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.     

Determination of pseudouridine at submicromolar concentrations by cathodic stripping voltammetry at a mercury electrode

Pseudouridine (5-ribosyluracil), uridine (N,1-ribosyluracil), deoxyuridine (N,1-deoxyribosyluracil) and uracil are investigated by means of d.c. polarography and by differential and normal pulse polarography. Pseudouridine, which is known to be a cancer marker, yields anodic polarographic currents in the pH range 7–11, whereas uridine and deoxyuridine are inactive under the same conditions. The polarographic response of pseudouridine obtained is due to the formation of a sparingly soluble mercury compound. Pseudouridine can be determined by differential pulse polarography in the concentration range 2–6 × 10^?6 M and by differential-pulse cathodic stripping voltammetry at concentrations two orders of magnitude lower. Small excesses of uridine, deoxyuridine or proteins do not interfere with the determination.     

A study of the electrochemical reduction of bilirubin and its derivatives

The electrochemical behaviour of bilirubin on mercury electrode in aqueous solution at physiological pH has been investigated by differential pulse polarography. The reduction of bilirubin is marked by two polarographic peaks at about −1.30 and −1.42V versus s.c.e. which however depend on the nature of the substituents in the biladiene skeleton.     

Polarographic analysis of corticosteroids: Part 6. Mechanism of polarographic electroreduction of some Δ4-3-ketosteroids and Δ1,4-3-ketosteroids

Prednisolone and dexamethasone give waves in d.c. and normal pulse polarography and peaks in differential pulse polarography which correspond to a one-electron uptake. The transfer of the first electron is reversible and at pH < 7 (i₁) is preceded or at pH >7 (i₃) is followed by a proton transfer. Protonation occurs on the carbonyl group and pinacol is formed. At pH >7, wave i₃ is followed by wave i₄ which involves transfer of another proton and electron on the carbonyl group and yields the unsaturated alcohol. At pH <3, the radical formed in the first electron uptake is further protonated and is reduced in wave (i₁₊₂). In tetramethylammonium hydroxide solution, reduction of the side chains on C-17 occurs at more negative potentials. Reduction of testosterone and hydrocortisone follows a similar mechanism (A)—(G), but the second electron uptake in wave i₄ at pH >7 is not observed.     

A polarographic study of some N-oxygenated products of N-ethyl-β-methoxy-β-(3'-trifluoromethylphenyl)-ethylamine (sk and f 40652a)

The polarographic behaviour of N-hydroxy-β-methoxy-β-(3'-trifluoromethylpbenyl)-ethylamine, N-ethyl-N-hydroxy-β-methoxy-β-(3'-trifluoromethylphenyl)ethylamine and (3-methoxy-β-(3'-trifluoromethylphenyl)acetaldoxime has been studied over the pH range 0—14. The hydroxylamines gave rise to anodic and cathodic behaviour whereas the oxime gave only a cathodic wave. The mechanism of the oxidation and reduction processes was investigated by d.c. polarography and preparative micro-coulometry. The optimum pH values for analytical purposes were 7, 8 and 4 for the two hydroxylamines and the oxime, respectively. The polarographic behaviour of a mixture of the three compounds was studied and the determination of traces of such compounds by differential pulse polarography is discussed.     

Polarographic analysis for corticosteroids: Part 1. The electroanalytical properties of corticosteroids

The electroanalytical behaviour of corticosteroids has been studied in different supporting electrolytes. The reduction patterns are established by examining pH profiles, by constant-potential coulometry, fast-sweep voltammetry, potentiometry with ion-selective electrodes and differential pulse polarography. Dimerization and alcohol formation take place with the reduction of the C-3 keto group resulting in one or two peaks, depending on the number of double bonds in the A-ring and the pH. The C-20 keto group is reduced to an alcohol. Both reduction steps can be used for analytical purposes. The differential pulse peak height is linear with the concentration down to 10^-6 M steroid.     

Effect of methyl viologen on the electrochemical behavior of denatured cytochrome c at mercury and solid electrodes

Denatured cytochrome c has been studied in 9.5 M urea medium in the presence of methyl viologen, using differential pulse polarography (DPP) and voltammetry (DPV), alternating current polarography (ACP) and cyclic voltammetry (CV). The DPP and CV curves exhibit an additional peak, which is assigned to the catalytic reduction of denatured cytochrome c.     

Electrochemical reduction of nitrotetralones

The electrochemical reduction of three nitrotetralone derivatives by tast and differential pulse polarography and cyclic voltammetry in a wide pH range was studied. In aqueous media, the reduction of mono- and dinitrotetralone follows the general pattern of reduction of aromatic nitro compounds: the nitro group is reduced in a four-electron step to a hydroxylamine group. However, in mixed media this reduction differs from that of other nitrobenzenes, due to the fact that the formation of the nitroradical anion was not observed. The reduction of the acetoxy derivative was studied only at alkaline pH, because it suffered acid hydrolysis. The ionization pK of the protonatable groups were polarographically obtained.     

Electrochemical study and analytical applications for new biologically active 2-nitrophenylbenzimidazole derivatives

The present study addresses the electrochemical behavior and the analytical applications of six 2-nitrophenylbenzimidazole derivatives with activity against Trypanosoma cruzi. When studied in a wide range of pH, by differential pulse polarography, tast polarography and cyclic voltammetry, these compounds exhibited two irreversible cathodic responses. With analytical purposes, the differential pulse polarography mode was selected, which exhibited adequate analytical parameters of repeatability, reproducibility and selectivity. The percentage of recovery was in all cases over 99%, and the detection and quantitation limits were at the level of 1 × 10⁻⁷ mol L⁻¹ and 1 × 10⁻⁶ mol L⁻¹, respectively. In addition, the differential pulse polarography method was successfully applied to study the hydrolytic degradation kinetic of one of the tested compounds. Activation energy, kinetic rate constants at different temperatures and half-life values of such application are reported.     

Differential pulse polarographic determination of procymidone in formulations and wine

The dicarboximide fungicide procymidone was studied systematically by using direct current polarography, cyclic voltammetry, differential pulse polarography (DPP), controlled potential electrolysis, and millicoulometry in the universal buffer medium with dimethylformamide as the solvent. Procymidone exhibited a single well-defined polarographic wave in the pH range 2.0-6.0, leading to the formation of the hydroxy compound. The overall reduction process was diffusion-controlled and adsorption-free. The variation of half-wave potential with pH, the concentration of the analyte, and other experimental conditions are described. The reduction mechanism proposed is an overall 4-electron process, in which the dicarboximide group is reduced. DPP was used to determine procymidone in agricultural formulations and wine at the optimum conditions found; a detection limit of 2.4 x 10(-9) M was estimated. The results obtained by the proposed method were also compared with those obtained by other methods.     

The analytical performance of direct current,normal pulse and differential pulse polarography with static mercury drop electrodes

The recently developed static mercury drop electrode (SMDE) provides a fundamentally new approach to electrodes for polarography. An analytical evaluation of the electrode is presented. For a range of electrode processes, current-sampled d.c. polarography at the SMDE is useful down to at least the 10^-7 M concentration level when short drop times and fast potential scan rates are used. The improvement in the limit of detection for d.c. polarography is therefore very substantial. Improvements in sensitivity associated with normal pulse and differential pulse polarography at the SMDE compared with the dropping mercury electrode (DME) are marginal. It is concluded that at the SMDE, the analytical performance and response characteristics of d.c., normal pulse and differential pulse polarography tend to converge.     

Glycine adsorption on a mercury electrode modified by neodymium

Ranges of neodymium reduction potentials in aqueous solutions of potassium chloride at pH = 2 and in solutions containing glycine were determined by differential pulse polarography. The glycine complex with neodymium is not formed in the range of the amino acid concentration 0.001–0.12 M. Glycine adsorption on a mercury electrode and on an electrode modified with neodymium was studied by the cyclic voltammetry method.     

溴甲酚紫的极谱研究

溴甲酚紫是一应用广泛的有机式剂^[1-4]。关于羟基三苯甲烷类染料的极谱研究报导不多^[5]。Ghoneim^[6]曾报导溴甲酚紫在pH<7时产生一个2电子还原波,当pH大于10时,可产生两个1电子还原波。Goel^[7]只研究了第一波的还原机理。本文用普通极谱、脉冲极谱、循环伏安等方法较详细地研究了溴甲酚紫的电还原。提出一电还原机理,解释了半波电位随pH变化的特殊规律。     

Electrochemical study and polarographic determination of ranitidine

In the polarographic reduction of ranitidine, an H₂-antagonist of histamine, three waves are observed; their half-wave potentials and limiting currents depend strongly on the pH of the solution. The first and second waves are due to reduction of teh protonated, CHNO₂H⁺, and unprotonated, CHNO₂, nitroethene group of ranitidine, respectively; the origin of the third wave is unknown. The characteristics of the second and third waves are studied in acetic acid/acetate buffer at pH 5.5; the first wave does not appear at this pH. The second wave (E_{$\text{1}\text{2}$} = ?0.90 V, vs. Ag/AgCl) is useful for determining ranitidine in the range 2.4–4.9 × 10^?4 M by direct current polarography and in the range 2.5 × 10^?7?2.05 × 10^?5 M by differential pulse polarography.     

455 — Effect of pH on the electroactivity of horse heart cytochrome c

The effect of pH on the electroactivity of horse heart cytochrome c has been studied by differential pulse polarography, differential pulse voltammetry at the gold electrode, cyclic voltammetry and spectro-photometry; the gold electrode has been activated by 4.4′-bipyridine. The experimental results have been interpreted on the basis of the existence of a pH-dependent equilibrium between two electroactive cytochrome c forms; in agreement with the Lambeth et al.'s scheme the existence of transient species can be postulated.From spectrophotometric and differential pulse voltammetric experiments the pK_a value of 8.1±0.1 has been calculated for the equilibrium between the neutral and alkaline cytochrome c forms: state III ? state IV+H⁺.     

An electrochemical study of the stability of cytochrome c3 from Desulfovibrio desulfuricans Norway strain

The electrochemical behaviour of cytochrome c₃ from Desulfovibrio desulfuricans, Norway strain, remains unchanged within the pH range 5–9.5. Differential pulse polarography peaks disappear at pH < 5 and two transitions are noted at alkaline pH (10.5 and 12). In denaturation experiments, guanidinium chloride has a more marked effect than urea on differential pulse polarography and cyclic voltammetry peaks.The study of absorption spectra shows that the δ band at 351 nm of the oxidized form increases above pH ∼ 10.Neither absorption spectra nor polarograms are modified after the treatment of cytochrome c₃ for 10 min at 100°C.It is concluded that hydrogen bonds must play a prominent role in maintaining the tertiary structure of the protein. Cytochrome c₃ from D. desulfuricans Norway is a particularly stable molecule as regards pH, denaturing agents, temperature and ageing.     

Electrochemical reduction of nicergoline and its analytical determination in dosage forms

Electrochemical reduction of nicergoline was studied at different pH and concentrations using differential pulse polarography and linear sweep voltammetry. Both techniques reveal that the reduction process occurs with strong adsorption of the product. Nicergoline is an excellent model for the previously developed theory related to the effects of strong adsorption of electroactive species in voltammetry. At concentrations below 0.1 mM, the adsorption prepeak is linearly dependent on nicergoline concentration. This peak was used to develop a new differential pulse polarographic method for the determination of the drug in pharmaceutical dosage forms. The method is simple and not time-consuming because nitrogen purging of samples and previous separation of the excipients were not needed. A comparative UV spectrophotometric assay was applied. Recovery data and composite and uniformity content studies for both methods are reported.     

Electroanalytical studies of the pesticides menazon and its hydrolysis products

An electroanalytical study based on d.c. polarography, differential pulse polarography (d.p.p.) and coulometry is described for Menazon, S-[4, 6-diamino-1,3,5-triazin-2-yl)- methyl]-O,O-dimethylphosphorodithioate, and its alkaline hysdrolysis products in a (1 + 4) methanol/water medium. The responses of the different species are studied for analytical utility. Menazon itself produces two diffusion-controlled cathiodic waves or peaks at pH 4.78; the determination limit is 5.5 × 10^-7 M for the d.p. peak at about ?0.8 V vs. Ag/AgCl. The hydrolysis products obtained in 0.1 M sodium ydroxide give rise to four anodic waves. Acidification of the alkaline medium to pH 4.3 produces three well-defined waves; two cathodic and one anodic. The responses for the cathodic processes are linear with concentration over 2–2.5 orders of magnitude; the anodic wave is adsorption-controlled and provides linear response only at low concentrations. The detection limit for the hydrolysis products is 0.17 × 10^-6 M. Reaction mechanisms are proposed.     

On the electroreduction of 4-chloro-2,6-diisopropylamino-s-triazine (propazine) on mercury electrodes

This paper presents a polarographic (dc and differential pulse (DP)) study of the reduction of the s-triazine derivative propazine (4-chloro-2,6-diisopropylamino-s-triazine). The study is performed in acidic media (from solutions 2.25 M in H₂SO₄ to pH 5) because no signals were obtained above pH 5 (even at pH values of 11–12). In the pH range 2–4 the polarograms decreased until they vanished. In DP polarography, two main reduction peaks were observed, accompanied by a pre-peak at less negative potentials, and a post-peak at more negative potentials, due to the adsorption of propazine on the electrode. The main peaks corresponded to two-electron reduction processes. At pH below the protonation pK of the triazine ring (about 1.7), the results showed that, in a first stage, propazine suffers a cleavage of the Cl atom via a CEC process (electron transfer placed between two chemical reactions) to yield a dechlorinated intermediate, which is reduced through an irreversible two-electron process, the rate-determining step (r.d.s.) being the second electron transfer. At pH above the pK, a protonation of the triazine ring precedes the reduction process, this reaction being also responsible for the observed decrease in limiting current.     

Polarographic determination of deltamethrin

The electrochemical characteristics of deltamethrin have been determined by means of electrochemical techniques such as d.c. polarography, cyclic voltammetry and differential pulse polarography over a wide range of pH from 2.0-12.0. The title compound exhibits a single well defined peak due to the reduction of the CC moiety present in deltamethrin. The overall reduction process is diffusion-controlled and adsorption free in nature. The variation of half-wave potential with the pH, concentration of the title compound, and other experimental conditions is described. A possible mechanism for the reduction is suggested. The number of electrons involved in the electrode reduction is two. Investigation was also undertaken for the determination of deltamethrin in vegetables and in storage bags of rice and wheat under FCI's storage system.