Amalgam Electrodes for Electroanalysis

Liquid mercury is a unique material for the indicator electrode in voltammetry. One reason for this is the high overvoltage for hydrogen formation, thus extending the actual potential window. Diluted amalgams are important reaction products in voltammetric (polarographic) processes, however liquid amalgams are rarely used directly as electrode material for analytical purposes. Because of the fact that voltammetry is very suitable for field and remote monitoring, issues concerning the use of mercury electrodes in environmental analyses have led to considerable research effort aimed at finding alternative tools with acceptable performance. Solid electrodes are such alternatives. Different types of electrodes are reviewed. In particular, solid amalgam electrodes are very promising, with acceptable low toxicity to be used for field measurements. Solid amalgam electrodes are easy and cheap to construct and are stable over a reasonable time up to several weeks. Assessment of the toxicity risk and the long time stability for remote and unattended monitoring is discussed. The differences between solid dental amalgam electrodes, made by using techniques known from dental clinical practice, and mercury film or mercury layer electrodes on solid substrates are reviewed. In particular the dental technique for constructing solid amalgam electrodes gives advantage because it's fast and inexpensive. Also the technique for making dental amalgam has been explored and optimized over years by dentists, giving advantage when the same technique is used for constructing electrodes. Dental amalgam electrodes has been found to act similar to a silver electrodes, but with high overvoltage towards hydrogen. This make it possible to use the dental amalgam electrode for detection of zinc, cobalt and nickel in additions to other metals like lead, copper, thallium, cadmium, bismuth, iron etc. Also the use for reducible organic compounds is expected to be promising.     

Alloy electrodes with high hydrogen overvoltage for analytical use in voltammetry. Some preliminary results

Liquid mercury and liquid diluted mercury amalgams have been the major electrode systems employed in voltammetry and related methods. This is mainly due to their high overvoltage to hydrogen, which enables the determination of heavy metals (zinc, nickel, cobalt, etc.) and other species with high negative half-wave potentials; the toxicity of mercury and liquid diluted mercury leads to ever increasing restrictions in their use. The use of such systems may even be forbidden in the future, at least in online systems for work in the field. Recent work, carried out in our laboratory, has demonstrated that a non-toxic solid dental amalgam may be used as the electrode material, conveniently replacing mercury. An extension of this work has shown that electrode materials comprising a metal or a compound with low hydrogen overvoltage change their hydrogen overvoltage properties substantially when contaminated with even small amounts of metals or compounds which show high hydrogen overvoltage. This extends greatly the range of potentially available electrode systems and thereby analytical possibilities of voltammetry. This new discovery also makes it possible to produce solid electrodes that have high overvoltage to hydrogen without any use of mercury.     

Comparison of Mercury Vapor Pressure of Silver Amalgam-Based Electrode Materials Using AAS

Nowadays, several amalgam-based electrodes are commonly used in electrochemistry as a less toxic alternative to mercury electrodes. Therefore, a comparison of the mercury vapor pressure of several silver amalgam-based electrode materials with the mercury vapor pressure of liquid mercury and of dental amalgam using AAS was done in this study. Method was optimized to get the highest mercury signal. Results showed that the mercury vapor pressure of amalgam-based electrode materials not containing liquid mercury is far lower than the mercury vapor pressure of liquid mercury (about two orders of signal magnitude) and comparable with mercury vapor pressure of dental amalgam.     

Working Electrodes from Amalgam Paste for Electrochemical Measurements

Paste electrode with paste amalgam as an active electrode material is described here for the first time. Designed electrode from silver paste amalgam (AgA‐PE) is solely metallic and does not contain any organic binder. Mechanical surface regeneration of AgA‐PE is performed in the same way as for classical carbon paste electrodes and reproducibility of such regeneration is about 10%. Electrochemical surface regeneration appeared very efficient for most measurements. In dependence on paste metal content, the electrode surface can be liquid (resembling a film) or rather solid. The hydrogen overvoltage on AgA‐PE is high, and the electrode allows measurements at highly negative potentials. AgA‐PE is well suited for study of reduction or oxidation processes without an accumulation step. Anodic stripping voltammetry of some metals tested on the electrode is influenced by formation of intermetallic compounds. The measurement based on cathodic stripping voltammetry (adenine, cysteine) and on catalytic processes from adsorbed state (complex of osmium tetroxide with 2,2′‐bipyridine) can be performed on AgA‐PE practically under the same conditions as found earlier for HMDE and for silver solid amalgam electrode. The working electrode from paste amalgam combines the advantages of paste and metal electrodes.     

Dental Amalgam in Voltammetry. Some Preliminary Results

ABSTRACT

Liquid mercury and liquid diluted mercury amalgams have been superior as electrode material for the use of voltammetry for analytical purposes. This is mainly due to the high overvoltage to hydrogen, which enables one to use a wide potential range for the measurements. Due to the toxicity of mercury and liquid diluted mercury compounds, the use of such compounds are increasingly restricted, and cannot be included in voltammetric devices for field and online applications.

The present authors have studied the properties of dental amalgam and related solid amalgams as electrode material in voltammetry. Due to the special properties of dental amalgam compared with mercury itself it is not toxic.

It has been found that dental amalgam and related solid amalgams have a very high overpotential to hydrogen, allowing one to carry out trace analyses at potentials sufficiently negative to allow determination of e.g. zinc, cobalt, nickel and iron at trace levels. This have previously been difficult except when using a mercury or a mercury film electrode.

The present preliminary paper describes such electrodes and some practical applications for trace heavy metal analyses, using differential pulse stripping voltammetry. Such determinations are very important for field and online analyses of pollutants in soil and groundwater, and the electrode can be used repeatedly. Further improvements can obviously be obtained by optimising the composition of the alloy.     

Metal Exchange Reactions in Metallothioneins Explored by Electrochemical Methods

Metallothioneins (MTs) are widely occurring, small, cysteine‐rich proteins, important for essential metal (Zn, Cu) homeostasis and transport and for heavy metal (Cd, Hg) detoxification. In buffered solutions of mammalian MT, voltammetry and potentiometric striping analysis (PSA) can distinguish different coordination of bound metals or follow their exchange, especially that of zinc and cadmium for copper, silver, and cobalt. The examples of different electrode applications as of hanging mercury drop electrode (HMDE), of silver solid amalgam (AgSAE) electrode, and of silica gel modified carbon paste electrode (SiO₂‐CPE) are given.     

Silver based mercury film electrode: Part II. The influence of aging of electrode on the electrode processes of various metal ions

The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10^?3M range show a prepeak-postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer 1 μm thick.     

Crystallic silver amalgam--a novel electrode material

A crystallic silver amalgam was found to be a suitable working electrode material for voltammetric determination of electrochemically reducible organic nitro-compounds. Optimum conditions for crystal growth were found, the crystal surface was investigated by atomic force microscopy in tapping mode and single crystals were used for the preparation of quasi-cylindrical single crystal silver amalgam electrode (CAgAE). An electrochemical behavior of this alternative electrode material was investigated in aqueous media by direct current voltammetry, cyclic voltammetry (CV), differential pulse voltammetry (DPV) and adsorptive stripping voltammetry (AdSV) using 4-nitrophenol as a model compound. Applicable potential windows of the CAgAE were found comparable with those obtained at a hanging mercury drop electrode, providing high hydrogen overpotential, and polished silver solid amalgam electrode. Thanks to the smooth single crystal electrode surface, the effect of the passivation is not too pronounced, direct DPV determination of 100 μmol l(-1) of 4-nitrophenol at CAgAEs in 0.2 mol l(-1) acetate buffer pH 4.8 provides a RSD around 1.5% (n = 15). DPV calibration curves of 4-nitrophenol are linear in the whole concentration range 1-100 μmol l(-1) with a limit of quantification of 1.5 μmol l(-1). The attempt to increase sensitivity by application of AdSV was not successful. The mechanism of 4-nitrophenol reduction at CAgAE was investigated by CV.     

Evaluations of solid electrodes for use in voltammetric monitoring of heavy metals in samples from metallurgical nickel industry

Evaluation of different solid electrode systems for detection of zinc, lead, cobalt, and nickel in process water from metallurgical nickel industry with use of differential pulse stripping voltammetry has been performed. Zinc was detected by differential pulse anodic stripping voltammetry (DPASV) on a dental amalgam electrode as intermetallic Ni–Zn compound after dilution in ammonium buffer solution. The intermetallic compound was observed at –375 mV, and a linear response was found in the range 0.2–1.2 mg L^–1 (r²=0.98) for 60 s deposition time. Simultaneous detection of nickel and cobalt in the low g L^–1 range was successfully performed by use of adsorptive cathodic stripping voltammetry (AdCSV) of dimethylglyoxime complexes on a silver–bismuth alloy electrode, and a good correlation was found with corresponding AAS results (r²=0.999 for nickel and 0.965 for cobalt). Analyses of lead in the g L^–1 range in nickel-plating solution were performed with good sensitivity and stability by DPASV, using a working electrode of silver together with a glassy carbon counter electrode in samples diluted 1:3 with distilled water and acidified with H₂SO₄ to pH 2. A new commercial automatic at-line system was tested, and the results were found to be in agreement with an older mercury drop system. The stability of the solid electrode systems was found to be from one to several days without any maintenance needed.     

Rapid potentiometric methods for uranium and for analysis of mixtures using potassium bromate

This investigation is an application of the method involving determination of uranium(VI) based on reduction to uranium(IV) with zinc metal in acidic medium, and then oxidation by adding a known excess of bromate. Boiling to expel the liberated bromine then adding sulfite and 1 ml 10% sulfuric acid, expel excess sulfite, cooling and titrating the liberated bromide with silver(I), using silver amalgam as indicator electrode in conjunction with calomel electrode. Binary and ternary mixtures are successfully analyzed. The present methods have advantages over the classical ones of being simple, rapid, and reliable.     

Renewable silver amalgam film electrodes in electrochemical stripping analysis—a review

The success of a voltammetric sensing procedure depends mainly on the proper choice of the working electrode. This is because its ability to accumulate the analyte determines the sensitivity of the method. The main criterion of the selection of the proper working electrode is the available potential window. A variety of conductive materials have been used for the preparation of working electrodes. Of these, two kinds of mercury electrodes—hanging mercury drop and film—were used because of their excellent voltammetric performance and, in particular, their high overpotential of hydrogen reduction. The significant drawbacks of mercury electrodes, however, are the toxicity of the material and the instability of liquid mercury films. To overcome these disadvantages, less toxic mercury-containing materials have been used, such as amalgams and amalgam film electrodes. This group includes renewable silver amalgam film electrodes used for electrochemical stripping sensing purposes. These electrodes have successfully been applied for anodic, adsorptive, cathodic, catalytic voltammetric, and potentiometric stripping determination of trace amounts of inorganic cations and organic compounds in various natural matrices. In this review, the electrode design, characteristics, and application of two kinds of renewable silver amalgam film electrodes are discussed in detail.     

A study on galvanic degradation of dental amalgam using activation analysis assessments

The method of neutron activation and gamma-ray spectrometry was used for studying galvanic corrosion of dental amalgam. The behaviour of galvanic couples of zinc containing and zinc free amalgam with dissimilar electrodes from other restorative material was investigated in various electrolytic media, in absence and presence of oxygen. Severe corrosion of amalgam takes place when coupled with gold electrodes.     

Silver Amalgam Tubular Detector Combined with Platinum Auxiliary Electrode for Electrochemical Measurements in Flow Systems

A new type of tubular detector with platinum auxiliary electrode inside the silver amalgam tube (TD+AuxE) was proposed, fabricated, tested and compared with a typical silver amalgam tubular detector developed earlier. Non‐stop‐flow differential pulse voltammetric anodic stripping method (AS‐DPV) and amperometric method in a glucose oxidase biosensor arrangement were tested. Both detectors were applied for AS‐DPV detection in flow systems for the first time. Solutions of zinc and cadmium cations were used as the testing species for voltammetry, and detection of oxygen concentration was used for amperometry. All these experiments require application of highly negative potentials, which is possible to realize with detectors made of silver solid amalgam. The proposed combination of TD+AuxE provides a much greater current response than the arrangement with three individual electrodes. All relevant parameters were optimized for the developed TD+AuxE. The simple and fast measuring protocol for the determination of the zinc content in commercial food supplement tablets has been developed.     

Combinatorial electrochemistry using metal nanoparticles: from proof-of-concept to practical realisation for bromide detection

Principles and practical application of combinatorial electrochemistry in search for new electroactive materials in electroanalysis have been explored. Nanoparticles of three different metals: silver, gold and palladium have been independently synthesized on the glassy carbon spherical powder surface by electroless deposition process and characterized using both spectroscopic and electrochemical techniques. These three materials were then combined together onto basal plane pyrolytic graphite electrode surface and the application of the combinatorial approach to find the electrode material for bromide detection as model target analyte was demonstrated. The component electroactive for bromide detection was next identified and it was found that silver nanoparticles were the active ones. A composite electrode based on silver nanoparticle modified glassy carbon powder and epoxy resin was then fabricated and it was found to allow accurate determination of bromide. The electroactivity for the bromide determination of the composite electrode was compared with that of a bulk silver electrode and it was shown that the composite electrode is very efficient with a comparable electroactivity with only a portion of precious metals being used for its construction.     

Potentiometric determination of mercury(ii) with ethylenediaminetetraacetic acid : Analysis of binary mixtures

Potentiometric determination of mercury(II) with EDTA using silver amalgam as indicator electrode, is suitable for 200 μg to 100 mg of mercury. Binary mixtures of mercury (II) with several other metals can be analysed by different methods involving masking agents, selective pH and differential titrations.     

A Cu/Nafion/Bi electrode for on-site monitoring of trace heavy metals in natural waters using anodic stripping voltammetry: An alternative to mercury-based electrodes

Stripping analysis has been widely recognised as a powerful tool for trace metal analysis. Its remarkable sensitivity is attributed to the combination of a preconcentration step coupled with differential measurements that generate an extremely favourable signal-to-noise ratio. Mercury electrodes have been traditionally employed for achieving high reproducibility and sensitivity of the stripping technique. However, because of the toxicity of mercury, new alternative electrode materials are highly desired, particularly for on-site environmental monitoring of trace pollutants. Bismuth is an electrode material characterized by its low toxicity and its ability to form alloys with some metals of interest like cadmium, lead or zinc, allowing their preconcentration at the electrode surface. We present here the preparation of Cu/Nafion/Bi electrodes and their application to heavy metal analysis by anodic stripping voltammetry. First, the main limitations of the basic Cu/Bi electrode for on-site monitoring in natural waters are highlighted. Then the modification of the Cu/Bi electrode by a Nafion membrane is presented. The analytical performances of this new electrode for trace cadmium and lead analysis were evaluated in non-deaerated solutions. Linear calibration curves were obtained in synthetic solutions for concentrations ranging from 2 to 12 and 2 to 18 μg L⁻¹ for cadmium and lead, respectively, with relative standard deviations lower than 5% (n = 15). The analytical methodology was then successfully applied to monitor the Cd²⁺ and Pb²⁺ content in real samples such as ground water and aquatic plant extracts. The results favourably compared to those obtained using a mercury drop electrode and were validated by ICP-MS.     

Rapidly renewable silver amalgam annular band electrode for voltammetry and polarography

This work describes a novel type of working electrode for use in voltammetry and polarography — the renewable silver liquid amalgam film–modified silver solid amalgam annular band electrode (AgLAF–AgSAE). The electrode is produced by mechanically refreshing the silver liquid amalgam film (AgLAF) before each measurement. The main constituents of the electrode are: a specially constructed silver solid amalgam annular band electrode (AgSAE), two silicon O-rings, silver liquid amalgam and a polypropylene electrode body. Contaminants from the analyzed solution are removed and the AgSAE surface is covered with a thick layer of fresh amalgam while pulling the AgSAE into the sensor body. During movement in the reverse direction AgLAF is formed and homogenized. The time needed to refresh the film is less than 1 s. The electrode is characterized by excellent surface repeatability (～ 1%) and long-term stability (over ten thousand measurement cycles).     

Voltammetric detection of metals and metal compounds on surfaces following a new sampling technique using alkali amalgam droplets

We propose a new method for the voltammetric detection of metals and metal compounds on surfaces of sold materials. The method is based on the reduction of oxidized metal compounds using an alkali amalgam droplet, the dissolution of the metals in the amalgam droplet, finally followed by recording the voltammetric anodic dissolution of the metal. The sample collection and all the following steps, including the voltammetric measurement, are performed at the same site of the sample and without any temporal interruption as one and the same electrode system is used for all steps. The proposed method is as an easy to handle, non-destructive express analysis for a wide range of samples.     

Voltammetric determination of traces of silver in some metals after dissolution of the sample in mercury

The general possibility of analysing metals and alloys by dissolution of the sample in mercury and recording the anodic voltammogram is examined for the determination of silver in some metals. In order to obtain good separation of the silver peak from the anodic limit, acetonitrile is used in the supporting electrolyte. If the main component of the sample is more noble than mercury, analysis is simple and takes ? 20 min. Significant amounts of base metals in samples must be removed from the amalgam prior to the anodic stripping; optimum conditions for the removal are given. The detection limits found for the determination of silver in gold and lead and in indium amalgam are 4 × 10^?3, 4 × 10^?4 and 4 × 10^?6 % respectively. Dissolution of the lead button in mercury seems to be a successful alternative to the cupellation procedure. Silver in mercury does not form intermetallic compounds with gold.     

Lab‐on‐a‐Chip Sensor with Evaporated Bismuth Film Electrode for Anodic Stripping Voltammetry of Zinc

In this work, we report on the development of a lab‐on‐a‐chip electrochemical sensor that uses an evaporated bismuth electrode to detect zinc using square wave anodic stripping voltammetry. The microscale electrochemical cell consists of a bismuth working electrode, an integrated silver/silver chloride reference electrode, and a gold auxiliary electrode. The sensor exhibits a linear response in 0.1 M acetate buffer at pH 6 with zinc concentrations in the 1–30 μM range and a calculated detection limit of 60 nM. The sensor successfully detected zinc in a bovine serum extract and the results were corfirmed by independent AAS measurements. Our results demonstrate the advantageous qualities of this lab‐on‐a‐chip electrochemical sensor for clinical applications, which include small sample volume (µL scale), reduced cost, short response time and high accuracy at low concentrations of analyte.