Anodic behaviour of the (222) macrobicyclic ligand on mercury electrode in propylenecarbonate

The anodic behaviour of the free macrobicyclic ligand (222) was examined by various electrochemical techniques on mercury and on platinum electrodes. From results on platinum electrode using cyclic voltammetry it appears that the anodic oxidation of (222) proceeds irreversibly at potentials more positive than +0.7 V (aq. SCE), however, its mechanism could not be determined because the anodic signal was not well developed. Under polarographic conditions a reversible anodic diffusion controlled wave at E_1/2=+0.08 V (aq. SCE) was observed, corresponding to a complex formation of the ligand (222) with the ions of mercury formed by anodic polarization of mercury electrode. In excess of mercuric cations a cathodic wave at slightly more positive potentals was found.     

Electrodimerization of the pyridiniumcarboxylic acids homarine and trigonelline

Homarine (1-methyl-2-carboxypyridinium ion) and trigonelline (1-methyl-3-carboxy-pyridinium ion) are reduced at mercury electrodes in alkaline solutions by a one-electron transfer to the pyridinium ring. Analysis of polarographic wave shapes using deviation-pattern recognition, peak widths of cyclic voltammograms, analysis of the products of bulk electrolyses and dependences of E_1/2 and E_p on scan rate, pH and concentration indicated that these reductions proceed by an EC² mechanism, where the second-order chemical reaction is a homogeneous dimerization. The zwitterionic form of the pyridiniumcarboxylic acid, which predominates in alkaline solutions, undergoes reduction to form a pyridinyl anion radical. Two radicals then couple to yield a dimer dianion which is subject to protonation. The dimer can be oxidized at mercury electrodes.     

Organic halide electroreduction on silver

Silver, whose extraordinary electrocatalytical properties for organic halide reduction have been recently evidenced, has been used as cathode material for systematic preparative electrolyses in membrane-divided cells. To better elucidate the substrate role on the remarkable positive shift of reduction potentials, and on the "cage effect" i.e. the promotion of intermolecular reaction on adsorbed intermediates, three halide substrate patterns are here compared in terms of both voltammetric characterization and preparative electroreduction products: aliphatic halides (adamantanes), aromatic halides (phenols) and anomeric glycosyl halides. The preparative electroreductions result mainly in dimerization in the case of glycosyl halides, in H-->Br substitution in the case of bromophenols, in dimerization + substitution in the case of haloadamantanes. The product analysis, both at the end of the reaction and at intermediate times, allows discussing the reaction pathways in terms of intermediate stability and of active surface accessibility. The possibility of complete dehalogenation on a wider substrate variety with remarkably lower energy consumption and almost quantitative current yields makes the process potentially very interesting for environmental purposes.     

Nitroazines. 4. Polarographic behavior of 5-nitropyrazolo[3,4-b]pyridines and the esr spectra of their electrochemically generated free radicals

1-Methyl- and 1-phenyl-substituted derivatives of 5-nitropyrazolo[3,4-b]pyridines are reduced at a dropping mercury electrode in a single one-electron wave with the formation of radical anions in interval of potentials from –0.94 to –1.06 V. Compounds unsubstituted at the N(₁) nitrogen atom give an additional polarographic wave at E_1/2 = –1.4 V due to the reduction of the deprotonated form (anion). The potentials and HFS constants of the ESR spectra of the corresponding electrochemically generated free radicals are given.For Communication 3, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 689–693, May, 1985.     

Solvent extraction studies of phosphonium salts and their analytical applications : Differentiation of n- and isopropyl halides

A test is described for identification and differentiation of n-propyl and isopropyl halides. The halide is reacted with triphenylphosphine to form the phosphoniun salt, and thiocyanate and copper or cobalt solutions are added, Extraction of the colored precipitate into a suitable solvent indicates n-propyl halide whereas isopropyl halides yield unextractable products. The test is applicable to solutions of each halide in the corresponding alcohol as well as to mixtures of both halides.     

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.     

Spurenanalyse von huminstoffen im trinkwasser: The d.c. polarographic determination of traces of humic substances in potable waters

(The d.c. polarographic determination of traces of humic substances in potable waters) The inhibiting effect of a tri-n-butylphosphate layer adsorbed at the mercury drop on the polarographic wave of copper(II) is reduced by humic substances. This effect can be utilized to determine humic substances in the range 0.05–1 mg l^-1. The standard substance used was isolated from peaty water. Humic and fulvic acids are not differentiated but amino acids, peptides and polyhydroxy compounds do not interfere.     

A direct electrochemical method for the determination of allopurinol and uric acid mixtures

Uric acid and allopurinol are both electrochemically oxidized at the pyrolytic graphite electrode (P.G.E.) in aqueous solution, the oxidation of allopurinol occurring at more positive potentials. Allopurinol is polarographically reducible at the dropping mercury electrode, and the wave is not affected by uric acid. Uric acid is strongly adsorbed at the P.G.E., so that concentrationvs. anodic peak current curves are not linear, but when the solution is saturated with allopurinol, these plots become linear. and changes in scan rate have less effect. It is shown that allopurinol is adsorbed at the P.G.E., and in saturated solutions, displaces adsorbed uric acid from the electrode surface. Allopurinol (0.1–1 mM) can be determined via its polarographic reduction wave at the D.M.E.; the solution is then saturated with allopurinol and uric acid (0.05–0.5 mM) is determined via its anodic peak at the P.G.E. The procedure is satisfactory in supporting electrolyte systems of pH 0–6.     

Investigation of the anodic processes on platinum in perchlorate and chlorate solutions by linear sweep voltammetry using the rotating double-ring electrodes

The polarographic behavior of chloramine-T has been investigated over a wide pH range. Chloramine-T gives two waves at low concentration. In acidic solution, the second wave is due to the formation of mercurous chloride, whereas, in alkaline solution, the second wave is due to the formation of the mercuric salt of p-toluenesulfonamide. The diffusion current of the total wave is due to a two-electron reduction, is proportional to the concentration and is stable, particularly in alkaline solution. The polarographic behavior of the anodic wave of p-toluenesulfonamide has also been investigated.     

Generation of ground electronic state haloalkyl radicals in the gas phase

Gaseous haloalkyl radicals were prepared by the photolysis of iodohaloalkanes in Pyrex vessels containing mercury (I) halides. Cleavage of the carbon-iodine bond gave mercury (II) halide and a radical which was subsequently shown to react on the ground state electronic energy surface. The usefulness of this method for chemical activation rate constant studies is illustrated by measurement of unimolecular rate constants for decomposition of CH₂ClCH₂Cl and CF₃CH₃. Possible mechanisms for photodecomposition of iodoalkanes in the presence of mercury (I) halides are discussed.     

Polarography of some sulphur-containing compounds : Part XII. Anodic waves of monoalkyldithiocarbamates and their reactions with heavy metals

Monoalkyldithiocarbamates give two anodic polarographic waves, corresponding to a one-electron and a two-electron process. A reaction scheme is proposed. The unusual behaviour observed during controlled-potential electrolysis in sodiumhydroxide media is interpreted as a chemical reaction of the primary electrolysis product; the species formed gives a wave similar to the original compound. Different adsorption phenomena are probably caused by varying orientation of the mercury compound on the electrode surface. The adsorbed layer formed in sodium hydroxide solution is so close-packed that it prevents hydroxyl ions from penetrating and thus giving an anodic wave. For analytical purposes, 0.1 M sodium hydroxide is the most suitable supporting electrolyte; linear calibration graphs can be obtained over the range 5 · 10^-5–1 · 10^-3M.     

Electrochemical behaviour of organolead compounds: Part I. Triphenyl-lead acetate

The electrochemical behaviour of triphenyl-lead acetate in 50% (v/v) ethanol has been investigated using various electrochemical techniques including polarography, cyclic voltammetry and controlled potential coulometry. It has been found to give three/four polarographic waves. The first normal wave involves an one-electron irreversible reduction of triphenyl-lead ions giving triphenyl-lead free radicals which are strongly adsorbed at the DME giving rise to an adsorption prewave. The triphenyl-lead free radicals produced in the normal reduction step immediately react with mercury of the DME producing phenylmercury radicals and metallic lead. Wave II has been ascribed to the simultaneous reduction of triphenyl-lead free radicals and phenylmercury free radicals. The metallic lead produced in these processes undergoes oxidation at the electrode and “distorts” the “normal” adsorption prewave of step I. A mechanism of reduction is proposed and polarographic methods have been developed for determining triphenyl-lead compounds at ordinary level and down to submicromolar level.     

The CuII−CuI−Cu0(Hg) system in the presence of benzotriazole: Part II. A chronocoulometric investigation

Both the oxidation of Cu⁰ at dropping amalgam electrodes immersed in solutions of benzotriazole (BTA) and the reduction of Cu^II at a dropping mercury electrode from BTA solutions have been investigated by the single potential-step chronocoulometric technique. The dependence of the charge Q(t) flowing as a consequence of a given potential jump E_i→E_f upon the initial and final potentials E_i and E_f, as well as upon the time t elapsed from the instant of the potential jump provides direct evidence for the presence of a single adsorbed monolayer of a Cu^I compound on a mercury electrode immersed in a Cu^II solution containing BTA, at applied potentials positive to ≈?0.4 V/SCE. Analogous measurements carried out at dropping amalgam electrodes reveal the presence of a single adsorbed monolayer of a Cu^I compound, or else of an adsorbed multilayer, depending on the potential range investigated. The results of the chronocoulometric measurements are in agreement with those of the polarographic measurements of Part I.     

The effect of size and location of adsorbed intermediates in the double layer on the tafel slope the reduction of hydroxylamine on the dropping mercury electrode

The reduction of hydroxylamine on the dropping mercury electrode was studied. Kinetic parameters were determined from measurements at the foot of the polarographic wave. Tafel plots were extended to higher current densities with the use of the Koutecký correction.This molecule was chosen as an example of a relatively small species for which the effect of competition with water is small and the importance of the location of the adsorbed intermediate in the double layer can be demonstrated. The experimentally observed value of b = 96 ± 3 mV (combined with reaction orders of unity both with respect to hydroxylamine and hydrogen ions) was found to be consistent with the notion of an adsorbed intermediate whose reduction is brought about by a potential difference φ_M - φ_x, where φ_x has an intermediate value between φ_M and φ_s. These results were compared with earlier results found in a study of the reduction of nitroalkanes under similar conditions.The advantages of making measurements on the DME at the foot of the polarographic wave are emphasized.     

Quantum chemical studies of the chemisorption of water and of unhydrated and hydrated halide ions on mercury

Local structures on electrode interfaces can be explored by quantum chemical investigation of medium-sized systems consisting of a cluster of substrate (metal) atoms, one or several solvent molecules, and/or at least one ion to be adsorbed at the interface. For the study of water adsorption and halide ion adsorption (unhydrated as well as hydrated) on a mercury surface, we have used the standard CNDO method together with geometrical optimization of the atom positions.In this paper, the following topics have been treated: (a) adsorption of a single water molecule in different positions on a close-packed plane cluster of seven mercury atoms; (b) adsorption of unhydrated halide ions (Cl^?, Br^?, I^?) in the “on-top” or hollow position on the mercury surface; (c) adsorption of monohydrated halides on the mercury surface. Further studies including solvation by six water molecules are discussed.The calculations provide information about minimum-energy geometries, energetic data, and local charges. Furthermore, they allow some conclusions about water mobility and reorientation on a close-packed metal surface, water orientation under the combined influence of an adsorbed ion and the metal surface, and trends of charge distribution in the halide series to be drawn. Calculations are critically discussed in the light of experimental and other quantum chemical data.     

Reactions of tricarbonylpyridine complexes (NN)(py)M(CO)3 (M = Mo, W; NN = 2,2′-bipyridine, 1,10-phenanthroline) with mercuric derivatives HgX2 (X = Cl, CN, SCN)

Treatment of mercury(II) halides and pseudohalides with complexes (NN)(L)M(CO)₃ (L = py; NN = 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen); M = Mo, W) gives new tricarbonyl complexes. In all cases elimination of the pyridine ligand occurs and in some cases there is partial displacement of halogen from the mercuric halide. Treatment of bipy(py)W(CO)₃ with mercuric chloride gives only an adduct. Conductivity, IR and electronic absorption are given, and possible formulations suggested.     

Determination of trace amounts of nitrite by single-sweep polarography

A single-sweep polarographic determination of nitrite in 0.2 M sulphuric acid medium containing nickel(II) sulphate and ammonium thiocyanate is described. The ternary complex (NiSCNNO)⁺ which is formed in the solution is strongly adsorbed on the surface of the mercury electrode and an adsorptive polarographic wave at ?0.57 V (vs. SCE) is related to the concentration of nitrite in the range 2.0 × 10^?8-1.0 × 10^?6 M. The detection limit is 8 × 10^?9 M. The relative standard deviation is 1.5% and the regression coefficient is 0.998. Most common anions and cations do not interfere. The mechanism of the electrode process was studied by several electrochemical methods. The polarographic wave is attributed to the reduction of nitrogen monoxide in the adsorbed (NiSCNNO)⁺ complex to hydroxylamine. The procedure was applied to the determination of trace amounts of nitrite in sausage, water and nitrate.     

Photochemical vs. electrochemical electron-transfer reactions. one-electron reduction of aryl halides by photoexcited anion radicals

Electrochemical reduction of aryl halides generally leads to expulsion of halide ion. The product aryl radical is unavoidably further reduced. In contrast, reduction of aryl halides by photoexcited anion radicals may be stopped at the aryl radical stage owing to the bimolecular nature of electron-transfer reactions. We have tested this hypothesis by photoinducing electron-transfer from anthraquinone anion radical to several aryl halides. For each halide it was possible to trap the corresponding radical by anthracene forming stituted 9-phenylanthracenes.     

Voltammetric Determination of Mercury(II) at Poly(3‐hexylthiophene) Film Electrode. Effect of Halide Ions

The well‐known method for the determination of mercury(II), which is based on the anodic stripping voltammetry of mercury(II), has been adapted for applications at the thin film poly(3‐hexylthiophene) polymer electrode. Halide ions have been found to increase the sensitivity of the mercury response and shift it more positive potentials. This behavior is explained by formation of mercuric halide which can be easily deposited and stripped from the polymer electrode surface. The procedure was optimized for mercury determination. For 120 s accumulation time, detection limit of 5 ng mL^?1 mercury(II) has been observed. The relative standard deviation is 1.3% at 40 ng mL^?1 mercury(II). The performance of the polymer film studied in this work was evaluated in the presence of surfactants and some potential interfering metal ions such as cadmium, lead, copper and nickel.     

Polarographic behaviour of phenothiazine

Summary Phenothiazine oxidizes at the dropping mercury electrode in waterethanolic solutions, containing 7 N sulphuric acid, giving a wave corresponding to the formation of free cation radicals.The normal potential of the system phenothiazine/radical, against the reference electrode formed from mercuric sulphate in the presence of 10 N H₂SO₄ and 30% ethanol, is –240±20 mV.In alkaline solutions of pH 12.5 phenothiazine also gives a polarographic wave. This wave corresponds to the insoluble anions of compounds formed with mercury. The conditions for the quantitative polarographic determination of phenothiazine are also given.

---

Zusammenfassung Es wurde bewiesen, daß in äthanolisch-wäßrigen Lösungen, die 7 n H₂SO₄ enthalten, Phenothiazin an der Quecksilber-Tropfelektrode oxydiert wird. Als Reaktionsprodukt entsteht ein freies Kation-Radikal. Das Normalpotential des Systems Phenothiazin/Radikal gegen die gesättigte Quecksilbersulfatelektrode, die 10 n H₂SO₄ und 30% C₂H₅OH enthält, beträgt –240 ± 20 mV.In alkalischen Lösungen bei pH 12,5 konnte man eine polarographische Welle registrieren, deren Eigenschaften schwerlöslichen Anionen einer Quecksilberverbindung entsprechen.Die Bedingungen für eine quantitative polarographische Phenothiazinbestimmung werden angegeben.

---

---

Dedicated to Prof. Dr. M. von Stackelberg on his 70th birthday.