Comparison of surface activity of organic compounds by the influence of surfactants on reduction peaks by a.c. polarography

Summary Surface activity of closely related organic compounds are compared by utilising the optimum concentrations corresponding to more negative peaks and concentrations required to remove or to suppress less negative reduction peak due to organic compound and due to metal ion to maximum extent instead of tensammetric peak by a. c. polarography. Surface activity measurements are p_H dependent. The correlation between optimum concentrations of surface-active substance and the amounts of reducible species would be of great significance in suppressing the polarographic maxima in d. c. polarography.

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Zusammenfassung Es wurden die Oberflächenaktivitäten eng verwandter organischer Verbindungen durch tensammetrische Studien mit der Methode der Wechselstrompolarographie bestimmt.

     

Electrogeneration of Hg(+) in the presence of tryptophan

In the presence of l-tryptophan, a one-electron oxidation of mercury takes place at the dropping mercury electrode at 0.265 V vs. SCE in 0.1M potassium nitrate. The electrogeneration of the monomeric Hg(+) ion has been established by a.c. polarography, cyclic voltammetry and ultraviolet spectroscopy. A paramagnetic Hg-trp compound has been electrochemically synthesized and characterized.     

Pulse-polarographic analysis of nucleic acids and proteins

Nucleic acids and proteins were studied by means of derivative and normal pulse polarography, and d.c. and a.c. polarography in connection with the dropping mercury electrode. It was shown that natural ribonucleic acids, as transfer, ribosomal and viral RNAs yield derivative pulse-polarographic peaks; from their heights and potentials conclusions can be made about their content of ordered structure in solution, similarly as in the case of deoxyribonucleic acids studied earlier. Synthetic single-stranded polyribo-cytidylic acid yields a well developed peak, whereas in the double-helical complex with polyriboguanylie acid it is inactive when using either derivative pulse polarography or d.c. polarography. Well developed peaks were obtained also with albumin (a protein containing reducible?S?S? groups), while only an inflex was observed on the d.c. polarogram. Proteins were also studied in media containing cobalt (Brdi?ka's solution) or nickel and it was shown that derivative pulse polarography due to its high sensitivity and accuracy enables us to carry out the measurements even in less common media than Brdi?ka's solution. This fact could be exploited in clinical chemistry as well as in the investigation of the nature of catalytic currents of proteins. The currents of double-helical polynucleotides obtained by means of normal pulse polarography exhibit a marked dependence on the initial potential and cannot represent a reliable indicator of structural changes of biopolymers in solution. They can however, be used in studies of the influence of the polynucleotide adsorption at different potentials on the subsequent reduction.     

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.     

Polarographic determination of thiabendazole

Summary Polarographic Determination of Thiabendazole Thiabendazole is used as a fungistat on citrus fruit to protect it from decay.In order to work out a polarographic determination the electrochemical behaviour of thiabendazole was investigated by sampledd c polarography and differential pulse polarography. Thiabendazole shows one wave or peak in the polarogram, when a short drop time (0.4 s) is used. Detection is most sensitive at pH 8. The current measured is proportional to the concentration. The detection limit is 0.5 ppm.By evaluating further experimental data it was possible to conclude, that mercury is first oxidised. Thereby two electrons are exchanged. Then mercury(II) ions formed in this way react with two molecules thiabendazole. Two protons are involved in this chemical process.Applying drop times greater than 0.4 s and concentrations higher than 1.5·10^–4 M a polarogram with two peaks is formed. This is due to the adsorption of the reaction product.For quantitative determination several citrus fruits were peeled and the peels were extracted with ethylacetate. After removing interfering substances by shaking with sodium hydroxide solution, thiabendazole was extracted by diluted hydrochloric acid and quantitatively determined by differential pulse polarography. The recovery of thiabendazole is 70.0% with a relative standard deviation of 2.9%. For all samples investigated the thiabendazole concentrations were below the permitted value.Cordially dedicated to Univ.-Prof. Dr. Dr. h. c. K. Kratzl on the occasion of his 70th birthday.     

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.     

Mechanism and analytical aspects of the polarographic maximum wave of tellurium

The polarogram of tellurium(IV) in weakly basic solution has a sharp maximum on the diffusion current plateau. The electrode process causing this maximum has been examined by means of various techniques such as semiconductor catalysis, "block" polarography and ultraviolet irradiation of the dropping mercury electrode. The maximum was found to be caused by the catalytic wave of hydrogen produced by the tellurium deposited on the mercury surface. Part of the tellurium is reduced to hydrogen telluride by nascent hydrogen formed catalytically. The brown mist which usually appears in the vicinity of the surface of the dropping mercury cathode consists of elemental tellurium produced by the decomposition of the hydrogen telluride near the electrode. The sudden decrease in the current maximum at a potential more negative than the peak potential of the maximum is due to desorption of tellurium, caused by movement of mercury.     

Preconcentration with chromatomembrane cell and adsorptive polarographic determination of fluorine in air

The present paper describes a procedure in which fluorine in the air was preconcentrated in a chromatomembrane cell and its content was determined by adsorptive polarography. In a pH 4.90 buffer solution the fluorine ion can form a ternary complex with La(III) and ALC. The complex can be adsorbed at the mercury electrode and yields a sensitive oscillopolarographic wave at -0.67 V, which can be sensitized by Triton X-100. Over the range 3.0x10(-8)-1.60x10(-6) M, the peak currents are linearly proportional to the concentration of the fluoride. The detection limit is 1.0x10(-8) M. First the fluorine in the air samples was preconcentrated in the chromatomembrane cells using 0.10 M NaOH solution, then its content was determined by complex-adsorptive polarography.     

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.     

Homocysteine Voltammetry at a Mercury Electrode in the Presence of Nickel Ions

At a mercury electrode, Hcy and Cys yield similar cathodic stripping peaks connected to the reduction of the pertinent mercury thiolate. However, due the different behavior as a ligand for nickel ion, the above compounds perform very differently in the presence of this ion. Whereas the nickel ion at a high enough concentration suppresses the Cys peak, in the case of Hcy it causes the cathodic peak to shift to more negative potentials. The peculiar behavior of Hcy is due to the stabilization of the mercury thiolate surface layer by intermolecular Ni²⁺ bridges within the surface layer. Conversely, in the case of Cys, the nickel ion strongly competes with the mercury ion and leads to the formation of a surface adsorbed bis‐cysteinatonickelate complex, which prevents the formation of mercury cysteinate. Such a difference allows determining Hcy by cathodic stripping voltammetry in the presence of nickel ion with no Cys interference.     

Polarographic study of uracil derivatives

Eighteen uracil derivatives were studied by d.c. polarography, differential-pulse (d.p.)polarography and d.p. cathodic stripping voltammetry. In a borax buffer at ca. pH 7.6, uracil, thymine and derivatives such as 5-halouracils, 5-trifluoromethyl-,5-aza-,5-acetyl-, 5-formyl- and 5-vinyl-uracil produced well-defined peaks at potentials between 0 V and ca. 160 mV vs. silver/ silver chloride (satd. KCl). The peaks are ascribed to the formation of sparingly soluble mercury salts. For the other derivatives tested (e.g., 5-nitro- and 5-ethynyl-uracil and 6-substituted uracils), the peaks were less well-defined and in some cases the polarographic curves were very complex. 2-Thiouracil produced a single peak at ca. ?400 mV, but only at pH 12.2. The shapes, heights and potentials of the peaks depended on the kind and position of the substituent on the pyrimidine ring. Rectilinear relationships of peak current vs. concentrations were found for most compounds (10^?5-10^?4 M) by d.p. polarography; d.c.polarography was not tested quantitatively. For 5-fluorouracil and 5-vinyluracil, linear calibrations were found for concentrations of 0.5–5 × 10^?7 M by d.p. cathodic stripping voltametry. Interference studies showed that small amounts of chloride and phosphate did not interfere but 5-fluorodeoxyuridine, which did not itself produce a peak, and proteins interfered seriously.     

A study of the electrochemical characteristics of some thiols by differential pulse polarography and other electrochemical techniques

A study of some low-molecular-weight odorous thiols at mercury electrodes has been made by differential pulse polarography (d.p.p.), d.c. polarography, linear-sweep voltammetry, and coulometric measurement on linear-sweep reduction. Linear current versus concentration plots are found for individual thiols, in aqueous 0.2 M sodium hydroxide by using d.p.p. at thiol concentrations from ca. 2 × 10^-7 M to 10^-4 M. A linear trend between peak potential and molecular weight was found for homologous series, but the method cannot be used for qualitative identification because of peak shifts in multicomponent mixtures. The electrode reactions show quasireversible behavior. At concentrations of thiol greater than 10^-4 M, the single peak splits into two peaks and film formation occurs. Charge density versus concentration plots indicate that the quantity of adsorbed material is several monolayers.     

Electrochemical Reduction of Cyclohex-2-enones

The electrochemical reduction of the cyclohex-2-enones 1a–1e (mercury cathode, CH₃CN, Bu₄NBF₄) was studied by means of cyclic voltammetry, d.c. polarography, coulometry and chemical product analysis. Compounds 1a–1c give a mixture of the hydrodimers 4 and 5 via formation of the radical anion 2 by an irreversible one electron transfer, followed by protonation and dimerization of the allylic radical 3 . The 6-halocyclohex-2-enones 1d and 1e exhibit two distinct reduction waves. The first corresponds to an irreversible two electron transfer with formation of the halide anion and the enolate anion 6 which gives 1b by protonation. The second wave corresponds to a quasi-reversible one electron transfer to 6 to afford the radical dianion 7 (Scheme 2).     

Determination of some methylcarbamate insecticides by a.c. polarography and cyclic voltammetry

The study of adsorption-desorption phenomena represents an important extension to the range of organic compounds that can be determined by electrochemical methods. The present work has utilized this approach for the determination of a range of methylcarbamate insecticides. The tensammetric behaviour of this class of compound at the mercury electrode has been investigated by a.c. polarography and the optimum experimental conditions for their analytical determination have been derived. Cyclic voltammetry has also been used to study the electrode process and by using the peak obtained on the cathodic sweep it was possible to extend the limit of detection to the ppm level.     

Comparison of fundamental and second-harmonic a.c., and normal,derivative and differential pulse linear-sweep and stripping voltammetric methods

Linear-sweep and stripping a.c. and pulse voltammetric methods have been compared for a variety of electrodes and electrode processes. Each of the linear-sweep techniques is readily used systematically because, in contrast to d.c. linear-sweep voltammetry, the theory for reversible electrode processes is basically analogous to that for polarography at a dropping mercury electrode. In stripping analysis, some departures are found at a hanging mercury drop electrode because of spherical diffusion effects. For reversible electrode processes, the limits of detection for a.c. and pulse methods are comparable. However, a.c. methods offer advantages over pulse methods in discriminating against irreversible electrode processes and permit the ready use of faster scan rates. Pulse methods are more sensitive for irreversible electrode process. Normal pulse polarography is particularly favourable in minimizing undesirable phenomena arising from adsorption or deposition of material on electrodes.     

8—氮鸟嘌呤的极谱伏安行为

用循环伏安法（ＣＶ）、电流采样极诸法（ＳＣＰ，即ＴＡＳＴ）、常规脉冲极谱法（ＮＰＰ）、微分脉冲极谱法（ＤＰＰ）、线性扫描伏安法（ＬＳＶ）、Ｏｓｔｅｒｙｏｕｎｇ方波伏安法（ＯＳＷＶ）和计时库仑法（ＣＣ）等电化学技术研究了抗癌药物８－氮鸟嘌呤（８－ａｚａｇｕａｎｉｎｅ，ｇｕａｎａｚｏｌｏ，简称８－ＡＧ）的极谱伏安行为．在 ０． １ｍｏｌ／Ｌ Ｈ２ＳＯ４底液中，８－ＡＧ有一良好的还原峰，峰电位（Ｅｐ）在－０． ９５Ｖ（ｖｓ．Ａｇ／ＡｇＣｌ，下同）附近，８－ＡＧ浓度在４×１０－６～８×１０－４ｍｏｌ／Ｌ范围内．峰高与浓度有良好的线性关系，线性相关系数ｒ＝０．９９９９～０．９９１０，检出限为１× １０－６ｍｏｌ／Ｌ．实验证明了该峰具有吸附性．本文提出了电极反应机理，它包括：酸性介质中８－ＡＧ的质子化、质子化的８－ＡＧ在汞电极上吸附以及完全不可逆的两电子还原过程．同时用量子化学计算方法（全略微分重叠法即ＣＮＤＯ／２）对８－ＡＧ和鸟嘌呤的各原子的净电荷以及Ｗｉｂｅｒｇ键级进行了计算，从理论上解释了８－ＡＧ的电化学还原机理。     

Application of reverse pulse polarography to the determination of substances which form films electrochemically on the mercury electrode

The application of reverse pulse polarography to the determination of substances which form films electrochemically on the mercury electrode is illustrated with penicillamine and cysteine. The dependence of the peak current on several variables is reported and compared with theoretical predictions. It is shown that under optimal instrumental conditions (long drop times and short effective pulses) reverse pulse polarography compares favourably with both normal pulse polarography and differential pulse polarography for the determination of penicillamine and cysteine, concentrations of penicillanline as low as 5 x 10(-8)M being readily determined in the presence of copper(II).     

Formation constants of some mercury(II) complexes determined from their anodic polarographic signals

The formation constants of some Hg(II) complexes have been determined from the anodic polarographic signals for the oxidation of mercury in the presence of the following ligands: thiourea and its phenyl- and diphenyl-derivatives, thiocyanate, ethylenediamine, EDTA, methylthioacetic acid, 2,2'-thiobisacetic acid, 3,3'-thiobispropanoic acid, 2,2'-[1,1-methandiylbis(thio)]bisacetic acid and 2,2'-[1,2-ethandiylbis(thio)]bisacetic acid. The use of differential pulse polarography and a.c. polarography instead of d.c. polarography increases the accuracy and precision of the potential measurements and, as a consequence, of the stability constants determined. The results obtained by the different methods are compared.     

The determination of some 2-thiobarbiturates by cathodic stripping voltammetry

Pulse polarography and cyclic voltammetry are employed in studies of the electrochemical behaviour of 5-ethyl-5'-(l-methylbutyl)-2-thiobarbituric acid (I), l-methyl-5-ethyl-5'-(l-methylpropyl)-2-thiobarbituric acid (II) and l,3-dimethyl-5-ethyl-5'-p-chlorophenyl)-2-thiobarbituric acid (III) in the pH range 4–12. All three compounds show anodic and cathodic waves or peaks in this pH range. Compounds (I) and (II) are oxidized at mercury indicator electrodes to produce mercury salts which can adsorb thereon and are thus amenable to cathodic stripping voltammetric analysis (c.s.v.) down to concentrations of the order of 10^-6 M, which is superior to the sensitivities obtained by differential pulse polarography (d.p.p.) based on a reduction peak. Compound (III) oxidizes to produce sulphur which is subsequently plated as HgS. Again the sensitivity of the c.s.v. method is of the order of lO^-6 M and analytically superior to d.p.p. The optimum pH for the three determinations is 8. The determination of (II) in the presence of its oxygenated analogue and metabolite, phemitone, and the effect of chloride ions are reported.     

Electrochemical behavior of decyl mercuric halides at mercury cathodes in dimethylformamide

Polarograms for decyl mercuric halides in dimethylformamide containing tetraalkylammonium perchlorates exhibit two waves. When large-scale electrolyses of decyl mercuric halides are performed at potentials corresponding to the first polarographic wave, the couldometric n value is unity and didecylmercury is obtained in quantitative yield; electrolyses carried out at potentials on the plateau of the second polarographic wave afford only decane and the n value is essentially 2. Double-potential-step chronocoulometry and staircase voltammetry indicate that, at potentials corresponding to the first polarographic wave, the decyl mercuric halide (which is itself adsorbed onto mercury to the extent of less than a monolayer) undergoes reversible one-electron reduction to adsorbed decyl mercury radicals and to adsorbed decyl mercury radical “polymer”; the adsorbed radicals have a lifetime of approximately 10^?3s and disproportionate into didecylmercury and elemental mercury. In the presence of electrolytically released halide ion, the adsorbed radicals are reoxidized to the decyl mercuric halide; alternately, the adsorbed species are reoxidized to decyl mercury cations at a potential approximately 600 mV more positive than that required for reoxidation to the decyl mercuric halide. At potentials corresponding to the second polarographic wave, reduction of decyl mercuric halides is an irreversible process producing decyl carbanions which are protonated by traces of water in the solvent to give decane.