Electrogenerated chemiluminescence in mechanistic investigations of electroorganic reactions: Part III. Reduction of some disulfides at the dropping mercury electrode

In the simultaneous cathodic reduction of diphenyl disulfide (Ph?S?S?Ph) or dibenzoyl disulfide (Ph?CO?S?S?CO?Ph) and a series of fluorescent aromatic hydrocarbons A at the dropping mercury electrode in dimethylformamide the emission of A is observed. This electrogenerated chemiluminescence (ECL) originates from the electron transfer between A^? and the radicals Ph?S or Ph?COS, which are formed in a one-electron reductive cleavage of the disulfide bond by A^?. As an intermediate the anion radical Ph?S?S?Ph^? or Ph?CO?S?S?CO?Ph^? is assumed. In the case of Ph?S?S?Ph the ECL intensity is enhanced by proton donors (water or benzoic acid), which increase the cleavage rate of Ph? S?S?Ph^? in an electrophilic attack by the proton. The relatively negative threshold reduction potential of A (?1.4 to ?1.6 V) for the ECL in comparison with the half-wave potential (?0.85 V) supports a mercury-assisted heterogeneous reduction mechanism of Ph?S?S?Ph. The intensity-potential curves and the intensity—time curves at the mercury drop were measured for different concentrations of Ph?S?S?Ph and A and for different mercury pressures. No luminescence could be detected with o,o′-dinitrodiphenyl disulfide and diethyl disulfide.     

A study on the adsorption voltammetry of cobalt(II)—dime thylglyoxime system

The behaviour of the cobalt complex with dimethylglyoxime (DMG), Co(II)A₂, at the mercury electrode has been investigated in details. The adsorption phenomena have been observed by both normal pulse polarography and voltammetry with linearly changing potential. Experimental results show that, under the condition of adsorption potentials ranging from ?0.60 to ?0.9 eV (vs. S.C.E.), Co(II)A₂ can be adsorbed on the surface of hanging mercury drop electrode (HMDE) very well. The superficial concentrations represents a Langmuir isotherm with both concentration of Co(II)A₂ and the preconcentration time. The superficial concentration equation for adsorption voltammetry, corresponding to the condition of the low coverage of the electrode surface, is deduced. The equation has been verified experimentally. The sensitivity of the proposed method, which has been analysed theoretically, is independent on the scan rate and the surface area of HMDE, but depends on the preconcentration time and the diffusion layer thickness. For the 120 sec accumulation, the lower limit of determination is 1.10^?9 M.     

Dependence of the energy of surface-active substances/metal interaction on their ionization potentials. Evaluation of hydrophilicity of metal

The function Δ(ΔG _A ⁰), which is the difference of Gibbs energies characterizing surface-active substance (surfactant, SAS) adsorption at metal/solution and air/solution surfaces, has been introduced. The equation connecting the function Δ(ΔG _A ⁰) with SAS ionization potential has been obtained using the elementary theory of donor-acceptor interactions. Published experimental data on SAS adsorption at mercury, bismuth and gold have been used for Δ(ΔG _A ⁰) calculation. The dependence of Δ(ΔG _A ⁰) on ionization potentials can be described by an equation derived in this work. It has been demonstrated that the value of the hydrophilicity of gold is much higher than the values for mercury and bismuth. The lifetime of SAS molecules at a metal surface has been estimated. The question of the possibility of theoretica l estimation of standard energies ΔG _A ⁰ characterizing SAS adsorption at a metal/solution surface has been discussed. Received: 9 December 1996 / Accepted: 13 January 1997     

The electrocapillarity of polyacrylates

Summary The electrocapillary properties of polyacrylic acid have been studied by two methods. Exploratory measurements have been made of the effect of the polymer on the differential capacity of a mercury drop in 0.1 m sodium perchlorate. They showed that the polymer was strongly adsorbed over a wide range of potentials but that it did not appear to form a monolayer. The surface excess of polymer obtained from drop weight data showed a maximum at very low concentrations and then a decline at higher concentrations. The bulk of the work was carried out by making surface tension measurements, using a sessile mercury drop, in solutions of a fraction of polyacrylic acid (mol. wt. 7.02×10⁴) in potassium chloride at 0.01, 0.1, 0.2, and 0.5 m at 25°C.The data have been used to evaluate the surface excesses of the polymer and of the inorganic ions. The distribution of K⁺ and Cl^– in the electrical double layer and the contact adsorption of Cl^– on the mercury were very little affected by the presence of the polymer. The surface excess of polymer was always found to be greatest at low concentrations, to decrease steeply at first as the concentration was increased and then to decrease more slowly at higher concentrations.Possible explanations of this behaviour are discussed and it is concluded that the rapid decrease is a consequence of molecular weight dispersion and the stronger adsorption of high molecular weight polymer. The slow decrease in surface excess at higher concentrations may be a result of configurational changes of the polymer molecules.Surface pressure data show that, despite this decrease in the surface excess, the surface coverage reaches a high level at very low polymer concentrations and then continues to increase slowly as the concentration of polymer is increased. This apparent contradiction is due to changes in configuration of the adsorbed polymer molecules. At higher bulk concentrations the chain configurations are more compact and each adsorbed molecule makes more contacts with and so occupies a greater area of the mercury surface than at low concentrations.The conclusion is reached that the surface excess of polymer is mostly contained in a layer probably more than 1000 Å thick. It consists of a concentrated and entangled mass of polymer chains. Relatively few of these chains are in contact with the mercury at any istant. The concentration in this surface layer decreases steadily with increasing distance from the mercury surface and it merges without discontinuity into the bulk solution.With 10 figures in 22 details     

Professor K.S.G. Doss in honour of his reaching his seventieth year

Recent studies have clearly demonstrated the importance of including the sphericity of the dropping mercury electrode in the theoretical analysis of certain polarographic phenomena. Experimental effects of sphericity on second harmonic a.c. polarograms¹ and on polarograms for reversible electrode reactions with amalgam formation^2.3 have been successfully described using a theoretical treatment augmented by computer simulation. The simulation procedure we present here allows one to simulate diffusion phenomena at the growing mercury drop (sphere) electrode without modification of other simulation operations, e.g. computation of surface boundary conditions⁴ or of homogeneous kinetics⁵. We treat two cases: “External” diffusion where the diffusing species, A, is in the solution phase with the boundary condition A_x=0=0 for t>0; and “internal” diffusion where the diffusing species is in the mercury phase with the same boundary condition.     

Determination of copper in sea water by cathodic stripping voltammetry of complexes with catechol

A reduction current is obtained when an aqueous solution of copper and catechol is subjected to differential-pulse cathodic stripping voltammetry (d.p.c.s.v.) because of the reduction of copper(II)—catechol complex ions which adsorb onto the hanging mercury drop electrode (HMDE). The most likely form of the adsorbed complex ions is CuL^2?₂ (L being catechol). A.c. polarographic measurements showed that these complex ions adsorb more strongly onto the drop than free catechol ions. Monolayer adsorption density is obtained at 2.1 × 10^?10 molecules/cm², equivalent to a surface area of 78 A² complex ion, which agrees well with the molecular surface area calculated from the bond lengths. Analytically useful currents are obtained at very low metal concentrations, such as in uncontaminated sea water. The possible interference by other trace metals, major cations, and organic complexing ligands is investigated. Competition for copper ions by natural organic complexing ligands is evident at low concentrations of catechol. Analysis of the dissolved copper concentration in sea water by d.p.c.s.v. at the HMDE (at neutral pH) compares favourably with the d.p.a.s.v. technique at a rotating disk electrode (at low pH) because of the shorter collection period and greater sensitivity.     

Voltammetric properties at pH 14 of electrodes modified by azobenzene polymers

Azobenzene polymers were prepared by condensation of p-phenylazobenzoyl chloride and poly(ethylenimine). Their loadings in electroactive sites range from 5 to 95%. They were adsorbed on a glassy carbon electrode or on a hanging mercury drop electrode (HMDE) whose area could be expanded after the adsorption. The voltammetric behavior of the polymeric films is described at pH 14. The azobenzene sites which are in the vicinity of the electrode surface are electroactive, but the electrochemical reaction does not propagate to the bulk of the coating. When the loading of the polymer is not too high, the expansion of the HMDE causes an increase in the number N of azobenzene double bonds which are reduced, N remaining proportional to the drop area A, because-the film is sufficiently flexible to cover the new surface which appears (soap bubble effect). For the highly loaded polymers (loading larger than about 50%), conversely, N remains nearly constant, owing to the rigidity of the film, which causes it to break up when the drop is expanded. The reversibility of the electrochemical reaction depends both on the loading and on the expansion, which could be due to changes in the orientation of the azobenzene molecules at the surface of the electrode.     

Determination of heavy metals in real samples by anodic stripping voltammetry with mercury microelectrodes : Part 2. Application to Rain and Sea Waters

Differential-pulse anodic stripping voltammetry at a mercury microelectrode is applied to determine labile and total zinc, cadmium, lead and copper in samples of rain and sea water. The low ohmic drop associated with microelectrodes permits reliable measurements in rain water without addition of supporting electrolyte. The values found in a typical sample were 0.95 μg l^?1 Cu, 0.38 μg l^?1 Pb, 0.01 μg l^?1 Cd and 0.95 μg l^?1 Zn, with relative standard deviations in the range 4–18%. The small effects of organic matter at microelectrodes, compared with those at a hanging mercury drop electrode, allow sensitive and reliable measurements of labile metals in surface sea water. Total metal concentrations are determined after acidification to pH 1.5 with hydrochloric acid. The results are compared with those obtained with atomic absorption spectrometry and with differential-pulse anodic stripping voltammetry at conventional mercury electrodes. Satisfactory results were obtained for a reference sea water.     

Polarographic maxima and solution movements

Solution movements during the existence of polarographic maxima have been made evident by suspending carbon black in the solution. Only upward motion is observed with maxima occurring on the negative side of the electrocapillary zero. Both upward and downward motion occur with positive maxima. Examples are found in which both upward and downward motion occur during the life of a single drop. Upward motion is associated with the early life of the drop during which the rate of surface growth is greater and also with drops having a shorter life time and hence a greater growth rate. For the two ions Fe(CN)₆^-3, and Cu⁺², a critical ratio of concentration to growth rate exists below which upward motion prevails and above which downward motion prevails. The motions and their directions are considered due in part to the negative character of the mercury surface as it leaves the glass capillary, the time interval for the mercury surface to lose its negative orientation, and the attraction of the ions of the solution by the non-uniformly charged surface.     

Voltammetry of biologically active species and surfactants on new miniaturized and contractible (compressible) mercury electrodes

The adsorption of the polyether-antibiotic monensin from an aqueous solution on mercury was used to investigate the effect of the decreasing size of a stationary mercury drop electrode on the shape of the voltammetric desorption peak of the surfactant. The change of the i-E curve indicated an acceleration of the transport of the surfactant to the electrode as well as of time-dependent changes in the adsorption layer. A decrease of the radius of the hanging mercury drop electrode from 220 μm to 80 μm at a constant accumulation time of t_ac = 70 s resulted in an about 4-fold increase of the evaluated signal (i-E pre-wave) of monensin. A 7-fold increase of the voltammetric desorption peak of monensin at conc. 5 · 10^–7 mol/L was observed as result of a compressive accumulation of the surfactant due to a contraction of the mercury drop electrode. A scheme of an apparatus for voltammetric/polarographic measurements by means of the contractible (compressible) mercury drop electrode is described. The controlled contraction of the electrode surface is presented together with preliminary results covering a new way of accumulation of surfactants, new accumulation effects, effective in adsorptive voltammetry, and other electroanalytical techniques.     

Determination of uranium(VI) in sea water by cathodic stripping voltammetry of complexes with catechol

Polarographic (d.c.) measurements showed that complex ions of uranium(VI) with catechol adsorb on the dropping mercury electrode. This effect is used to determine uranium(VI) directly in sea water. Optimal conditions include pH 6.8, 2 × 10^?3 M catechol, and a collection potential between ?0.1 and ?0.4 V (vs. Ag/AgCl) at a hanging mercury drop electrode. The cathodic scan is made with the linear-scan or differential-pulse mode (d.p.c.s.v.). The detection limit with the d.p.c.s.v, mode is 3 × 10^?10 M after a collection period of 2.5 min. Between pH 6 and 8, the peak height increases with pH and with catechol concentration up to 5 × 10^?3 M. There is linear relationship between the collection time and the measured peak height until the drop surface becomes saturated. With a collection period of 3 min, the reduction current increases linearly with the metal concentration up to about 5 × 10^?3 M U(VI). The maximum adsorption capacity of the mercury drop is 4.4 × 10^?10 mol cm²; each complex ion then occupies 0.38 nm², equivalent to the size of about one catechol molecule. Interference by high concentrations of Fe(III) is overcome by selectively adsorbing U(VI) at a collection potential near the reduction potential of Fe(III). Organic surfactants reduce the peak height for uranium by up to 75% at unnaturally high concentrations only (4 mg l^?1 Triton X-100). Competition by high concentrations of Cu(II) for space on the surface of the drop is eliminated by addition of EDTA.     

Adsorptive Stripping Voltammetric Determination of Albendazole at a Hanging Mercury Drop Electrode

ABSTRACT

Albendazole is determined by differential-pulse adsorptive cathodic stripping voltammetry at a hanging mercury drop electrode using the reduction peak of its copper(II) complex at ?0.28V at an accumulation potential 0.0V vs. Ag/AgCl electrode. The optimum conditions of pH, accumulation potential and accumulation time were studied. The calibration graph for the determination of albendazole was linear in the range 3.0X10^?8 - 9X10^?7M with a relative standard deviation of 2.8%. The detection limit was 1.0X10^?8M after 180s accumulation at 0.0V. The effect of common excipients and metal ions on the peak height of albendazole was studied. The presence of Cu²⁺ ions forms a stable complex with albendazole which is strongly adsorbed at the mercury electrode surface. The method was applied to the determination of the drug in commercially available dosage forms.     

Voltammetry of biologically active species and surfactants on new miniaturized and contractible (compressible) mercury electrodes

The adsorption of the polyether-antibiotic monensin from an aqueous solution on mercury was used to investigate the effect of the decreasing size of a stationary mercury drop electrode on the shape of the voltammetric desorption peak of the surfactant. The change of the i-E curve indicated an acceleration of the transport of the surfactant to the electrode as well as of time-dependent changes in the adsorption layer. A decrease of the radius of the hanging mercury drop electrode from 220 μm to 80 μm at a constant accumulation time of t_ac = 70 s resulted in an about 4-fold increase of the evaluated signal (i-E pre-wave) of monensin. A 7-fold increase of the voltammetric desorption peak of monensin at conc. 5 · 10^–7 mol/L was observed as result of a compressive accumulation of the surfactant due to a contraction of the mercury drop electrode. A scheme of an apparatus for voltammetric/polarographic measurements by means of the contractible (compressible) mercury drop electrode is described. The controlled contraction of the electrode surface is presented together with preliminary results covering a new way of accumulation of surfactants, new accumulation effects, effective in adsorptive voltammetry, and other electroanalytical techniques. Received: 26 March 1998 / Revised: 14 July 1998 / Accepted: 15 July 1998     

Electrogenerated chemiluminescence in mechanistic investigations of electroorganic reactions: Part I. Cathodic cleavage of bis-(2,4,5-triphenylimidazolyl)-1,2′ (dilophyl)

In the simultaneous cathodic reduction of bis-(2,4,5-triphenylimidazolyl)1,2′ L₂ and some aromatic hydrocarbons A in dimethylformamide the emission of A is observed. This electrogenerated chemiluminescence (ECL) originates from the electron transfer between A^?. and the imidazolyl radicals L^., which are formed by a cleavage of L₂^?. into L^. and the imidazolyl anion L_.^? By detecting L₂^?. and L^. as intermediates, this ECL study provides evidence for an ECE mechanism of the cathodic cleavage of L₂.     

Determination of nitrate in natural waters by voltammetry at a stationary mercury drop electrode

Nitrate can be determined by second-sweep cyclic voltammetry at a stationary mercury drop electrode utilizing the autocatalytic effect of the hydroxyl ions formed at the surface of the electrode during the reduction of nitrate in the presence of an excess of trivalent cations. The reduction current in the second sweep with the same drop is proportional to the nitrate concentration in the range 1–1500 μmol l^?1 in natural waters. The humic substances present in natural waters have a favourable effect on the determination of nitrate. The method is applied to the determination of nitrate in drinking and river waters.     

The electrochemical reduction and determination of reduced nicotinamide adenine dinucleotide in acidic media

The apparent reduction of reduced nicotinamide adenine dinucleotide (NADH) in acidic media at a static mercury drop electrode was investigated. A simple, quick pretreatment procedure was developed to convert the NADH to its acid-hydrated form. This adsorbs on the mercury surface during a film deposition time and the film is then reduced. The adsorption is diffusion-controlled and hence the peak currents for square-wave and linear-scan voltammetry are proportional to Ct^1/2_pAf and Ct^1/2_pAv, respectively, where t_p is the effective film deposition time, C the concentration of NADH, A the electrode area, f the square-wave frequency, and ν the linear scan rate. Several electrochemical techniques were compared for the determination of NADH; the method of choice is square-wave voltammetry, although staircase or linear scan voltammetry can also be used. The detection limit is less than 7 nM, and the range of linear response covers 2–3 orders of magnitude of NADH concentration.     

Chronoamperometric Determination of Lecithin

Abstract

A rapid, sensitive and reproducible chronoarnperometric method for the determination of lecithin is described. The method is based on the inhibition of the oxygen reduction process by the adsorption of lecithin at the mercury -solution interface. Current-time curves recorded during the growth of single mercury drops at ?0.200 V (vs SCE) in aqueous-ethanolic solutions of lecithin are employed. The adsorption effect may be evaluated by measuring the time necessary for a complete drop surface coverage, “t_o”, interpolating the descending branch of the i-t curves linearly to zero current. The influence of the variables that affect the inhibition process is studied and a linear relationship between t^1/2 _o and lecithin concentration in the 20–80 μg/ml range is found. The relative standard deviation of the proposed method is 1.3%.     

Electrochemical Behavior and Determination of Nicardipine

Abstract

The voltammetric behavior and measurement of nicardipine at the glassy carbon, carbon paste and hanging mercury drop electrodes are discussed. Cyclic voltammetry is used to elucidate the redox mechanism. Nicardipine is shown to adsorb on carbon electrodes, with the surface species retaining its electroactive characteristics. The adsorptive accumulation serves as a preconcentration step which improves the voltammetric measurement with respect to selectivity and sensitivity. Coupling this with a medium-exchange step eliminates interferences due to solution-phase electroactive species and permits direct measurement in urine. The inherent sensitivity of differential pulse voltammetry at the mercury electrode permits convenient measurement at the submicromolar level, with detection limit of 2 × 10^?8M. Amperometric detection for a flow injection system is illustrated.     

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.     

Electrochemical studies of homocysteine and homocystine at mercury electrodes

The behaviour of homocysteine and cysteine at mercury electrodes is compared. The one-electron oxidation associated with thiols is shown to be the same for both compounds in acidic phosphate buffer, giving rise to an adsorbed thiol—mercury complex, (RS)₂Hg, at the electrode surface. Formation of this complex is utilized in the cathodic stripping voltammetric determination of homocysteine; the detection limit is 10^?9 M after a deposition time of 90 s at a hanging mercury drop electrode. The similar E_{$\text{1}\text{2}$} values for homocysteine and cysteine mean that prior separation is needed for their individual determination. Amperometric detection with a mercury-coated goal electrode after separation by cation-exchange liquid chromatography provides a method for the simultaneous determination of both compounds. Reduction of homocystine at the mercury electrode is also compared to that of cystine. The more negative reduction potential, and the maximum observed for homocystine on d.c. polarograms, which is not seen for cystine, is attributable to different reaction kinetics at the mercury electrode; the products of both the 2-electron reductions are the corresponding thiol-containing amino acids.