Voltammetric Determination of 4‐Nitrophenol Using a Novel Type of Silver Amalgam Paste Electrode

A differential pulse voltammetric (DPV) method was developed for the determination of 4‐nitrophenol (4‐NP) at a newly developed silver amalgam paste electrode (AgA‐PE) in Britton–Robinson buffer pH 3.0. The electrode is based on a disposable plastic pipette tip filled with paste amalgam based on a mixture of mercury and fine silver powder (9 : 1, w/w). The experimental parameters, such as pH of Britton–Robinson buffer and activation and regeneration potential of the electrode surface were optimized. The reduction peak current dependences were linear for the concentration of 4‐NP from 0.2 to 100 μM. The method showed reproducible results with RSD (n=45) of 1.7%. The limit of determination (LOD) was 0.3 μM. The method was successfully applied for the direct determination of 4‐NP in drinking water.     

Electrodes of Nontoxic Solid Amalgams for Electrochemical Measurements

The preparation, activation and electrochemical pretreatment of electrodes based on nontoxic solid amalgams were described. Testing of metal solid amalgam electrodes (MeSAEs) proved their broad applicability in many respects, e.g., as to the range of working potentials and the level of background currents, well comparable with those of the hanging mercury drop electrode (HMDE). A regeneration of their surfaces before each measurements could be simply automatized using a PC‐controlled system providing a reasonable repeatability of voltammetric measurements down to 3% RSD. Combination with stripping techniques at accumulation times t_ac=300 s the detection limit amounted to the concentration level of 1 ppb Cu(II), Pb(II), Cd(II), Zn(II), etc. Best electrochemical properties were exhibited by the silver solid amalgam electrode (AgSAE). For example, polished AgSAE (p‐AgSAE), completely free of liquid mercury, proved satisfactory even at more negative potentials enabling the determination of Zn(II), Mn(II), IO , etc. Moreover, even better repeatability of mercury meniscus modified AgSAE (m‐AgSAE) was due to better quality and renewability of its surface. In many cases further testing confirmed that under appropriate conditions MeSAEs represent good, often cheaper and more users‐friendly alternatives to HMDE.     

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.     

A Novel Voltammetric Method for the Determination of Maleic Acid Using Silver Amalgam Paste Electrode

An efficient voltammetric method was developed for the determination of maleic acid at a silver amalgam paste electrode (AgA‐PE) in Britton–Robinson buffer pH 2.0. The experimental parameters, such as pH of Britton–Robinson buffer, type of the supporting electrolyte and activation of the electrode surface were optimized. Under the optimal conditions, a linear response was observed over the 2×10^?6–1×10^?4 mol L^?1 maleic acid concentration range, determination limit being 5×10^?7 mol L^?1. A highly stable response, with a relative standard deviation (RSD) of 1.6% for 45 repetitive measurements of 1×10^?4 mol L^?1 maleic acid showed that there was no apparent surface passivation indicating the suitability of the method. The method was successfully applied for direct determination of maleic acid in drinking and river water.     

Cathodic stripping voltammetry of cysteine using silver and copper solid amalgam electrodes

Silver and copper solid amalgam electrodes (modified with mercury meniscus and based on amalgamation of fine metallic powder) have been successfully tested for cathodic stripping voltammetry of cysteine. In the case of the silver solid amalgam electrode AgSAE the relative standard deviation (RSD) and the detection limit (3 SD) reached +/-2.3% and 3x10(-9) mol l(-1) cysteine, respectively.     

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.     

Detecting DNA Damage with a Silver Solid Amalgam Electrode

Mercury electrodes modified with supercoiled (sc) DNA have been used as highly sensitive tools for the detection of DNA strand breaks or as sensors for DNA cleaving substances. In this paper we show that silver solid amalgam electrode (AgSAE), in connection with alternating current voltammetry, provides similar information about DNA damage as the hanging mercury drop electrode. The AgSAE can be used for the detection of enzymatic or chemical DNA cleavage in solution or at the electrode surface. AgSAE modified with scDNA can be utilized as a sensor for DNA nicking substances.     

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.     

Voltammetric Determination of Azidothymidine Using Silver Solid Amalgam Electrodes

A new simple and direct electroanalytical method was developed for the determination of azidothymidine in commercial pharmaceutical preparations. It is based on differential pulse voltammetry at silver solid amalgam electrode with polished surface (p‐AgSAE) or surface modified by mercury meniscus (m‐AgSAE). The electroreduction of azidothymidine in basic media at these electrodes gives rise to one irreversible cathodic peak. Its potential in 0.05 mol L^?1 borate buffer, pH 9.3 at ca. ?1050 mV is comparable to that using hanging mercury drop electrode (HMDE). Achieved limits of quantitation are in the 10^?7 mol L^?1 concentration range for both amalgam electrodes. According to the procedure based on the standard addition technique, the recoveries of known amounts of azidothymidine contained in pharmaceutical preparations available in capsules were 101.4±1.8% (m‐AgSAE), 100.3±3.5% (p‐AgSAE) and 102.0±1.0% (HMDE) (n=10). There was no significant difference between the values gained by proposed voltammetric methods and the HPLC‐UV recommended by the United States Pharmacopoeia regarding the mean values and standard deviations.     

Nafion Coated Silver Amalgam Electrode for Determination of Trace Metals by Anodic Stripping Voltammetry

This paper describes characterization and application of Nafion coated solid silver amalgam electrodes to prevent surface fouling of surfactants in determination of trace metals by differential pulse anodic stripping voltammetry (DPASV). Polymer films of different thickness were tested using Nafion solutions in the range 0.25%–1%. Optimum thickness was archived using a 0.5% Nafion solution, resulting in both increased response and stability over time compared to uncoated electrodes. The influence of model surface active macromolecules was studied using triton X‐100, sodium dodecyl sulfate, dodecyl pyridinium chloride and bovine serum albumin as representatives for surfactants. The resistance to surfactants makes the studied Nafion coated solid silver amalgam electrodes an interesting alternative for practical use in environmental monitoring.     

Voltammetric Monitoring of Bivalent Iron in Waters and Effluents,Using a Dental Amalgam Sensor Electrode. Some Preliminary Results

A very sensitive method for detection of iron(II) in the ng/L range with a deposition time of 180 s by use of differential pulse anodic stripping voltammetry on a novel dental amalgam electrode is introduced and tested in different supporting media. A well‐defined peak for the oxidation of iron metal to ferrous ion was observed around.     

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.     

A Highly Sensitive Method for Determination of Monobutyltin in Aqueous Media by Stripping Voltammetry at a Renewable Silver Amalgam Annular Band Electrode

An adsorptive stripping voltammetric (AdSV) procedure for the determination of monobutyltin in aqueous media at a silver liquid amalgam film‐modified silver solid amalgam annular band electrode (AgLAF? AgSAE) is described. Determination of monobutyltin proceeds in two steps. At the beginning monobutyltin ions (BuSn³⁺) are accumulated from 0.1 M NH₄NO₃ and 10 % ethanol solution at a potential of ?0.2 V, than the BuSn⁰ film is preconcentrated at the working electrode surface at a potential of ?0.7 V. After this step the DP AdSV voltammogram is recorded. The analytical parameters and the procedure of the electrode regeneration and activation were optimized. The calibration curve of monobutyltin in the range 0.02–0.30 mg L^?1 is linear (r=0.9973). The detection limit for 5 s of preconcentration, calculated as 3σ of the blank was equal to 0.004 mg L^?1, repeatability of the peak current was 1.8 % (n=5). Repeatability and sensitivity of monobutyltin determination depends strongly on the analyzed solution properties, measurement conditions and the working electrode quality. The proposed procedure was tested by means of monobutyltin determination in tap waters.     

Voltammetry using a dental amalgam electrode for heavy metal monitoring of wines and spirits

We have introduced a non-toxic electrode material similar to dental amalgam for use in voltammetry. Its electrochemical properties are like a silver electrode. However, it possesses a higher overvoltage towards hydrogen than silver, and therefore enables detection of metals like zinc, nickel and cobalt. As such solid electrodes are found to give stable results over several weeks, without any maintenance, and because this method greatly facilitates monitoring of heavy metals, attempts to apply such methods to various samples have been are carried out. The present paper deals with the determination of zinc and lead at nanogram per milliliter levels in wines and spirits with only minor treatment of the samples. The procedure may easily be adapted to continuous monitoring.We have previously found that audible sound may greatly increase the voltammetric signal using liquid mercury as well as silver as electrode material. This is also applied to the actual systems.Finally, model determinations of thallium in brandy with the dental amalgam electrode are compared with atomic absorption spectrometric (AAS) measurements. It was found that the electrode could be used repeatedly, without fouling, and with results close to those found by the AAS method.     

Copper Solid Amalgam Electrodes

This work describes the behavior of copper solid amalgam electrodes (CuSAE). The applied potential range has been compared with that of the silver solid amalgam electrode (AgSAE) and the hanging mercury drop electrode (HMDE). In 0.05 M tetraborate buffer the applicable potential range of CuSAE is+0.945 V to ?1.75 V excluding ?0.2 V to ?0.5 V, where the anodic oxidation of copper occurs. CuSAE does not need other than electrochemical pre‐treatment, which has been documented by the evaluated repeatability of eleven voltammetric curves of Cd²⁺ (0.1 ppm), Pb²⁺ (0.1 ppm) and Mn²⁺ (0.5 ppm). The obtained results showed that CuSAE could substitute the solid copper, amalgamed copper or liquid copper amalgam electrodes, and can be applied for the study of systems needing an addition of Cu²⁺ ions into the measured solution.     

Voltammetric Determination of 4‐Nitrophenol and 5‐Nitrobenzimidazole Using Different Types of Silver Solid Amalgam Electrodes – A Comparative Study

The voltammetric behavior of two genotoxic nitro compounds (4‐nitrophenol and 5‐nitrobenzimidazole) has been investigated using direct current voltammetry (DCV) and differential pulse voltammetry (DPV) at a polished silver solid amalgam electrode (p‐AgSAE), a mercury meniscus modified silver solid amalgam electrode (m‐AgSAE), and a mercury film modified silver solid amalgam electrode (MF‐AgSAE). The optimum conditions have been evaluated for their determination in Britton‐Robinson buffer solutions. The limit of quantification (L_Q) for 5‐nitrobenzimidazole at p‐AgSAE was 0.77 µmol L^?1 (DCV) and 0.47 µmol L^?1 (DPV), at m‐AgSAE it was 0.32 µmol L^?1 (DCV) and 0.16 µmol L^?1 (DPV), and at MF‐AgSAE it was 0.97 µmol L^?1 (DCV) and 0.70 µmol L^?1 (DPV). For 4‐nitrophenol at p‐AgSAE, L_Q was 0.37 µmol L^?1 (DCV) and 0.32 µmol L^?1 (DPV), at m‐AgSAE it was 0.14 µmol L^?1 (DCV) and 0.1 µmol L^?1 (DPV), and at MF‐AgSAE, it was 0.87 µmol L^?1 (DCV) and 0.37 µmol L^?1 (DPV). Thorough comparative studies have shown that m‐AgSAE is the best sensor for voltammetric determination of the two model genotoxic compounds because it gives the lowest L_Q, is easier to prepare, and its surface can be easily renewed both chemically (by new amalgamation) and/or electrochemically (by imposition of cleaning pulses). The practical applicability of the newly developed methods was verified on model samples of drinking water.     

Voltammetric Determination of Trace Amounts of 2‐Methyl‐4,6‐Dinitrophenol at a Silver Solid Amalgam Electrode

The voltammetric behavior of 2‐methyl‐4,6‐dinitrophenol was investigated by differential pulse voltammetry (DPV) at a nontoxic mercury meniscus‐modified silver solid amalgam electrode (m‐AgSAE). Conditions have been found for its determination by DPV at m‐AgSAE in the concentration range of 0.2 to 1 μmol L^?1.     

Square Wave Stripping Voltammetry of Unlabeled Single‐ and Double‐Stranded DNAs

We show that, in difference to previously applied electrochemical methods working with stationary electrodes, square wave voltammetry produces well‐developed peaks II_SW (specific for dsDNA) and III_SW yielded by ssDNA at hanging mercury drop electrode (HMDE) and solid amalgam electrodes (SAEs). Using these peaks various kinds of DNA structural transitions can be studied, including unwinding of dsDNA at negatively charged electrode surfaces. The sensitivity of the DNA analysis is much better than that obtained with guanine oxidation signals at carbon electrodes. Both carbon electrodes and SAEs appear attractive as transducers in label‐free RNA and DNA sensors.     

Investigation of protein FTT1103 electroactivity using carbon and mercury electrodes. Surface-inhibition approach for disulfide oxidoreductases using silver amalgam powder

Recently, it was shown that electrochemical methods can be used for analysis of poorly water-soluble proteins and for study of their structural changes and intermolecular (protein–ligand) interactions. In this study, we focused on complex electrochemical investigation of recombinant protein FTT1103, a disulfide oxidoreductase with structural similarity to well described DsbA proteins. This thioredoxin-like periplasmic lipoprotein plays an important role in virulence of bacteria Francisella tularensis. For electrochemical analyses, adsorptive transfer (ex situ) square-wave voltammetry with pyrolytic graphite electrode, and alternating-current voltammetry and constant-current chronopotentiometric stripping analysis with mercury electrodes, including silver solid amalgam electrode (AgSAE) were used. AgSAE was used in poorly water-soluble protein analysis for the first time. In addition to basic redox, electrocatalytic and adsorption/desorption characterization of FTT1103, electrochemical methods were also used for sensitive determination of the protein at nanomolar level and study of its interaction with surface of AgSA microparticles. Proposed electrochemical protocol and AgSA surface-inhibition approach presented here could be used in future for biochemical studies focused on proteins associated with membranes as well as on those with disulfide oxidoreductase activity.     

Mercury meniscus on solid silver amalgam electrode as a sensitive electrochemical sensor for tetrachlorvinphos

The in-house prepared mercury meniscus modified solid silver amalgam electrode (m-AgSAE) was successfully applied for the detection of organophosphate pesticide tetrachlorvinphos in pH 7 buffer solution. The electrochemical performance of m-AgSAE for the reduction of tetrachlorvinphos was evaluated using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV), respectively. The surface morphology of solid silver electrode (AgE), as-amalgamated solid silver amalgam electrode (AgSAE), and polished solid silver amalgam electrode (p-AgSAE) was examined by field emission scanning electron microscopy (FESEM). Among the applied techniques, DPV and SWV analysis showed a remarkable increase in the reduction peak current and provided a simple, fast, and sensitive method for the determination of tetrachlorvinphos. The electrochemical impedance spectroscopy (EIS) was used to correlate the electrocatalytic activity of AgSAE, p-AgSAE and m-AgSAE with their interfacial charge transport capabilities. Under the optimized experimental conditions, the DPV and SWV responses were linear over the 1–9 μM and 10–50 μM concentration ranges with a detection limit of 0.06 μM for DPV and 0.04 for SWV. The estimation of tetrachlorvinphos in the ground and waste water samples with the proposed method was in good agreement with that of the added amount. The proposed electrochemical method not only extends the application of non-toxic m-AgSAE, but also offers new possibilities for fast and sensitive analysis of tetrachlorvinphos and its structural analogs in environmental samples.