Preconcentration and determination of mercury(II) at a chemically modified electrode containing 3-(2-thioimidazolyl)propyl silica gel.

A mercury-sensitive chemically modified graphite paste electrode was constructed by incorporating modified silica gel into a conventional graphite paste electrode. The functional group attached to the (3-chloropropyl) silica gel surface was 2-mercaptoimidazole, giving a new product denoted by 3-(2-thioimidazolyl)propyl silica gel, which is able to complex mercury ions. Mercury was chemically adsorbed on the modified graphite paste electrode containing 3-(2-thioimidazolyl)propyl silica (TIPSG GPE) by immersion in a Hg(II) solution, and the resultant surface was characterized by cyclic and differential pulse anodic stripping voltammetry. One cathodic peak at 0.1 V and other anodic peak at 0.34 V were observed on scanning the potential from -0.1 to 0.8 V (0.01 M KNO3; v = 2.0 mV s(-1) vs. Ag/AgCl). The anodic peak at 0.34 V show an excellent sensitivity for Hg(II) ions in the presence of several foreign ions. A calibration graph covering the concentration range from 0.02 to 2 mg L(-1) was obtained. The detection limit was estimated to be 5 microg L(-1). The precision for six determinations of 0.05 and 0.26 mg L(-1) Hg(II) was 3.0 and 2.5% (relative standard deviation), respectively. The method can be used to determine the concentration of mercury(II) in natural waters contaminated by this metal.     

Stripping Voltammetry of Mercury(II) with a Chemically Modified Carbon Paste Electrode Containing Silica Gel Functionalized with 2,5‐Dimercapto‐1,3,4‐thiadiazole

The accumulation voltammetry of mercury(II) was investigated at a carbon paste electrode chemically modified with silica gel functionalized with 2,5‐dimercapto‐1,3,4‐thiadiazole (DTTPSG‐CPE). The repetitive cyclic voltammogram of mercury(II) solution in the potential range ?0.2 to +0.8 V (vs. Ag/AgCl), (0.02 mol L^?1 KNO₃ ; v=20 mV s^?1) show two peaks one at about 0.0 V and other at 0.31 V. However, the cathodic wave peak, around 0.0 V, is irregular and changes its form in each cycle. This peak at about 0.0 V is the reduction current for mercury(II) accumulated in the DTTPSG‐CPE. The anodic wave peak at 0.31 V is well‐defined and does not change during the cycles. The resultant material was characterized by cyclic and differential pulse anodic stripping voltammetry performed with the electrode in differents supporting electrolytes. The mercury response was evaluated with respect to pH, electrode composition, preconcentration time, mercury concentration, “cleaning” solution, possible interferences and other variables. The precision for six determinations (n=6) of 0.05 and 0.20 mg L^?1 Hg(II) was 2.8 and 2.2% (relative standard deviation), respectively. The method was satisfactory and used to determine the concentration of mercury(II) in natural waters contaminated by this metal.     

Study of an organically modified clay: selective adsorption of heavy metal ions and voltammetric determination of mercury(II)

In this work, a hydrophilic clay, Na-montmorillonite from Wyoming, USA, was rendered organophilic by exchanging the inorganic interlayer cations for hexadecyltrimethylammonium ions (HDTA), with the formulae of [(CH₃)₃N(C₁₆H₃₃)]⁺ ion. Based on fact that organo-clay has high affinities for non-ionic organic molecules, 1,3,4-thiadiazole-2,5-dithiol was loaded on the HDTA-montmorillonite surface, resulting in the 1,3,4-thiadiazole-2,5-dithiol-HDTA-montmorillonite complex (TDD-organo-clay).The following properties of TDD-organo-clay are discussed: selective adsorption of heavy metal ions measured by batch and chromatographic column techniques, and utilization as preconcentration agent in a chemically modified carbon paste electrode (CMCPE) for determination of mercury(II).The main point of this paper is the construction of a selective sensor, a carbon paste electrode modified with TDD-organo-clay, its properties and its application to the determination of mercury(II) ions, as this element belongs to the most toxic metals. The chemical selectivity of this functional group and the selectivity of voltammetry were combined for preconcentration and determination.     

Voltammetric detection of lead(II) and mercury(II) using a carbon paste electrode modified with thiol self-assembled monolayer on mesoporous silica (SAMMS)

The anodic stripping voltammetry at a carbon paste electrode modified with thiol terminated self-assembled monolayer on mesoporous silica (SH-SAMMS) provides a new sensor for simultaneous detection of lead (Pb2+) and mercury (Hg2+) in aqueous solutions. The overall analysis involved a two-step procedure: an accumulation step at open circuit, followed by medium exchange to a pure electrolyte solution for the stripping analysis. Factors affecting the performance of the SH-SAMMS modified electrodes were investigated, including electrode activation and regeneration, electrode composition, preconcentration time, electrolysis time, and composition of electrolysis and stripping media. The most sensitive and reliable electrode contained 20% SH-SAMMS and 80% carbon paste. The optimal operating conditions were a sequence with a 2 min preconcentration period, then a 60 s electrolysis period of the preconcentrated species in 0.2 M nitric acid, followed by square wave anodic stripping voltammetry from -1.0 V to 0.6 V in 0.2 M nitric acid. The areas of the peak responses were linear with respect to metal ion concentrations in the ranges of 10-1500 ppb Pb2+ and 20-1600 ppb Hg2+. The detection limits for Pb2+ and Hg2+ were 0.5 ppb Pb2+ and 3 ppb Hg2+ after a 20 min preconcentration period.     

Coal as a New Carbon Paste Electrode Modifier with Sorption Properties

A new type of carbon paste electrode modified with subbituminous and bituminous coal is presented. The operability of the coal carbon paste electrode with respect to the working potential window attainable was tested in various electrolytes. Cyclic voltammetry of the reference redox system [Fe(CN)₆]^3?/4? was performed to evaluate electron transfer kinetics. Open‐circuit sorption of Cd(II), Pb(II), and Cu(II) with subsequent anodic stripping voltammetry was used to pilot coal sorption ability. The coal modified carbon paste electrode was also examined as a support for mercury film deposition and anodic stripping voltammetry of metals.     

Electrodeposition of mercury film on electrodes modified with clay minerals

A new type of carbon paste electrode modified with clay mineral and covered with a mercury film is presented in this work. Electrodeposition of the mercury film was performed on the carbon paste electrode modified with montmorillonite. The mercury film was deposited by both electrodeposition in situ and a preliminary electrodeposition. The pre-deposited film of mercury showed to be suitable for anodic stripping voltammetry. An open-circuit sorption of Cd, Pb, and Cu with subsequent anodic stripping voltammetry exhibited higher current responses of metals. Besides the enhanced sensitivity superior separation of the current responses during a simultaneous stripping of metals is expected to be achieved by means of the newly prepared electrode. Presented at the 57th Congress of Chemical Societies, Tatranské Matliare, 4–8 September 2005.     

Voltammetric determination of trace amounts of mercury with a carbon paste electrode modified with an anion-exchanger

Summary A carbon paste electrode modified with a liquid anion exchanger (Amberlite LA2) was used for the voltammetric determination of mercury(II). Mercury is preconcentrated, as tetrachloromercurate(II), onto the surface of the modified electrode only by the ion-exchange effect of the modifier without application of potential. After exchange of the medium the accumulated amount of mercury(II) is determined by differential pulse anodic stripping voltammetry in a blank electrolyte solution. The response depends on the concentration of mercury in the bulk solution, preconcentration time, and other parameters. The detection limit was 1 g Hg(II)/l when a suitable time for preconcentration was chosen. Preconcentration for 5 min yields a linear calibration graph for concentrations up to 1000 g Hg(II)/l. The effect of other ions on the determination of mercury and the applicability of the method to the analysis of phenylmercury compounds in pharmaceutical preparations were investigated.     

Fabrication and application of a new modified electrochemical sensor using nano-silica and a newly synthesized Schiff base for simultaneous determination of Cd,Cu and Hg ions in water and some foodstuff samples

A new chemically modified carbon paste electrode was constructed and used for rapid, simple, accurate, selective and highly sensitive simultaneous determination of cadmium, copper and mercury using square wave anodic stripping voltammetry (SWASV). The carbon paste electrode was modified by N,N′-bis(3-(2-thenylidenimino)propyl)piperazine coated silica nanoparticles. Compared with carbon paste electrode, the stripping peak currents had a significant increase at the modified electrode. Under the optimized conditions (deposition potential, −1.100 V vs. Ag/AgCl; deposition time, 60 s; resting time, 10 s; SW frequency, 25 Hz; pulse amplitude, 0.15 V; dc voltage step height, 4.4 mV), the detection limit was 0.3, 0.1 and 0.05 ng mL⁻¹ for the determination of Cd²⁺, Cu²⁺ and Hg²⁺, respectively. The complexation reaction of the ligand with several metal cations in methanol was studied and the stability constants of the complexes were obtained. The effects of different cations and anions on the simultaneous determination of metal ions were studied and it was found that the electrode is highly selective for the simultaneous determination of Cd²⁺, Cu²⁺ and Hg²⁺. Furthermore, the present method was applied to the determination of Cd²⁺, Cu²⁺ and Hg²⁺ in water and some foodstuff samples.     

Effect of the modification of mercuric oxide on the properties of mercury films at HgO-modified carbon paste electrodes

The influence of modifications of the mercuric oxide on the voltammetric properties of mercury film carbon paste electrodes was studied. The mercury film was formed electrochemically from the bulk red or yellow mercuric oxide-modified carbon paste electrodes. Differential pulse anodic stripping voltammetry and optical microscopy in polarised light were used to characterise the properties of the mercury films. The results were compared with those obtained using the conventional preparation of mercury-plated carbon paste electrodes when the mercury film is deposited on the surface of the electrode by reduction of Hg(II) ions in solution utilising a sufficient negative potential. It was shown that the mercury film formed from the yellow modification of the mercuric oxide provides better voltammetric characteristics than the red one owing to the high distribution of its small particles, i.e. the mercury droplets after electrochemical treatment. Such a mercury film has similar properties to those of a mercury film generated from solution. Received: 06 December 1999 / Accepted: 16 March 2000     

Evaluation of a carbon paste electrode modified with organofunctionalised SBA-15 nanostructured silica in the simultaneous determination of divalent lead, copper and mercury ions

The performance of a carbon paste electrode (CPE) modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol in the simultaneous determination of Pb(II), Cu(II) and Hg(II) ions in natural water and sugar cane spirit (cacha?a) is described. Pb(II), Cu(II) and Hg(II) were pre-concentrated on the surface of the modified electrode by complexing with 2-benzothiazolethiol and reduced at a negative potential (-0.80 V). Then the reduced products were oxidised by DPASV procedure. The fact that three stripping peaks appeared on the voltammograms at the potentials of -0.48 V (Pb2+), -0.03 V (Cu2+) and +0.36 V (Hg2+) in relation to the SCE, demonstrates the possibility of simultaneous determination of Pb2+, Cu2+ and Hg2+. The best results were obtained under the following optimised conditions: 100 mV pulse amplitude, 3 min accumulation time, 25 mV s(-1) scan rate in phosphate solution pH 3.0. Using such parameters, calibration graphs were linear in the concentration ranges of 3.00-70.0 x 10(-7) mol L(-1) (Pb2+), 8.00-100.0 x 10(-7) mol L(-1) (Cu2+) and 2.00-10.0 x 10(-6) mol L(-1) (Hg2+). Detection limits of 4.0 x 10(-8) mol L(-1) (Pb2+), 2.0 x 10(-7) mol L(-1) (Cu2+) and 4.0 x 10(-7) mol L(-1) (Hg2+) were obtained at the signal noise ratio (SNR) of 3. The results indicate that this electrode is sensitive and effective for simultaneous determination of Pb2+, Cu2+ and Hg2+ in the analysed samples.     

Sensitive Voltammetric Determination of Cadmium(II) with 8‐Hydroxyquinoline and Ionic Liquid Modified Carbon Paste Electrode

In this paper 8‐hydroxyquinoline (HQ) and ionic liquid (IL) modified carbon paste electrode was fabricated and used for the sensitive determination of cadmium(II) with differential pulse anodic stripping voltammetry (DPASV). The modified electrode was prepared by the addition of HQ and IL 1‐ethyl‐3‐methylimidazoliam ethylsulphate as the modifiers into the traditional carbon paste mixture. Cd(II) was preconcentrated and reduced on the surface of the modified electrode at the potential of ‐1.0 V (vs. SCE) by the co‐contributions from the formation of HQ‐Cd(II) complex and the accumulation effect of IL. Then the reduced Cd on the electrode surface was reoxidized by DPASV with a sensitive oxidation peak appeared at ‐0.79 V (vs. SCE). Under the optimal conditions the oxidation peak current was proportional to the Cd(II) concentration in the range from 0.03 to 2.0 mol/L with the detection limit as 5.0 nmol/L (3σ). The proposed method was successfully applied to the water samples detection with the recovery in the range from 95.6% to 96.6%.     

Palladium Particles‐Impregnated Natural Phosphate Electrodes for Electrochemical Determination of Mercury in Ambient Water Samples

We present here a simple procedure for the determination of mercury(II) using differential pulse anodic stripping voltammetry (DPASV) at palladium particles‐impregnated natural phosphate modified carbon paste electrodes (Pd‐NP‐CPE). The surface of modified electrode was characterized using SEM, infrared spectroscopy, X‐ray diffraction and electrochemical analysis. All experimental variables involved in the voltammetric stripping method were optimized. The detection limit was found to be 4.99×10^?8 mol L^?1 (S/N=3) that is not different to the permitted value for Hg(II) in water reported by the Environmental Protection Agency (EPA). The proposed electrode exhibits good applicability for monitoring Hg(II) in tap and wastewater.     

Quantitative Studies of Metal Ion Adsorption on a Chemically Modified Carbon Surface: Adsorption of Cd(II) and Hg(II) on Glutathione Modified Carbon

The adsorption behavior of model toxic metal cations namely Cd(II) and Hg(II) on carbon surfaces chemically modified by glutathione was investigated as a function of the concentration of Cd²⁺ and Hg²⁺ ions, time and the amount of modified carbon used. Square wave and linear sweep anodic stripping voltammetry was used to monitor the uptake of Cd(II) and Hg(II) ions respectively. Kinetic and adsorption isotherm studies reveal that both Cd(II) and Hg(II) ions undergo similar large adsorption with the modified glutathione carbon material (Glu‐carbon).     

Voltammetric study of the redox behaviour of the Hg(II)/Hg(I)/Hg system at a rotating metal-ring/glassy-carbon disc electrode

The reduction of Hg(II) at a glassy-carbon electrode in various electrolytes has been studied by rotating ring-disc voltammetry. Reduction proceeds directly to metallic mercury in a single 2-electron step. However, at the foot of the wave, and only during the first reduction sweep after pretreatment of the electrode surface, a small amount of Hg(I) species is detected at the ring. The appearance of an Hg(I) intermediate is most pronounced in sulphuric acid solution. The reduction of Hg(II) is found to proceed irreversibly and to be of first order. At sufficiently negative potentials the reduction is convective-diffusion controlled. Stripping voltammetric experiments indicate that the dissolution of mercury gives Hg(II) in complexing electrolytes. In non-complexing electrolytes the initially formed Hg(II) reacts with mercury atoms on the electrode surface to give Hg(I). During electrodissolution, two stripping peaks may be observed as a result of underpotential adsorption of mercury on glassy carbon. The difference in peak potential between the adsorption (mono) layer peak and the bulk mercury peak has been related to the difference in work functions of the deposit (mercury) and substrate (carbon). A rotating glassy-carbon electrode has been used for the anodic stripping determination of mercury. When an appropriate amount of a cation such as cadmium(II) or copper(II) is added to the test solution, mercury down to 2 x 10(-9)M (0.4 ng ml ) can be determined in acidified thiocyanate electrolyte with a relative standard deviation of about 22%.     

A sensitive mercury (II) sensor based on CuO nanoshuttles/poly(thionine) modified glassy carbon electrode

Shuttle-like copper oxide (CuO) was prepared by a hydrothermal decomposition process. The resulting material was characterized by scanning electron microscopy and X-ray diffraction. It was then immobilized on the surface of a glassy carbon electrode modified with a film of poly(thionine). A pair of well-defined and reversible redox peaks for Hg(II) was observed with the resulting electrode in pH 7.0 solutions. The anodic and cathodic peak potentials occurred at 0.260 V and 0.220 V (vs. Ag/AgCl), respectively. The modified electrode displayed excellent amperometric response to Hg(II), with a linear range from 40 nM to 5.0 mM and a detection limit of 8.5 nM at a signal-to-noise ratio of 3. The sensor exhibited high selectivity and reproducibility and was successfully applied to the determination of Hg(II) in water samples.     

Determination of traces of metals by anodic stripping voltammetry at a rotating glassy carbon ring—disc electrode: Part 1. Method and instrumentation with evaluation of some parameters

The equipment and procedure are described for the determination without preconcentration of several heavy metals based on d.c. anodic stripping voltammetry at a rotating ring—disc glassy carbon electrode with in situ mercury plating. During stripping of the metals deposited on the disc, the current from the reduction of the ions collected at the ring is measured. Some parameters (scan rate, thickness of the mercury film, electrode rotation and deposition time) influencing the ring collection peak current are examined experimentally. The results are compared with the theoretical considerations given by de Vries and van Dalen for anodic stripping voltammetry on a stationary mercury film electrode and by Bakanov et al. for a rotating mercury film electrode.     

Voltammetric determination of mercury(II) at a carbon paste electrode in aqueous solutions containing tetraphenylborate ion

The determination of mercury(II) ions can be achieved by monitoring the decrease in the oxidation peak of the tetraphenylborate ion in the presence of this metal ion at a carbon paste electrode. The reaction between mercury(II) and the tetraphenylborate ion results in the formation of diphenylmercury, thus providing the method with good selectivity over other metal ions. Using anodic stripping voltammetry in a neutral electrolyte, a linear dependence of the decrease of peak height was observed on increasing the mercury(II) concentration in the range 1 x 10(-6)-8 x 10(-9)M mercury(II). Zinc(II), cadmium(II), lead(II), nickel(II), cobalt(II), tin(II), potassium(I) and ammonium(I) ions did not interfere at a 1000-fold concentration excess. Iron(III) and chromium(III) did not interfere at a 250-fold and 50-fold concentration excess, respectively. Following masking procedures, copper(II), bismuth(III) and silver(I) did not interfere at a 100-fold concentration excess. The method can be used to determine the concentration of mercury(II) in natural waters contaminated by this metal.     

Stripping voltammetric determination of trace amounts of mercury using a carbon paste electrode modified with 2-mercapto-4(3H)-quinazolinone

A carbon paste electrode modified with 2-mercapto-4(3H)-quinazolinone was used for the voltammetric determination of mercury(II). Mercury was preconcentrated onto the surface of the modified electrode only by the complexing effect of the modifier without application of potential (i.e. in open-circuit conditions). After exchange of the medium, the accumulated amount of mercury(II) was determined by differential pulse anodic stripping voltammetry. The response depended on the concentration of mercury in the bulk solution, preconcentration time, and other parameters. The detection limit was 0.1 g 1^–1 Hg(II) for a preconcentration time of 15 min. Preconcentration for suitable times yielded a linear calibration graph from 0.5 to 6000 g 1^–1 Hg(II). For multiple determinations (5 runs), the relative standard deviation was 5% for a concentration of 100 g 1^–1 Hg(II). The proposed procedure was used to determine trace mercury in plant and sewage sludge samples with good results.On leave from Hainan University, Hainan Peoples Republic of China     

Stripping voltammetric determination of mercury(II) at antimony-coated carbon paste electrode

A new procedure was elaborated to determine mercury(II) using an anodic stripping square-wave voltammetry at the antimony film carbon paste electrode (SbF-CPE). In highly acidic medium of 1 M hydrochloric acid, voltammetric measurements can be realized in a wide potential window. Presence of cadmium(II) allows to separate peaks of Hg(II) and Sb(III) and apparently catalyses reoxidation of electrolytically accumulated mercury, thus allowing its determination at ppb levels. Calibration dependence was linear up to 100 ppb Hg with a detection limit of 1.3 ppb. Applicability of the method was tested on the real river water sample.     

Construction of Modified Carbon Paste Electrode for Highly Sensitive Simultaneous Electrochemical Determination of Trace Amounts of Copper (II) and Cadmium (II)

A chemically modified electrode was constructed for rapid, simple, accurate, selective and highly sensitive simultaneous determination of Cu(II) and Cd(II) using square wave anodic stripping voltammetry. The electrode was prepared by incorporation of SiO₂ nanoparticles, coated with a newly synthesized Schiff base, in carbon paste electrode. The limit of detection was found to be 0.28 ng mL^?1 and 0.54 ng mL^?1 for Cu(II) and Cd(II), respectively. The proposed chemically modified electrode was used for the determination of copper and cadmium in several foodstuffs and water samples.