Admittance measurements of kinetic and adsorption parameters of anodic mercury complex formation with macrobicyclic (222) ligand in propylenecarbonate and dimethylformamide

Admittance measurements were applied to investigation of the charge-transfer rate and mechanism of anodic complex formation between mercury and macrobicyclic ligand (222) as well as to the cathodic reduction of Hg²⁺-(222) complex formed in the bulk. From measurements in PC and DMF using adsorbable and non-adsorbable base electrolyte anions it was shown that the reactant adsorption effects are observable only if adsorption of ClO₄^? and (222) takes place at the same time. Corresponding charge-transfer rates were evaluated and potential dependence of the adsorption capacity for two ligand concentrations was given. At the half-wave potential apparent rate constants k_1/2 listed below were found (data from Fig. 9).     

Anodic behaviour of 2-mercaptoethanol at a mercury electrode

The anodic processes which occur at the mercury electrode in 2-mercaptoethanol solutions are studied by various polarographic techniques (d.c., a.c. and differential pulse), controlled-potential coulometry, cyclic voltammetry and differential capacity curves. Two steps are distinguished in the process: a one-electron charge transfer and a dismutation step leading to the formation of a mercury(II) mercaptide. The final product is isolated and analyzed. Differential-pulse polarography can be used to determine ?10^?4 M 2-mercaptoethanol with a limit of M.     

Electrochemical behavior of kryptand 222 on a mercury electrode

The electrochemical behavior of kryptand 222 at the interface between a mercury electrode and an aqueous solution is studied by the impedance spectroscopy, polarography, and cyclic voltammetry methods. It is established that kryptand 222 possesses high surface activity at this interface. Adsorption parameters of kryptand 222 are found on the basis of the model of two parallel capacitors supplemented by the Frumkin isotherm by the regression analysis method. The potential dependences of the differential capacitance (C,E curves), calculated using these parameters satisfactorily agree with experiment. It is shown that the maximum in the C,E curves in the region of negative potentials is caused by catalytic process of hydrogen evolution.     

Anodic stripping coulometry at a thin-film mercury electrode

Primary coulometric versions of d.c. anodic stripping voltammetry, performed in a microcell at a mercury-plated rotating glassy carbon electrode, are applied to the determination of cadmium and lead. With exhaustive preelectrolysis of the sample solution, 5–100 ng of the metal can be determined from the charge contained in the stripping signal; precision and accuracy are in the 5% range. An extrapolation procedure that requires only partial metal deposition is evaluated.     

Anodic stripping voltammetric determination of bismuth(III) using a Tosflex-coated mercury film electrode

A Tosflex-mercury film electrode (TMFE) was prepared by spin-coating a solution of the perfluorinated anion exchange polymer Tosflex onto a glassy carbon electrode surface followed by electrodeposition of mercury film on this electrode. This electrode was used for the determination of trace bismuth(III) which was preconcentrated onto the TMFE as anionic bismuth(III) complexes with chloride in a chloride medium. The preconcentration was carried out at a potential of-0.2 V, and the preconcentration of the bismuth(III) was enhanced significantly by the anion-exchange feature of Tosflex. The accumulated bismuth(III) was then determined by anodic square-wave stripping voltammetry (SWSV). Various parameters influencing the determination of bismuth(III) were examined in detail. With 2 min accumulation, the analytical signal versus concentration dependence was linear up to 50 ppb, and the detection limit was 0.58 ppb. This modified electrode showed good resistance to the interferences from surface-active compounds and common ions.     

Anodic stripping voltammetry at a nickel-based mercury film electrode

The electrochemical properties of the nickel-based mercury film electrode (Ni-MFE) were investigated with respect to application of the electrode in the anodic stripping voltammetry (a.s.v.) of heavy metal ions. The hydrogen overpotential at the Ni-MFE is higher than those at MFEs based on other metals, and high enough to get quantitative a.s.v. peaks of lead and cadmium. The mercury film of the Ni-MFE is stable both mechanically and chemically; a.s.v. peaks at a Ni-MFE which had been used fifty times within 300 h after its preparation were identical with those at the freshly prepared electrode. With the Ni-MFE, 5 × 10^?10–10^?7 M lead(II) and 2 × 10^?10–10^?7 M cadmium(II) in the solution can be determined with relative standard deviations of 11 and 12%, respectively. These results are comparable to those obtained by a.s.v. at an in situ mercury-plated g]assy carbon electrode.     

Anodic stripping voltammetry of germanium at the hanging mercury drop electrode

The Ge(IV)—Ge(0) system was investigated by cyclic and stripping voltammetry at HMDE in acidic pyrogallol medium and in phosphate, borate and carbonate buffers. It was found that germanium electrodeposited from dilute Ge(IV) solutions dissolved anodically forming two peaks corresponding to the oxidation of the unstable homogeneous and stable heterogeneous amalgams. Both peaks can be exploited analytically for the determination of traces of germanium but due to the complex nature of the germanium amalgam the sensitivity and reproducibility of the determinations are lower compared to the results obtained for metals well-soluble in mercury.     

Anodic stripping determination of mercury in air with a glassy carbon electrode

A technique for stripping determination of mercury traces in air employing a glassy carbon electrode is described. The sample is passed at 2 liters min^?1 for 2 hr through an absorber containing 0.2 M potassium permanganate and 10% $\text{w}\text{v}$ sulfuric acid (1:1). After reduction with hydroxylamine hydrochloride, the determination is carried out in 0.12 M potassium thiocyanate at pH 2.0 ± 0.2 in the presence of 0.2 ppm of cupric ions. Calibration curves were found to be linear in the range 20 ppb-1 ppm Hg(II) in the cell. The accuracy of the method was tested over simulated samples and it was found to be better than 95%; the relative standard deviation was 5% or less. The limit of detection of mercury in air was approximately 10 μg m^?3.     

Electrochemical behaviour of tilmicosin at the hanging mercury drop electrode

The electrochemical behaviour of tilmicosin (TIM) was investigated using cyclic voltammetry and square-wave voltammetry in Britton-Robinson buffers (pH 2–12). The voltammograms of TIM showed one-irreversible peak which attributed to reduction of the >C=C< group in the entire pH values. However, an additional peak was shown over the pH range 2.00 to 4.00. Its peak potential is more positive potential from that of >C=C< group, and its potential was not practically dependent on the pH. On the other hand, the peak potential of >C=C< group was shifted to the more negative values with increasing pH. According to the obtained voltammetric data, the probable reaction mechanism for the reduction of TIM was proposed. Moreover, the adsorption of TIM on the mercury electrode surface by means of square-wave adsorptive stripping voltammetry measurements is studied at different pH values. The peak current variations with the deposition time and TIM concentration were observed.     

Extraction studies of Am(III) and Eu(III) with a tris-bipyridyl macrobicyclic ligand

Complexation of Am(III) with a tris-bipyridine cryptand (L) has been carried out in a nonaqueous medium (CH₃CN–CHCl₃). Subsequently the complexation behaviour was investigated using the reverse extraction tracer technique with dinonyl naphthalenesulphonic acid (HD) in toluene as the organic phase and varying concentration of HCl (upto 2M) as the aqueous phase. Equilibrium is attained in the two-phase system at a rate dependent on the hydrogen ion concentration in the aqueous phase. Whereas it takes only a few minutes to attain the equilibrium state at pH 6.0, a phase contact period of 50 days was insufficient if the acidity is greater than 0.4M, presumably due to the slow dissociation of the cryptate formed. The large enhancement in the distribution ratio value in the synergistic system with 1M HCl as the aqueous phase under non-equilibrium conditions is employed for the analytical separation of Am(III) from Eu(III).     

Electrochemical behaviour of low-potential electron-transferring proteins at the mercury electrode

This paper reports on a pulse polarographic study of low-potential electron-transferring proteins at the mercury electrode. The proteins studied were the negatively charged iron-sulphur cluster containing ferredoxins from spinach, and from Megasphaera elsdenii, the iron-containing rubredoxin and the FMN-containing flavodoxin both from M. elsdenii. Furthermore, the positively charged, four haem-containing cytochrome c₃ from Desulfovibrio vulgaris strain Hildenborough was studied. It was observed that the electrode reaction of these proteins could be made much more efficient when a polymer or surfactant was added, with a charge opposite to the protein. The reduction efficiency of these proteins reaches an optimum when the net charge of the protein times its concentration is about equal to the same amount of opposite charges, which was added as a polymer or surfactant.     

The polarographic behaviour of tungstate at the dropping mercury electrode

Summary The polarographic behaviour of W(VI) is investigated in some supporting electrolytes of varying H⁺ concentration. In HCl solution the reduction yields two waves; the first one is essentially reversible and corresponds to the process W(VI) W(V). The second one is irreversible and composed of two strongly overlapping waves representing the stepwise reduction of W(V) to W(III). In H₂SO₄ solution, three irreversible waves of almost equal heights are observed. The reduction in H₃PO₄ solution indicated that different types of phosphotungstic acid (PTA) are formed which change to either a mixture of 9- and 12-PTA or to 9-PTA only, depending on the H₃PO₄ concentration. In HCl or H₃PO₄ solutions i _l is a linear function of the W(VI) concentration.

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Zusammenfassung Das polarographische Verhalten von Wolframat ist in einigen Trägerlösungen verschiedener H⁺-Konzentration untersucht worden. In HCl-Lösung ergibt die Reduktion zwei Stufen. Die erste Stufe ist reversibel und stellt die Reduktion W(VI) W(V) dar, während die zweite dem Reduktionsverlauf W(V) W(III) entspricht und aus zwei stark über-lappenden Stufen besteht. In H₂SO₄-Lösung sind drei Stufen mit fast gleichen Höhen zu erkennen. In H₃PO₄-Lösung scheint aus dem polarographischen Verhalten von Wolframat hervorzugehen, daß verschiedene Formen der Wolframatophosphorsäure (WPS) gebildet werden, welche sich entweder in eine Mischung der 9- und 12-WPS oder nur in die 9-WPS umwandlen. Die Art der Umwandlung hängt von der H₃PO₄-Konzentration ab. Der Grenzstrom in HCl- und H₃PO₄-Lösungen ist eine lineare Funktion der W(VI)-Konzentration.

     

A highly selective mercury electrode based on a diamine donor ligand

(H₂NCHMeCH₂NH₂)(H₂O)₂HgCl₂ (I) was synthesised, characterised and used for the fabrication of a potentiometric sensor for Hg²⁺ metal ions. Membrane having I as electroactive material, sodium tetraphenyl borate (NaTPB) as an anion excluder, dibutylamine (DBA) as plasticizer in PVC matrix in the percentage ratio of 10:3:150:150 (I:NaTPB:DBA:PVC) (w/w) exhibits a linear response to Hg²⁺ ions in a concentration range of 1.25 × 10⁻⁵ to 1.0 × 10⁻¹ M having a detection limit of 8.9 × 10⁻⁶ with a slope of 25 ± 0.1 mV over the pH range 6.6-9.3. Selectivity coefficients for Hg(II) relative to a number of interfering ions were investigated. The electrode is highly selective for Hg²⁺ ions over a large number of mono-, bi-, and trivalent cations. Normal interferents like Ag⁺ and Cd²⁺ do not interfere in the working of the electrode. The electrode has also been used successfully in mixtures having a 10% (v/v) methanol and acetone content without showing any considerable change in working concentration range or slope. These electrodes have been found to be chemically inert showing a fast response time of 10 s and were used over a period of 4 months with good reproducibility (s = ±0.2). The electrode was used for determination of mercury in binary mixtures with 100% recovery and thus the proposed sensor can be used for real sample analysis.     

Electrodeposition of mercury on glassy carbon electrode from very dilute mercury(II) solution

Flameless atomic absorption sepctrometry (AAS) has been applied to the investigation of the electroreduction of mercury at the glassy carbon (GC) electrode in dilute mercury(II) solution. The atomic mercury which is produced by electrolysis is found both in the electrolyte solution and on the electrode. The evaporation experiment combined with the flameless AAS clearly shows that mercury(0) deposits on the GC electrode as metallic mercury and adatoms depending strongly on the concentration of mercury(II) in the solution. The monolayer formation and underpotential deposition cannot be observed in the mercury(II)/GC electrode system.     

Anodic stripping voltammetry at a micro-disc mercury film electrode: Theory of the reversible case

For p=(ar²/D_O)^1/2<0.12 and H=al²/D_R<0.14, simple theoretical equations are proposed to characterize the reversible anodic stripping voltammetric peaks of amalgams obtained with a micro-disc mercury film electrode. The calculated, normalized peaks were higher, thinner and shifted towards more negative potentials compared with the peaks described for the regular size mercury film electrodes. The ADI method was used in calculations.     

Lead selective membrane electrode using cryptand(222) neutral carrier

The construction, performance characteristics and applications of a polymeric membrance electrode for lead(II) ion are reported. The electrode was prepared by incorporating cryptand(222) as the neutral carrier into a plasticized poly(vinyl chloride) membrane. The influence of membrane composition, pH and concentration of internal reference solution were investigated. The electrode exhibits a potentiometric response for Pb²⁺ ion over the concentration range 10^–1–10^–5 M with a detection limit of 5 × 10^–6 M Pb²⁺. It shows a relatively fast response time of about 30 s and can be used for about two months without any considerable divergence in potential. The proposed sensor revealed good selectivities for Pb²⁺ in the presence of several metal ions and could be used in the pH range of 2.5 to 7.5. It was used as an indicator electrode in the potentiometric titration of Pb²⁺ with EDTA. Received: 20 November 1998 / Revised: 26 March 1999 / Accepted: 31 March 1999     

Anodic stripping voltammetric determination of mercury by use of a sodium montmorillonite-modified carbon-paste electrode

A sodium montmorillonite (SWy-2)-modified carbon-paste electrode has been examined for determination of trace levels of mercury. Because of its strong cation-exchange and adsorptive characteristics, SWy-2 greatly improves the sensitivity of determination of Hg(2+). Hg(2+) is preconcentrated and reduced on the modified electrode surface at -0.40 V and then stripped from the electrode surface during the positive potential sweep. The conditions used for determination, e.g. supporting electrolyte, pH, amount of SWy-2, accumulation potential, and accumulation time, were optimized. The peak current was linearly dependent on the concentration of mercury from 1 x 10(-9) to 5 x 10(-7) mol L(-1). The detection limit (signal-to-noise ratio=3) was 1 x 10(-10) mol L(-1) after accumulation for 6 min. When the SWy-2-modified carbon-paste electrode was used to detect mercury in water samples the average recovery was 101.11%.     

Lead selective membrane electrode using cryptand(222) neutral carrier

The construction, performance characteristics and applications of a polymeric membrance electrode for lead(II) ion are reported. The electrode was prepared by incorporating cryptand(222) as the neutral carrier into a plasticized poly(vinyl chloride) membrane. The influence of membrane composition, pH and concentration of internal reference solution were investigated. The electrode exhibits a potentiometric response for Pb²⁺ ion over the concentration range 10^–1–10^–5 M with a detection limit of 5 × 10^–6 M Pb²⁺. It shows a relatively fast response time of about 30 s and can be used for about two months without any considerable divergence in potential. The proposed sensor revealed good selectivities for Pb²⁺ in the presence of several metal ions and could be used in the pH range of 2.5 to 7.5. It was used as an indicator electrode in the potentiometric titration of Pb²⁺ with EDTA. Received: 20 November 1998 / Revised: 26 March 1999 / Accepted: 31 March 1999