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.     

Reverse pulse voltammetry in ec2 reactions: Electrochemistry of 1-alkyl-4-t-butylpyridinium ions

The applications of the reverse pulse method are extended in order to distinguish between different paths of electrodimerization. The method for determination of rate constants of the chemical reaction coupled with charge transfer (ec₂) is further developed. Measurements of K′_d vs. t_p provide valuable information for the determination of rate constants.The reverse pulse method is applied in the study of the electrodimerization of 1-alkyl-4-t-butylpyridinium ions (1-alkyl=?CH₃; ?CH₂CH₃; ?CH(CH ₃)₂) in acetonitrile. It is found that the electrodimerization mechanism is consistent with the sequence: Py⁺ + e = Py ? $\text{1}\text{2}$ Py₂. The formal potential E^o′ is measured directly for 1-CH(CH₃)₂, and indirectly for the other pyridium ions. All values are identical within experimental error (E^o′ = ?1.835 V vs. Ag/Ag⁺).The values of k_d and k_m are measured. With an increase in the size of the l-alkyl group, k_d decreases and k_m increases. This is consistent with the expected “stabilization” of the radicals due to the increase of the l-alkyl group size which hinders the dimerization site (2- and 6-positions).     

Determination of nanomolar concentrations of lead and cadmium by anodic-stripping voltammetry at the silver electrode

Lead and cadmium have been determined by subtractive anodic-stripping voltammetry (SASV) in the square-wave mode at a silver electrode without removal of oxygen. The sensitivities and detection limits for the two metals differ considerably. Detection limits of 0.05 nM for lead and 1 nM for cadmium have been achieved following 90 s electrodeposition. The repeatability of consecutive SASV runs is good (for lead 0.5% at 20 nM for 30 s electrolysis, 5% at 0.3 nM for 60 s electrolysis; for cadmium 2.5% at 20 nM for 30 s electrolysis, 5% at 5 nM for 60 s). Hundreds of runs can be carried out without any pretreatment of the electrode. The high stability is attributed to renewal of the electrode surface that takes place during the electrodeposition step in a two-electrode cell: the silver counter/quasi-reference electrode generates silver ions that codeposit with lead and cadmium at the Ag-RDE, thus ensuring a continuity of the latter. Underpotential deposition (UPD) plays a central role in anodic-stripping voltammetry (ASV). During the deposition step, the adatom coverage of trace elements is in the range of 0.01-1% and no bulk deposition is invoked for metals that exhibit UPD. The UPD properties and, as a result, the ASV signals are strongly affected by the type and concentration of the supporting electrolyte. The effects of Cl⁻, Br⁻, SO₄²⁻ and NO₃⁻ are shown. The analysis of lead and cadmium in natural waters has been performed. Surfactants distort the SASV signal. In order to ensure surfactant-free solutions, the samples were pretreated by wet ashing.     

The coulostatic method: Mathematical development for multi-step processes

Analytical derivations for multi-step processes of the type: ν_O)+ne=ν_RR is carried for systems subjected to both activation and diffusion control. Expressions for η/t and c_i(x, t)/t are derived. It is shown that the surface concentration c_i(0, t) has an extremum in contrast with the monotonic changes characteristic of other transient methods. The current—potential relationship at small perturbations from equilibrium is discussed.     

BIMANES—26. AN ELECTRON TRANSFER REACTION BETWEEN PHOTOSYSTEM I1 AND MONOBROMOBIMANE INDUCES STATIC CHLOROPHYLL a QUENCHING IN SPINACH CHLOROPLASTS

Abstract— Monobromobimane in chloroplasts lowers both the quantum yield of system II photochemistry and the yield of chlorophyll a fluorescence. Illumination of the chloroplasts in the presence of monohromobimane is an absolute prerequisite to the manifestation of this phenomenon, which proceeds via the Photosystem II intermediate, the semiquinone radical anion, Q_A-. The latter transfers an electron to monobromobimane to yield an anion radical (mBBr^·), which may either lose bromide ion to yield a reactive radical (mB^·), or acquire a proton and undergo further reduction, eventually forming syn-(methyl, methyl) bimane. In turn, mB reacts with the protein of the light-harvesting complex, to form a product which acts as static excitation energy quencher in the chlorophyll pigment bed of photosystem 11. Polarographic reduction of monobromobimane shows an adsorption wave at O V and two reduction waves. Prolonged reduction in water at -0.5 V yields syn-(methyl, methyl) bimane (which is further reduced at more negative potentials) and bromide ion. Thus, both electrochemical and chloroplast-induced reduction produce syn-(methyl, methyl) bimane. Monobromobimane may then serve as a Photosystem II activated probe in elucidating the conformation of intrinsic thylakoid membrane polypeptides.     

Ensembles of microelectrodes: A digital- simulation

The rate of diffusion to ensembles of microelectrodes is calculated by digital simulation, based on the explicit finite-difference technique. Two limiting cases are shown in the current-time relationship at short times, when the diffusion layer is small in comparison to the radius of each microelectrode, the current behaves as in the case of semi-infinite planar diffusion to the active area on the surface. At long times, when the diffusion layers of adjacent electrodes overlap significantly, the current approaches the case of planar diffusion to the total area of the surface, including the non-active regions. The results are compared with previous calculations based on analytical solutions, which were derived on the basis of different approximations. Good agreement between reported experimental data and results based on simulation is observed.Edge effects arising as a result of contributions of radial components to the total diffusion rate are shown to be significant, even on relatively large electrodes. The errors which arise due to such effects are estimated as a function of the size of the electrode and the duration of measurement.     

Temperature dependence of the transfer coefficient. The reduction of iodate in alkaline media

The reduction of iodate on a dropping mercury electrode was studied over a wide range of temperatures, and compared with earlier studies on the reduction of bromate under nearly identical conditions. A fully computerized experimental setup was used. This allows collection of a large number of experimental points, for proper statistical analysis of the data. The transfer coefficient was found to be a function of temperature, decreasing by about 10% with increasing temperature over 80°C. This behavior is very different from that found earlier for the reduction of bromate, where α is strictly independent of temperature.The reduction of iodate occurs at less negative potentials than that of bromate, far from interference by solvent decomposition. This allows very accurate determination of the Tafel parameters, but makes diffuse double layer correction more difficult, since specific adsorption cannot be excluded. We have tested the effect of specific adsorption of I⁻ by changing the concentration of iodate and by adding as much as 10 mM of NaI. The effect of the chloride was tested by replacing it with fluoride as the anion of the supporting electrolyte. The effect of surface active agents was tested by adding Triton X-100. Small variations in the transfer coefficients were observed between these experiments, but the temperature dependence of α persisted in all cases. Correction for the diffuse double layer effect changes the value of the transfer coefficient, but does not eliminate its temperature dependence.It must be concluded that the transfer coefficient for the reduction of iodate is temperature dependent, and this dependence cannot be attributed to experimental inaccuracy or inadequate correction for the effect of the diffuse double layer resulting from specific adsorption. This is in contrast to our findings for the reduction of bromate, hydroxylamine and the H₃O⁺ ion, for all of which α is found to be strictly independent of temperature.