chemical reactions. Hence, the choice of the working electrode also depends on the reaction mechanism. For instance, the reduction of lead ions on a platinum electrode is complicated by nucleation and growth of lead micro-crystals, while on a mercury electrode lead atoms are dissolved in mercury and the reduction is fast and reversible. Similarly, the well-known pigment alizarin red S and the product of its reduction are both strongly adsorbed on the surface of mercury and carbon electrodes [17]. In this case, the liquid mercury electrode is analytically much more useful because the adsorptive accumulation on the fresh electrode surface can be easily repeated by creating a new mercury drop. However, on the solid electrode, the film of irreversibly adsorbed substance is so stable that it can be formed in one solution and then transferred into another electrolyte for the measurement of the kinetics of the electrode reaction. After each experiment... 

A PCR sequence was used related to Hepatitis B (HBV) virus genome. Optimization of hybridization detection was performed with 17-mer short oligonucleotides. Carbon paste electrodes (CPE) and hanging mercury drop electrodes (HMDE) were used as sensor surface, CV and DPV transduction of MDB accumulated after hybridization between 23-mer capture probe and HBV-PCR amplicon was monitored for the detection. By using MDB indicator. Herpes simplex (HSV) virus genome detection and discrimination of HSV type I and type II viruses were performed in PCR amplicon... 

An electroanalytical method based on adsorptive stripping voltanunetry that is claimed to be faster and cheaper was developed for the determination of uranium in river water samples and a seawater standard (Grabarczyk and Koper 2011). A hanging mercury drop electrode was used and accumulation was executed by pulsing the potential. Recovery of uranium spikes from river water samples was close to 100% with an RSD of about 5% at 1, 5, and 10 mnol L" (238,1190, and 2380 ng L, respectively). 

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