Polarization sensitive in situ infrared spectroscopy: the adsorption of simple ions at platinum electrodes

The application of a simplified spectrometric setup to the investigation of the electrochemical double layer using polarization sensitive in situ infrared spectroscopy is described. As examples, the adsorption of carbon monoxide, thiocyanate, and cyanate at platinum electrodes are investigated. Results are compared with previously published data. Differences are explained in terms of different experimental conditions, they are not caused by differences between the employed methods.     

The potential dependence and kinetics of HCOOH adsorption on rhodium electrodes

The formic acid adsorption on an electrochemically prepared rhodium electrode has been studied by the radiochemical method. Electrochemical properties of the rhodium electrode surface in 0.5 M H₂SO₄ have been investigated by cyclic voltammetry. It has been shown that starting from E=0.20 V the rate of HCOOH adsorption is markedly potential dependent being practically independent of the electrode potential up to E=0.20 V. It seems that the HCOOH adsorption process may be explained on the basis of the two-sites kinetics model. The data obtained for HCOOH adsorption on a rhodium electrode have been compared with those for a platinum electrode reported previously.     

Voltammetric determination of ubiquinone adsorption at mercury electrodes

Adsorption isotherms of the ubiquinones UQ₄, UQ₆, UQ₉ and UQ₁₀ have been determined by two independent voltammetric techniques at the stationary mercury/solution interface. While the redox behavior is essentially the same for all homologues investigated, the molar free enthalpy of adsorption Δ_AG increases steadily with the number of isoprenic units from UQ₄ to UQ₁₀. In case of UQ₁₀, the experiments have been carried out with three different potentiostatic systems. This allows one to estimate the accuracy of the methods which are direct ones and do not require calibration.     

The adsorption of the HgEDTA complex at mercury electrodes

While free EDTA has no tendency to adsorption on mercury surfaces, its complex with Hg(II) is adsorbed strongly. The coverage is very small in alkaline solutions where HgYOH^3? is present, reaches 60% at moderate pH, and is high at pH = 2, where the predominant species in solution is HgYH^?. Dependence of peak potential on pH for cathodic stripping voltammetry indicated that for pH > 3, HgY^2? is adsorbed at the surface, while at pH 2 the adsorbed complex is protonated. Cyclic chronopotentiometric experiments suggest formation of a coherent film of adsorbed material at pH 2. At pH = 2 adsorption of HgEDTA can be described by a Frumkin isotherm, and at pH = 4.8 by either a virial or HFL isotherm.     

Voltammetric detection of cadmium ions at glutathione-modified gold electrodes

An electrochemical sensor for the detection of cadmium ions is described using immobilized glutathione as a selective ligand. First, a self-assembled monolayer of 3-mercaptopropionic acid (MPA) was formed on a gold electrode. The carboxyl terminus then allowed attachment of glutathione (GSH)via carbodiimide coupling to give the MPA-GSH modified electrode. A cadmium ion forms a complex with glutathione via the free sulfhydryl group and also to the carboxyl groups. The complexed ion is reduced by linear and Osteryoung square wave voltammetry with a detection limit of 5 nM. The effect of the kinetics of accumulation of cadmium on the measured current was investigated and modeled. Increasing the temperature of accumulation and electrochemical analysis caused an increase in the voltammetric peak of approximately 4% per degrees C around room temperature. The modified electrode could be regenerated, being stable for more than 16 repeated uses and more than two weeks if used once a day. Some interference from Pb(2+) and Cu(2+) was observed but the effects of Zn(2+), Ni(2+), Cr(3+) and Ba(2+) were insignificant.     

Stripping voltammetry of silver ions at polythiophene-modified platinum electrodes

The present work describes the development of a modified platinum electrode for stripping voltammetric determination of silver. The deposition of films based on electropolymerisation of the monomer thiophene was carried out by cycling the potential towards positive values between 0 and 1.6 V.The preconcentration process of silver ions was initiated on the surface of the modified electrode by complexing silver with polythiophene (PTH) when a negative potential (−0.5 V) was applied; then the reduced products was oxidized by means of differential pulse stripping voltammetry and the peak was observed at 0.17 V. Parameters such as pH, supporting electrolyte and number of electropolymerisation cycles were studied. A linear relation between current peak and concentration of Ag(I) was obtained in the range 0.07-1.0 mg L⁻¹. The detection limit for Ag(I) was evaluated to be 0.06 mg L⁻¹. The reproducibility was tested carrying out 11 measurements at different electrodes and the relative standard deviation was 1.5%. The interference of several metals was investigated and showed negligible effect on the electrode response.     

Specific adsorption of azide ions on mercury electrodes from constant ionic strength solutions

The adsorption of azide ions on mercury from aqueous solutions of NaN₃+NaF at constant ionic strength has been studied by measurements of the double layer capacity. Taking into account changes in the structure of the diffuse layer, it was possible to show that the properties of the inner layer are similar to those observed previously in solutions of pure NaN₃, and that the same adsorption isotherm applies to both systems.     

Charge exchange mass spectrometry at high energy

It is shown that charge exchange mass spectra using various reagent ions can be determined on a commercial double focusing mass spectrometer. The experiment relies upon the presence of a collision region held at a potential above ground with the product ions being selected by their unique kinetic energies. Cyclohexene has been studied in detail and the results are in agreement with previous thermochemical and field ionization kinetics data. Agreement with reported charge exchange spectra of n-propanol, taken at lower energies, is good. The present results, including both the variation of spectra with reagent gas and the small range of kinetic energies present in the charge exchange beam, indicate that a resonant or near resonant process is involved. Thus, the internal energy of the nascent ion is defined within quite narrow limits, just as is the case for charge exchange at lower energy.     

Cyclic voltammetry and chronoamperometry of lead ions at carbon fibre electrodes

Cyclic voltammetry of reversible redox couples. e.g., hexacyanoferrate (III)/(II), at carbon fibre electrodes yields sigmoidal anodic and cathodic scans that are almost superimposed. However, the cyclic voltammogram of lead (II) ions was markedly different. The voltammogram, and results from chronoamperometric measurements, can be interpreted on the basis of lead nucleation.     

Charge propagation in "ion channel sensors" based on protein-modified electrodes and redox marker ions

The mechanism of charge propagation in "ion channel sensors" (ICSs) consisting of gold electrodes modified with a layer of charged proteins and highly charged redox-active marker ions in solution was investigated by electrochemical techniques, QCM and AFM. The study is based on seven proteins (concanavalin A, cytochrome c, glucose oxidase, lysozyme, thyroglobulin, catalase, aldolase, and EF1-ATPase) in combination with seven electroactive marker ions ([Fe(CN)6]3-, [Fe(CN)6]4-, [Ru(NH3)6]3+, mono-, di-, and trimeric viologens), as well as a series of suppressor and enhancer ions leading to the following general statements: (i) electrostatic binding of charged marker ions to the domains of the protein is a prerequisite for an electrochemical current and (ii) charge propagation through the layer consists of electron hopping along surface-confined marker ions into the pores between adsorbed proteins. It is further shown that (iii) marker ions and suppressor ions with identical charge compete for oppositely charged sites on the protein domain, (iv) electrostatically bound multilayers of marker or enhancer ions with alternating charge form on a charged protein domain, and (v) self-exchange and exergonic ET catalysis between adsorbed marker ions and marker ions in solution take place. In addition to fundamental insight into the mechanism of charge propagation, valuable information for the design, optimization, and tailoring of new biosensors based on the ICS concept is demonstrated by the current findings.     

Charge overlap interaction in quantum dot films: time dependence and suppression by cyanide adsorption

Chemical bath deposited films of CdSe nanocrystals (<4 nm) are shown to exhibit time-dependent spectral red shifts, caused by increasing overlap of the electron wave functions in adjacent nanocrystals. Treatment of these "aggregated" films with aqueous KCN solution results in repulsion of the wave functions due to the strongly adsorbed negatively charged cyanide and thus electronic decoupling of the physically connected nanocrystals. The previously reported band gap increase due to cyanide adsorption on nominally uncoupled nanocrystals is also described here in more detail.     

Charge trapping reactions in bilayer electrodes

The cyclic voltammetric peaks for charging trapping and untrapping reactions between the inner and outer redox polymer films of five bilayer electrodes are compared to a theory for control of the rate of charge trapping by electron diffusion rates in the inner polymer film. The five bilayer electrodes use various different redox polymer films (electropolymerized poly-pyridine complexes of Fe, Ru, and Os, and polyvinylferrocene) arranged in different orders. The currents on the rising edge of the bilayer trapping and untrapping peaks follow the electron diffusion theory up to ca. 80% of the peak current; currents thereafter are controlled by another process(es). The analysis yields values for the electron diffusion constants in the inner bilayer polymer films, which agree with one another for different bilayers having the same inner film polymer films and which also agree with independent determinations by other methods. Two of the bilayers are made from the same two polymers, arranged in different inner-outer order. These bilayers also illustrate the occurrence of a “leak reaction”, in which charge trapped in the outer film is discharged via a thermodynamically unfavorable electron transfer reaction with the inner polymer film.     

Modeling of specific adsorption of ions at electrodes during the passing from a mixed solution of constant ionic strength to a binary solution

Electrocapillary curves in the mc NaNO₃ + (1 ? m)c NaF mixed solutions are calculated in terms of the Alekseev-Popov-Kolotyrkin model. The analysis of limiting case m = 1 shows that in a NaNO₃ binary solution the model predicts quadratic dependence of the NO ₃ ^? anion adsorption energy on the potential, which contradicts to experimental data. A modification of the model is suggested, which allowed removing this contradiction.     

Impedance analysis of the transport of counter ions at polypyrrole-Nafion composite electrodes

The ion transport accompanying the redox reactions of polypyrrole at a polypyrrole-Nafion composite electrode was investigated with systems containing specifically selected electrolytes. From the cyclic voltammetric and impedance data obtained, transport of cations was found to be responsible for the charge transfer process and parameters such as the charge transfer resistance, R_ct, the low frequency polymer resistance, R_e, the limiting capacitance, C_e and the diffusion coefficient, D_ct for the related cations were estimated. The electrochemical behaviour (i.e., electronic insulation and electronic conductance) of polypyrrole-Nafion composite electrodes was found to be the same as that of the polypyrrole electrodes, except that they appeared in regions of more negative potentials.