Electrochemical behaviour of triphenylgermanium bromide

The electrochemical behaviour of triphenylgermanium bromide has been thoroughly investigated using various electrochemical techniques including polarography, cyclic voltammetry and controlled potential coulometry. It has been found that in non-aqueous solvents triphenylgermanium bromide gives only one small drawn-out wave, while in aqueous-organic media two reduction waves are observed. The first wave has been ascribed to adsorption of the products of the reduction step II. The triphenylgermanium free radicals have been postulated to combine rapidly with protons in acidic media or to abstract hydrogen from water in alkaline media. The protonated species has been found to be reduced at potentials at which normal reduction of triphenylgermanium bromide takes place giving rise to a superimposed catalytic proton-discharge wave. A mechanism of reduction of triphneylgermanium bromide at the DME has been postulated and analytical methods for the determination of triphenylgermanium compounds at the formulation and trace analysis levels have been developed.     

Electrochemical behaviour of organotin compounds: Part III. Dialkyltin mercaptocarboxylates

Electrochemical behaviour of di-n-octyltin dithioglycolate has been investigated in acetate buffer (pH 7.0) containing 80% (v/v) ethanol. It has been found to give two well-defined polarographic waves, one of which is anodic and involves two-electron oxidation of the mercaptide. The other wave has been found to be cathodic and is due to one-electron reduction of the dioctyltin compound to a free radical. Strong adsorption of the compound at the DME was indicated by depression in its drop time curve. A mechanism of the electrode process is postulated and a method for detection of these compounds down to p.p.m. level has been developed.     

The riboflavin—dihydroriboflavin system in acidic medium on mercury: Part II. A polarographic investigation

Polarographic current—potential characteristics and current—time curves for the reduction of riboflavin (RF) to dihydroriboflavin (DRF) in 0.01 M HClO₄ + 0.09 M NaClO₄ have been examined in detail. It has thus been shown that the RF adsorption prewave is due to the formation of two overlapping adsorbed monolayers of DRF molecules. Lateral interactions of RF molecules between themselves, of DRF molecules between themselves and of RF with DRF molecules in the first adsorbed monolayer are weak, whereas vertical interactions between overlapping DRF molecules are relatively strong. This explains the abrupt increase in the slope of the RF adsorption prewave and its shift towards more positive potentials as soon as the second adsorbed monolayer of DRF starts to form. The so-called “normal” wave for RF reduction has a half-wave potential E^II_{$\text{1}\text{2}$} = ?0.180 V/SCE, practically coinciding with the formal potential of the RF/DRF couple, and a slope corresponding to a reversible two-electron reduction unaffected by semiquinone formation. The shape of polarographic mean current vs. potential curves has been accounted for through an approximate solution of the corresponding diffusional problem.     

Studies on the polarographic behaviors of alizarin complexone and the adsorptive complex wave of thulium-alizarin complexone

In the 0.1M acetate buffer solution, at pH 5.9, alizarin complexone exhibits a reversible adsorptive prewave. On the surface of DME, the adsorption of alizarin complexone obeys Frumkin adsorption isotherm. In the same buffer solution, when the thulium was added, a sensitive adsorptive complex wave of thulium-alizarin complexone has been obtained by single-sweep polarograph. The composition of the complex and the mechanism of electrode processes were studied in detail.     

Fundamentals of lead-acid cells: Part XII. The reduction of lead sulphate

The reduction of lead sulphate grown anodically on planar and porous electrodes has been studied by a potentiostatic step technique. Both “planar” and “porous” electrodes behave in a similar manner and indicate that the electrode reduction reaction has a finite depth of penetration into the electrode. The kinetics of the formation of metallic lead on both electrodes appear to be instantaneous nucleation and two-dimensional growth processes with subsequent current limitations due to overlap and lead sulphate depletion. The current limitation processes are very complex and it has not been found possible to fit a satisfactory mathematical model.     

Voltammetric behaviour of the nitrosodisulfonate/hydroxylaminedisulfonate redox system

Electrochemical behaviour of dialkyltin compounds in 80% (v/v) ethanol has been investigated by different electrochemical techniques such as polarography, cyclic voltammetry and controlled potential coulometry and it has been found to give three polarographic reduction waves. The first step involves the formation of an ion-radical which is further reduced in the second step to the dibutyltin diradical. Dibutyltin diradical undergoes polymerization as well as further reduction in two parallel processes. A procedure has been developed for determining these compounds down to p.p.b. level.     

pH effects on the reduction of bilirubin in aqueous media

The electrochemical reduction of bilirubin at the mercury electrode was studied in aqueous media embracing an extended pH range; a variety of electrochemical techniques was used. Reduction in neutral solution results in three waves—the first a reversible adsorption prewave, followed by a pH dependent, reversible, two electron wave and finally by a one electron irreversible wave. In basic solution, only two waves are discernable: the reversible adsorption prewave, followed by a three electron wave resulting from the coalescence of the second and third waves seen in neutral solution. Familiarity with the detailed electrochemistry of bilirubin reduction provides the prerequisite basis for developing a sensitive analytical method for its determination.     

Electrochemical reduction of substituted pyrimidines in acetonitrile

Polarographic studies of several substituted pyrimidines were reinforced by the results from cyclic voltammetry, controlled-potential electrolysis, and spectrophotometric examination of electrolyzed solutions, as well as by the examination of model compounds. Pyrimidines substituted with non-reducible groups (amino, methyl) are reduced in a single, one-electron (1e), diffusion-controlled process, very similar to that for pyrimidine itself. Pyrimidine-4-carboxylic acid exhibits three reduction waves: a very drawn-out acid-reduction wave with unusual properties and at more negative potential, an adsorption prewave and a wave corresponding to the 1e reduction of the pyrimidine moiety, 2-Chloro-and 2-bromopyrimidine each exhibit two polarographic waves; the first, corresponding to irreversible scission of the carbon-halogen bond, has electrochemical properties quite different from those normally expected; the second is due to reduction of the electro-generated pyrimidine. Results are compared with those for the reduction of bromobutane, bromobenene, and 2-bromopyridine.     

Electrochemical behaviour of organotin compounds: Part IV. Phenyltin trichloride

The electrochemical behaviour of phenyltin trichloride has been studied using various electrochemical techniques including polarography, coulometry and cyclic voltammetry. Phenyltin trichloride has been found to show a single major polarographic wave corresponding to a three-electron reduction process to form phenyltin free radicals which rapidly polymerise. Slow hydrolysis of phenyltin trichloride to stannoic acid has been observed to be the major cause of the time-dependence of the limiting current. Analytical methods for the determination of PhSnCl₃ at formulation and trace levels and in the presence of di- and triphenyltin compounds have also been developed.     

Voltammetric study of the electrochemical reduction of lucigenin in aqueous medium

The electrochemical reduction of lucigenin (bis-N-methylacridinium nitrate) in aqueous solution was studied by d.c. Tast polarography at a dropping mercury electrode, by cyclic voltammetry at an amalgamated gold electrode and at a hanging mercury drop electrode, and by microcoulometry. The effects of pH, lucigenin concentration and temperature were studied, and special methods were applied to study the suspected adsorption and catalytic (regeneration) currents. A spectrophotometric study is also reported. It was found that lucigenin is reduced in two separate one-electron steps. An adsorption prewave accompanies the first step, while the second, below pH 3.5, is catalytic, owing to the chemical regeneration of the intermediate reduction product at the electrode surface.     

The riboflavin—dihydroriboflavin system in acidic medium on mercury: Part I. A chronocoulometric investigation

The riboflavin (RF)—dihydroriboflavin (DRF) system in 0.01 M HClO₄ + 0.09 M NaClO₄ has been studied on mercury by the single-step chronocoulometric technique. At ?0.040 V/SCE, where RF is still electro-inactive, this substance is adsorbed according to a Langmuir isotherm with an adsorption coefficient K_O = 5.2 × 10⁶ 1 mol^?1, giving rise to a single adsorbed monolayer. At potentials along the plateau of the RF polarographic adsorption prewave, as well as at more negative potentials, DRF is adsorbed with formation of two overlapping monolayers. This behaviour denotes strong attractive vertical interactions between overlapping adsorbed DRF molecules. The progressive shift in the chronocoulometric Q vs. E curve for electro-oxidation of adsorbed DRF towards more positive potentials with an increase in the surface concentration of DRF confirms the strength of these vertical interactions. The simultaneous presence, with formation of a charge-transfer complex, of adsorbed RF and DRF molecules along the plateau of the polarographic RF prewave, as postulated by Tedoradze and co-workers [21,22] is excluded.     

Electrochemical reduction of 5,5′-dichlorohydurilic acid. mechanism and analytical applications

The electrochemical reduction of 5,5′-dichlorohydurilic acid has been studied at the dropping mercury electrode (DME) and the pyrolytic graphite electrode (PGE). At the DME the single polarographic reduction wave observed at pH 6–11 involves a direct 4e—2H⁺ reduction of the carbon-halogen bond to give hydurilic acid and chloride. The state of hydration or ionization of the 5,5′-dichlorohydurilic acid has no effect on the electrochemical reaction. At the PGE, 5,5′-dichlorohydurilic acid shows two voltammetric peaks. Peak I_c, observed between pH 5 and 7, arises from an overall 4e—2H⁺ reduction of 5,5′-dichlorohydurilic acid via a mechanism that involves initial electron attack at a carbonyl group alpha to a carbon-halogen bond with simultaneous elimination of chloride ion. The peak II_c process involves an initial 2e—1H⁺ reduction of a partially hydrated form of 5,5′-dichlorohydurilic acid with only one unhydrated halocarbonyl moiety available for reaction. Attack is again via the carbonyl group with simultaneous elimination of chloride and formation of 5-chlorohydurilic acid. A chemical dehydration step then occurs with a rate constant of ca. 0.24 s^?1 at pH 8.2, with formation of a further reducible halocarbonyl group. This is again reduced in an overall 2e—2H⁺ reaction to give hydurilic acid and chloride ion. The peak II_c process hence proceeds via an ECE mechanism. The different mechanisms observed for reduction of 5,5′-dichlorohydurilic acid at mercury and pyrolytic graphite electrodes are unusual. Analytical methods have been developed for the polarographic determination of 5,5′-dichlorohydurilic acid via its reduction wave at the DME, and for the voltammetric determination of hydurilic acid via its first oxidation peak at the PGE.     

Pulse polarograpmc study of the electrochemical reduction of lucigenin in aqueous medium

The electrochemical reduction of lucigenin (bis-N-methylacridinium nitrate) in aqueous solution was studied by normal pulse polarography, normal pulse polarography with differential detection of the current, and differential pulse polarography with cathodic and anodic pulses at several pulse amplitudes. The effects of pH and lucigenin concentration were studied. In confirmation of an earlier d.c. polarographic study, lucigenin is shown to be reduced in two separate one-electron steps. An adsorption peak accompanies the first step, while the second, below pH 3.5, is catalytic owing to chemical regeneration of the intermediate reduction product at the electrode surface.     

Polarographic reduction of some mesoionic 2,3-diaryl-2 H-tetrazolium-5-thiolate compounds

The polarographic behaviour at the DME of 2,3-diphenyl-, 2,3-di(4-methylphenyl)- 2,3-di(3-chlorophenyl- and 2,3-di(2-chlorophenyl)-2 H-tetrazolium-5-thiolate compounds was investigated in 10% (v/v) aqueous methanol over a wide range of pH. The reduction proceeded through one four-electron irreversible wave to give the corresponding thiocarbazide. A mechanism for this reduction was proposed and subsequently substantiated by a controlled potential electrolysis of a mesoionic compound whereby the identify of the final reduction product has been proved spectrophotometrically to be the thiocarbazide.     

A detailed analysis of the polarographic behaviour of methylene blue in phosphate buffer on mercury

Polarographic current-potential characteristics and current-time curves for the reduction of methylene blue (MB) to methylene blue leucoform (MBL) in a pH 7.9 aqueous phosphate buffer have been examined in detail over a wide concentration range. It has thus been shown that the so called “normal” or “main” reduction wave of MB actually consists of two separate steps, the former with a half-wave potential practically coinciding with the formal potential E₀=?0.250 V/SCE of the MB/MBL couple and the latter with a half-wave potential of about ?0.310 V/SCE. As soon as the well-known MB adsorption prewave has attained its maximum height (which occurs at a MB bulk concentration c₀^*?5×10^?5M), a further slight increase in c₀^* causes the appearance of the wave with E_1/2=?0.310 V. The height of the linear potential-sweep voltammetric peak corresponding to the latter wave increases proportionally to the sweep rate, thus revealing the “adsorption” nature of this wave. A comparison with a previous chronocoulometric investigation of the MB/MBL system has permitted us to conclude that the wave with E_1/2=?0.310 V is due to reduction of the MB molecules which, after having reached the surface of the dropping electrode by diffusion, are adsorbed at the top of the adsorbed monolayer of MBL in direct contact with the electrode and remain in this adsorbed state after reduction. For c₀^*>7×10^?5M the wave with E_1/2?E₀=?0.250 V starts to develop. This wave is due to the electroformation of MBL molecules which diffuse back into the solution. The shape of polarographic current-potential characteristics and current-time curves has been accounted for semiquantitatively through an approximate solution of the corresponding diffusional problem.     

Identification Des Produits De La Reduction Electrochimique d'Un Nouvel Antiinflammatoire: Le Piroxicam

Abstract

The electrochemical reduction of piroxicam has been investigated on a mercury pool in acidic and alcaline media.

After exhaustive electrolysis, the extraction followed by chromatographic separation of the reaction products gives the reduced compounds which have been identified by spectroscopic techniques. From the analysis of the NMR ¹H, ¹³C and IR spectra and the results of our preceding electrochemical investigation, the polarographic behaviour of piroxicam has been elucidated. The process occurs by the irreversible reduction of the double bond of the enol function over the total pH range investigated. However, in alkaline and highly acidic solutions, a preceding two-electron step occurs, giving rise to the opening of the thiazine ring.     

ESR spectra of free radicals of 3,3-disubstituted 1,2,3,4-tetrahydropyridines produced during electrochemical reduction

The polarographic properties (potentials, number of electrons, reversibility) of the electrochemical reduction of 3,5-diethoxycarbonyl-3-(p-nitrobenzoyl)-6-(p-nitrophenyl)-1,2,3,4-tetrahydropyridine and its derivatives in dimethylformamide have been determined. In the course of the electrochemical generation, ESR spectra of primary radical anions of p-nitrobenzoyl structure were obtained along with ESR spectra of nitrophenyl-p-substituted free radicals formed as a result of further reduction.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1103–1110, August, 1998.     

Electroreduction of Ammonia and Hydroxyammonia Complexes of Divalent Metals: Effect of the Ligand Concentration and the EDL Structure

Kinetics and mechanism of electroreduction of complexes Pd(NH₃)₄ ²⁺ on a dropping mercury electrode (DME) and a Pd electrode, as well as ammonia complexes of Co(II), Ni(II), and Zn(II) and hydroxyammonia complexes of Zn(II) on DME at different concentrations of ammonia and supporting electrolytes and different pH values are discussed. The half-wave potentials of electroreduction of ammonia complexes of Pd(II) and Ni(II) on DME in the absence of a polarographic maximum obey an equation that takes into account the effect the EDL structure has on the rate of a slow outer-sphere electrochemical stage. As opposed to Pd(II) complexes, the reduction of the other complexes involves preceding reversible chemical stages, which yield diammonia complexes undergoing a direct reduction on DME. The reasons for the emergence of a polarographic maximum upon an increase in the concentration of reduced complexes and the time of recording an instant current are discussed.     

ELECTRON SPIN RESONANCE STUDIES OF ULTRAVIOLET IRRADIATED KERATIN AND RELATED PROTEINS

Abstract— The formation of free radicals in keratin and related proteins by exposure to u.v. light has been examined by electron spin resonance spectroscopy. From a comparison of the spectra obtained and the effects of different wavelengths and the stabilities of the radicals produced, it has been shown that several species of free radicals are produced by the u.v. irradiation of keratin. However it has been possible to identify only those free radicals associated with the cystine residues. The free radicals produced in keratin by exposure to shorter wavelength irradiation (below 3250 Å) were found to be quite different to those produced at longer wavelengths.     

The reducibility of Ru,Pt/Ru and Pt oxide electrocatalysts as measured by temperature-programmed reduction and cyclic voltammetry

The electrochemical reduction of alkaline cryptates (222, M)⁺ has been studied on mercury electrode by normal pulse polarography, potentiostatic coulometry and cyclic voltammetry in propylene carbonate as solvent. The corresponding kinetic parameters have been calculated and compared with those obtained on solvated alkaline cations in the same medium. A more detailed study of the electrochemical reduction mechanism of the cryptate (222, K)⁺ shows that the primary product of the reduction is the unstable (222, K^o), and that the final stable products are the free ligand (222) and the amalgam K^o (Hg). The alkaline cations, when complexed by the same (222) ligand, exhibit close values of the polarographic diffusion coefficients. The specific polarographic behaviour of the cryptate (222, Cs)⁺ is described and its stability constant calculated in propylene carbonate. An analytical application of the electrochemical reduction of cryptates is also proposed.