Mercury-Based Reference Electrodes

The calomel electrode was introduced by Ostwald in 1890. It is an electrode of the second kind (cf. Chap. II.9). As a reference electrode of fixed, well-known and very reproducible potential, it is still important today. In the simplest case, a single drop of mercury is placed in a small tube and is covered by mercury(I) chloride (calomel, Hg2Cl2) (see Fig. III.2.2 a). Another possibility is to fill a small glass tube with a paste of mercury, mercury(I) chloride and potassium chloride solution (Fig. III.2.2b). The paste is in contact with a potassium chloride solution of constant activity. Mostly, a saturated potassium chloride solution is used and the paste additionally contains solid potassium chloride. The electrode net reaction can be formulated in the following way  

the potential of this electrode against the standard hydrogen electrode is given by the equation  

The back reaction by cooling down the electrode is very slow so that a hysteresis of the electrode potential occurs. This is the reason why it is recommended that the calomel electrode only be used at lower temperatures, in maximum up to 70 °C. The thermal coefficient is smallest for a calomel electrode with 0.1 M KCl, but it is easier to handle the saturated calomel electrode. 

Another mercury-containing reference electrode of the second kind is the mer-cury/mercury(I) sulphate electrode. In principle the construction is the same as for the calomel electrode. The electrolyte solution consists either of potassium sulphate in a certain concentration or sulphuric acid. The electrode net reaction is  

The electrode potentials of this electrode at different temperatures are given in Table III.2.3. The mercury/mercury(I) sulphate electrode with sulphuric acid is useful as a reference electrode in solutions containing sulphuric acid. 

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The portable instrumentation and low power demands of stripping analysis satisfy many of the requirements for on-site and in situ measurements of trace metals. Stripping-based automated flow analyzers were developed for continuous on-line monitoring of trace metals since the mid-1970s [16,17]. These flow systems involve an electrochemical flow detector based on a wall-jet or thin-layer configuration along with a mercury-coated working electrode, and downstream reference and counter electrodes. 

Bubble electrodes — Mercury electrodes constructed in such a way that an electrolyte solution is entering a mercury-filled vessel through one or many small orifices in the bottom (cf. Fig. 1). The solution bubbles grow and will finally ascend in the mercury. The counter and the reference electrode have to be situated in the tubing fed to the orifice. Bubble electrodes have been developed as flow-through detectors for HPLC and FIA [i-iii,vi], as electrodes to generate ESR-active species [iii-iv], for - tensammetry [vii], and for electrosynthesis [v]. The fact that they are mercury based impeded further developments of this electrode type. 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

To create a scale of electrode potentials based on eq. 1 A. 12, we need to know the contact potential difference between the solution phase of a suitable reference electrode system and the electrode itself. The most accurate measurements of this type have been made by using the standard hydrogen electrode as the reference electrode and mercury, the work function of which is known to good accuracy (although it is not normally used in the SHE), as the reference metal. Equation 1A.12 then gives... 

As stated earlier, the reference electrode in a cell used for electroanalysis is designed so that its potential is independent of the composition of the test solution. There are several general properties that reference electrodes should have in order to be useful in analysis (1) they should be reversible with an electrode potential which is independent of time and reproducible (2) they should have a small temperature coefficient (3) they should be ideally non-polarizable with negligible effects from the flow a small current through the system and (4) they should be easily constructed. The most commonly used reference electrodes are those based on on the mercury calomel system and the silver silver chloride system. The electrolyte most commonly used in these systems is KCl. Relevant parameters for commonly used reference electrodes are given in table 9.4. 

For the determination of tin(ll) in kits an EG 8c G polarographic analyzer (model 384) with a static mercury drop electrode (model 303) and a cabinet reference electrode was used. Model 384 is a microprocessor-based polarographic analyzer with built-in floppy disk memory to store and recall analytical curves. By controlling each step of the analysis, the microprocessor automates polarographic and voltammetric measurements. All experimental parameters may be chosen by the operator. Concentrations are computed automatically aud recorded iu the rauge from 0.001 ppb to 9999 ppm. 

Linear sweep voltammetry is based on the potential being ramped up between the working and auxiliary electrodes as current is measured. The working electrode is usually a SMDE nowadays, in which case this technique would be called linear sweep polarography. In this set-up, the auxiliary electrode is a mercury pool electrode and may also serve as the reference electrode. The resultant current-potential recording (the polarogram) can yield much information which can be used to qualitatively identify the species and the medium in which it is determined as well as calculate concentrations. Analysis of mixtures is also possible. The detection limit is of the order of 10 M. 

A number of commonly used reference electrodes based on mercury belong to this class of electrodes. 

With the liquid level above the analyte solution, some contamination i>f the sample is inevitable. In most instances, the amount of contamination is too slight to be of concern. In determining ions such as chloride, potassium, silver, and mercury, however, precaution must often be taken to avoid this source of error. A common w-ay is to interpose a second salt bridge between the analyte and the reference electrode this bridge should contain a noninierfering electrolyte, such as potassium nitrate or sodiujn sulfate. Double-junction electrodes based on this design are offered by several manufacturers. 

Under experimental conditions the SHE is rarely used. Reference electrodes of a second kind are used instead, which are simpler to handle and are commercially available. The Ag/AgCl electrode was already mentioned. Other examples are the calomel electrode based on Hg/Hg2Cl2/KCl (for instance, as saturated calomel electrode (SCE)), the mercury sulfate electrode Hg/Hg2S04/H2S04 (0.5 mol 1 ), and the mercury oxide electrode Hg/HgO/ NaOH (Imol 1 ). Potentials of some reference electrodes versus the SHE are shown in Table 3.2. 

This electrode of the second kind is the most frequently used reference electrode in practical measurements, because the construction is very simple, the potential is very well reproducible, and last, but not least, this electrode is free of mercury Normally, a silver wire is covered with silver chloride, which can be achieved electrochemically or thermally [1]. Electrochemically produced films are thinner than thermally produced films. The construction of a commercially available sil-ver/silver chloride electrode is similar to the calomel electrode (see Fig. III.2.3). A very simple method for preparing a silver/silver chloride electrode has been described by Thomas [4]. Because reference systems based on silver/silver chloride can be produced in a very small size, they are often used in microsystems [5-9]. The electrolyte solution in these reference systems is normally a potassium chloride solution (mostly saturated or 3 M), and only seldom sodium or lithium chloride. The electrode net reaction is... 

Concentric cylindrical gauze electrodes ensure uniform potential and current distributions. When a reference electrode is employed - historically calomel or mercury/mercury sulfate, more recently (for occupational health reasons) using silver-based couples and occasionally a wire of the metal to be plated - placement of the tip of the salt bridge close to the working electrode minimizes the ohmic potential drop. 

These electrodes have nitrate-sensitive ion-exchange material incorporated into poly(vinyl chloride)-based membrane electrodes. Care is necessary to avoid contamination by the chloride from the saturated calomel reference electrode and a mercury/me-rcurous sulfate electrode is preferable as a reference electrode. Industrial monitors using nitrate ion-selective electrodes are commercially available. 

Most commonly used reference electrodes are based on mercury (calomel electrode) and silver, in equilibrium with the saturated solution of the corresponding chlorides. Their potential will be constant if the chloride ion concentration around the electrode is constant. The mercury-mercury(I) sulfate electrode is used instead of the calomel electrode when the presence of chloride is undesirable. 

Reference electrodes of mercury have been used by several investigators in an attempt to measure single electrode potentials. Stastny and Strafelda (5 ) concluded that the zero charge potential of such an electrode in contact with an infinitely dilute aqueous solution is -0.1901V referred to the standard hydrogen electrode. Hall ( ) states that the potential drop across the double layer under these conditions is independent of solution composition when specific adsorption is absent. Daghetti and Trasatti (7, ) have used mercury reference electrodes to study the absolute potential of the fluoride ion-selective electrode and have compared their estimates of ion activities in NaF solutions with those provided by other methods. Their method is based on the assumption that the potential drop across the mercury I solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy-Chapman theory. 

To monitor mercury deposition in situ, a microscope reaction cell can be used. The working Pt UME and counter electrode are inserted through a hole at the base of the cell while the reference electrode is positioned in a side compartment as shown in Figure 6.3.8.1. Once mounted on an optical miCToscope equipped with a water immersion objective (Olympus FLxw40), a camera and personal computer can then be used to record images of the mercury deposition. 

Japanese workers had independently reached similar qualitative conclusions based on limited series of oligomeric polystyrene derivatives. They utilized the related measurement technique of mercury contact charging. Later work using a complex frictional charging apparatus in vacuo confirmed our results as shown in Figure 5, a plot of their reported contact potential difference between a metal reference electrode and the films versus Hammett constant. Note the plateau effect at High potential differences. 

The potential for the direct-current mode was selected from a so-called hydrodynamic flow voltammogram (Figure 5), The potential was chosen on the plateau, before the reduction wave of sulphite ions (Figure 6), Between pH 7 and pH 9 MMC produces only one reduction wave[23], From Figure 5 the pulse base and the pulse height for the differential pulse mode were selected. The optimum that could be achieved was -390 mV and -100 mV, To overcome changes in the reference solution of the reference electrode[21], the potential was checked every day and the inner solution was refreshed every day, if necessary. Most of the mercury detectors described, make use of very short drop times, resulting in small electrode-surfaces, fast renewal of the surface and a low noise level due to low capacity current. 

The polarographic cell assembly comprising cell base and cover with a dropping mercury electrode as the cathode, a platinum wire auxiliary electrode as the anode and a saturated calomel reference electrode making solution contact via a liquid junction bridge. The mercury column is kept at a constant height, to have a natural drop time of about 4 s. The instrumental conditions for the differential pulse technique were 0.5 s drop time, -10 mV/S scan rate, 50 mV modulation amplitude. 

Reference electrodes based on mercury are second-kind electrodes composed of pure mercury and a sparingly soluble mercury salt [98, 99]. Due to environmental concerns in recent years these electrodes are less frequently used compared to their... 

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