Showing papers on "Polarography published in 1974"

The electrochemical behaviour of DNA at electrically charged interfaces.

Abstract: The adsorption and the reduction of native and denatured DNA has been studied at the hanging mercury drop electrode (HMDE) by triangular sweep polarography. It has been established from the dependence of the reduction peak of denatured DNA (Fig. 2) on the waiting time at the starting potential E8 of the sweep (Fig. 3) where the adsorption takes place and on pH (Fig. 4) that a protonated form of denatured DNA is reduced from the adsorbed state in a totally irreversible electrode process at a pH lower than 7.5. From the logarithmic analysis of the beginning part of the reduction peak the charge transfer rate constant and the charge transfer coefficient were determined (Fig. 5). The reduction products are interconnected to a high molecular network strongly adhering to the surface of the electrode and blocking it for a repeated reduction on the same drop. In neutral media the protonation of DNA becomes rate-determining. The adsorption is diffusion controlled and from the time dependence of the coverage the diffusion coefficient of DNA could be estimated (Fig. 6) to 5.10−7 cm2 s−1 in 1 M KCl at 25 ‡C and at pH 6.7 for denatured DNA of the mean molecular weight 106 g. The protonated molecules of DNA are adsorbed via strong induced dipole interactions of theπ-electron system in the heteroaromatic ring of the bases with electrons of the metal while the other parts of the DNA molecule including the solvated sugar and phosphate groups are orientated to the solution. From the time integral of the reduction peak (Fig. 6) the maximal surface concentration of the electrochemically active monomeric units in DNA was estimated to 1.4·10−10 mol cm−2 with an average area per mononucleotide of about 60 a2. Both values vary only within 10% in the temperature range 5 to 25 ‡C, in the concentration range of DNA 10 to 360 Μg/ml and for starting potentials Es −0.2 to −1.3 V vs. SCE. Native DNA is also adsorbed at the electrode and reduced in acid solutions in the same irreversible electrode reaction as denatured DNA. However, before a partial opening of the helix is induced by the strong electric field at the interface leading with respect to helix-coil transition to an effect equivalent to partial melting which has to precede the electron transfer step. The extent of unscreened reducible groups and their orientation with the electrochemically reactive sites towards the electrode depends on pH (Fig. 8), on electrical parameters of the interface as the starting potential Es (Fig. 7) and on the portion of the DNA molecule which comes into the region where the electric field is acting. The maximal extents of unscreened reducible groups are 85% for native DNA of the mean molecular weight of 2.5·106 g and 46% for native DNA of the mean molecular weight 107 g (Fig. 6). The results have with respect to the physicochemical properties of DNA general significance also for the behaviour of nucleic acids at charged biological interfaces as for instance the membrane of living cells.

Synthesis and electrochemical properties of heteropolymolybdates

Abstract: The beta form of 12-molybdosilicic acid was prepared and isolated as a yellow hygroscopic solid from a 1:1 mixture of water-dioxane. The identity of the beta- isomer was established polarographically and spectrophotometrically. The range of stability of the beta-isomer as a function of hydrogen ion concentration was determined. In addition, techniques were developed for preparing, in pure form, several heteropoly compounds used for polarographic and conductivity measurements. The solvolytic stability, electrolyte strength and oxidation-reduction behaviour of several heteropoly compounds were determined by pH, conductivity, and polarographic measurements. Heteropolymolybdate oxidation—reduction behaviour and stability in solution were examined by direct current and alternating current polarography and cyclic voltammetry in aqueous and water-dioxane solutions. The reduction of the anions proceeds in several steps for which the half-wave potentials were determined. Conductivity and pH measurements on the heteropoly acids H4 [PMo11V040]and H5 [PMo10V2O40]at 25 °C in aqueous solution, and in mixed oxygen-containing solvents, show that the vanadium compounds exhibit behaviour typical of unsymmetrical strong electrolytes in oxygen-containing solvents and are stable in these media.

Electrochemical Studies on Ag, Fe, and Cu Species in AlCl3 ‐ NaCl Melts

Abstract: The electrochemical behavior of the Ag/Ag+, Fe/Fe2+, Fe2+/Fe3+ , and Cu/Cu+, Cu+/Cu2+ systems in the equimolar region of has been investigated using potentiometry, pulse polarography, cyclic voitammetry, chronoamperometry, and chronopotentiometry. In contrast to the Al electrode, the emf of the Ag+/Ag electrode was found to be virtually independent of the melt composition. The Ag+ was found to undergo reversible deposition at a tungsten electrode. However, in cyclic voltammetric experiments, the stripping process showed a marked dependence on Ag+ concentration, temperature, and sweep rate. Possible explanations for this dependence are presented. The diffusion coefficient for Ag+ at 175δC was found to be . The Fe3+/Fe2+ couple was found to be reversible while the Fe2+/Fe couple showed typical deposition‐stripping behavior. Above an initial concentration of 5 mM the deposition process became concentration independent while the stripping process was found to involve two different electrode reactions. This behavior was consistent with the low solubility of in the equimoiar region. Nernst plots for the anodization of Cu gave low n‐values in Cl‐rich melts, while n‐values of 1 were obtained in ‐rich melts. These results are best explained by introducing the following equilibrium reaction for which a value of was calculated for . The two Cu couples were well behaved electrochemically.

The Reduction of [Ru(Hedta)(H2O)] at the Mercury Electrode and Subsequent Reactions

Abstract: The reduction of [Ru(Hedta)(H2O)] at the mercury electrode and the subsequent chemical reactions in buffer solutions were investigated by AC and DC polarography and by large-scale electrolysis. Two polarographic steps were observed; the more positive step is discussed in this paper. The step was a diffusion-controlled, one-electron reduction of ruthenium(III) to ruthenium(II). For [H+]=(10−3–10−5) mol dm−3 the reduction was reversible, and it became less reversible if [H+]<10−5 mol dm−3. Large-scale electrolysis in formate buffer solutions at the potential of the polarographic diffusion current plateau confirmed that the reduction is a one-electron process. In formate buffer, the reduced form of the complex is deactivated. Furthermore, in the presence of perchlorate ions the reduced form is reoxidized to the original form. The rate constant for the reoxidation reaction at 25.0 °C was k2=(1.38±0.26)×10−3 mol−1 dm3 s−1. The activation parameters for the reoxidation reaction were ΔH\eweq=(94.1±17.6)×103J mol...

Portable polarographic analyzer and quick polarographic determinations

Abstract: A special purpose portable polarographic instrument and method for the rapid repetitive quantitative determination of polarographically reactive species in aqueous solution utilizing a sample cell in which are placed a conducting sample solution, a suitable reference electrode and a fixed area working electrode and, connected to the electrodes, a special circuit for (1) impressing across the cell a reducing or oxidizing voltage throughout a time linear pre-selected range of scanning potential, (2) proportionately converting the consequent current flow to a potential, and (3) measuring the potential. The maximum potential is retained in a peak seeking voltage circuit and is read on a voltmeter in a voltage follower circuit. The polarographic test utilizing the instrument is made more specific for O PARALLEL -C- compounds, such as aldehydes, by reacting the test sample in aqueous medium with a hydrazine acid addition salt at a pH of about 3 to 6.5 to form the hydrazone in aqueous medium which is detectable with great sensitivity and specificity. The working electrode may be underlying or may be suspended, but in any event, is best replaced or renewed or cleaned between determinations.

Application of high-speed liquid chromatography to organic microanalysis. II

Abstract: The construction of a simple, inexpensive system for high-speed liquid chromatography is described. The system consists of a microdetector based on the principle of polarography with a dropping mercury electrode, an electronic measuring circuit, a gas displacement pump, and highly efficient chromatographic columns. The operation of the system is described and tested. All components of the system are readily available or easily fabricated, and the cost, exclusive of strip-chart recorder, is approximately $ 200.

Standard Electrode Potentials of R--/R+ Redox Couples and Medium Effects in Non-aqueous Solvents

Abstract: The standard electrode potentials of R-/R+ redox couples may be estimated from polarographic reversible half-wave potentials of R/R+ and R-/R couples. When R+ and R- are large ions, similar in size and structure, the standard potential may be assumed as a solvent-independent reference point of electrode potential in various solvents. A few possible candidates for the "reference redox couple" have been discussed.

Pulse radiolytic polarography. Competitive oxidation and reduction of hydroxycyclohexadienyl radicals at the mercury drop electrode in aqueous solutions

Abstract: Hydroxycyclohexadienyl are produced in the puls e irradiation of aqueous N/sub 2/O-staurated solutions of aromatic compunds by a 20-nsec pulse of high- energy electrons in the vicinity of a handing mercury drop electrode. The polarographic current is recorded as a function of timee and at constant potential. Simultaneously, the changes in the opticsl absorption of the radicals with time in the bulk of solution are measured. Short-time polerograms of radicals are obtained by plotting the current at about 20 mu sec after the pulse vs. the electrode potential. Many hydroxycyclohexadienyl radicals show a steep polarographic wave with an anodic part immediately followed by a cathodic part. These waves are interpreted in terms of competitive oxidation and reduction of radicals. In addition, the problem of disproportionation of hydroxycyclohexadienyl radicals via electron transfer is discussed. Both electrode processes can occur with large rate constants in a certain potential range. Shifts of the waves to more negative potentials in the case of phenol and cresol are explained by the existence of mesomeric structures of alpha -alcohol radicals in the dihydroxycyclohexadienyl radicals formed. In strongly alkaline solutions, O/sup -/ reacts with toluene and benzylalcohol to form the benzyl radical and the benzyl alcohol radicalmore » anion, respectively. The polarograms of these radicals have well separated anodic and cathodic waves. The current vs. time curve of some radicals show a chasge in the sign of the current signal at potentials close to the foot of the polarographic wave. This effect is attributed to temporary changes in the relative concentrations of adsorbed and mobile radicals in the vicinity of the mercury drop electrode. (auth)« less