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The electrochemical impedance of anodically grown passive films on glassy metal Fe 80 B 20 in borate buffered solutions, was measured in the range between +0.50 V vs. sce , the potential of surface film formation, and +1.10 V, which is deep in the range of the oxygen evolution reaction (OER). By modelling, numerical fitting, and validation procedure, equivalent circuits in particular ranges of potentials were established. The values of elements of these were determined and their potential dependence analysed. Results of both the experimental and modelling procedures show that: (i) at passive potentials, between +0.50 and +0.70 V, the electrode impedance exhibits pure capacitance behaviour due to the non-separable film, and Helmholtz double-layer capacitance, with a low ion transfer anodic current density, i a ; (ii) at E ⩾ +0.75 V, the pure capacitance model is complemented by surface (adsorbed) state impedance; (iii) at potentials between +0.95 and +1.05 V the impedance data and a steep increase in i a point towards two-step kinetics of the OER, in which surface (adsorbed) states mediate electron transfer. Comparison of impedance data for Fe 80 B 20 with those obtained for Fe indicates a lower rate constant for the formation of surface (adsorbed) states, a different control step and a lower electrocatalytic efficiency for the OER on Fe 80 B 20 .

Anodic films formed by electrochemical passivation of Fe80B20 ribbons in borate-buffer and in acid sulfate solutions, were studied using impedance spectroscopy, in a broad range of frequencies, at potentials within a range of constant passive currents. The impedance data were analysed according to the general, multiparameter electrode admittance equation, which takes into account at the most, four relaxation processes. An insulator or n-type semiconductor barrier layer impedance determines the total electrode impedance of the passive film formed in the borate buffer solution. In acid sulfate solutions, the total electrode impedance is determined by both an ion-conducting inner layer and a precipitated outer layer. The impedance of the outer layer is characteristic for diffusion in a finite length region, coupled with an ion accumulation hindered kinetics at the inner/outer layer boundary at moderate frequencies, and modulation of the outer layer thickness at very low frequencies. In the borate buffer solution, there is negligible influence of boron on the corrosion rate. Films formed in acid sulfate solutions show a pronounced effect of boron, through formation of a more ion conductive inner layer. Higher corrosion rate of the substrate is caused by a higher dissolution rate of the anodic film.

Summary Cyclic chronopotentiometry has been applied in studies of the kinetics of homogeneous chemical reactions coupled to electrode reaction, and as a diagnostic technique for distinguishing between various mechanisms. Working curves, the plots of relative transition time a n vs. the kinetic parameter, were obtained by digital simulation. For the e.c.e. mechanism, which includes the equilibrium constant between the initial and final species, the information obtained from the third transition time is most important. Within the limits of the equilibrium constant K eq of 10 −5 to 10 4 the resolution was obtained either in a 2 or in a 3 for a whole range of probable kinetic parameters. In real experimental conditions one can obtain the rate coefficient for the first order chemical reaction between 0.05 and 200 s −1 . For the e.c.e. and the disproportionation mechanisms, which are quite similar in many respects, the working curves offer a means of distinguishing between these: if there is overlapping in the a 2 points the a 3 points for the two mechanisms are perfectly separated.

Summary The dismutation of uranium(V) in carbonate-bicarbonate solutions has been studied by means of cyclic chronopotentiometry. The aim of the investigation has been to elucidate the actual reaction mechanism. There is evidence from the data on the concentration and pH dependence of the rate coefficients for the dismutation of uranium(V) that the disproportionation reaction is paralleled by an e.c.e. mechanism. The e.c.e. path contributes significantly to the overall reaction rate for concentrations of free carbonate ion of 5×10−3 M and of U(VI) of 1 and 3×10−3 M. For higher concentrations of either the free carbonate or of U(VI) in the bulk of solution the disproportionation is the predominant mechanism. It is difficult, if not even impossible, with the presently available electrochemical techniques, to estimate the relative contributions of either reaction mechanism.

Adsorption of lower monofunctional alcohols from hexane solutions on controlled pore glass was studied using flow-microcalorimetry. Original and silicone treated glass surfaces were compared for their adsorption properties. For butan(1)ol adsorption densities of 0.26 and 0.71 nm 2 per molecule were found, respectively, the major difference in the energetics of adsorption on these two types of surfaces being entropic in nature.

The reduction of Ni 2+ ions at mercury electrodes in acidic perchlorate solutions, at perchlorate concentrations below 0.2 M , is characterized by absence of kinetic control in the preceding step, and by a complex reaction mechanism following the electron transfer. This reaction sequence is known to involve intermetallic compound formation between Ni and Hg and is best described, as shown here, by a parallel second and third order kinetic scheme. Apparent rate coefficients for this kinetic scheme were determined using cyclic chronopotentiometric data and fitting by digital simulation. A linearization test of computed kinetic rate coefficients versus the number of transitions permits quantitative tests of validity of assumptions made.

The electrokinetic phenomenon of surface conductivity was studied for capillaries of pure fused silica and of a borosilicate glass, in water, dilute aqueous solutions of HCl and NaCl, in methanol and acetonitrile. Measurements were done in the temperature range between 5 and 30°C by a dc linear voltage sweep technique at a rate of 0.2 V/sec or less, and with applied polarization voltages up to ±40 V. Surface conductivity was calculated from the total specific conductivity measured by two bright platinum electrodes at the mouths of the capillary and from the specific bulk conductivity of the liquid medium measured by a separate pair of Pt electrodes at the same temperature. Absolute values of the surface conductivity were between 10 −9 and 10 −10 ohm −1 . Energies of activation were calculated from the slopes, S , of log κ vs 1 T curves, observed within a temperature range from 5 to 30°C. The energies E defined as S = E / k were: 45–55 kJ/mole for the borosilicate glass, and 25–34 kJ/mole for fused silica in water, in aqueous solutions, and in methanol. In acetonitrile they were 105 kJ/mole for borosilicate glass and 59 kJ/mole for fused silica. The results were interpreted on the basis of the assumption that protonic conductivity is the predominant mode of surface charge transfer, with the rotation of water molecules in the first liquid layer as the rate-determining step.

The electrochemical reduction of the uranyl (VI) ion at Pt, gold-plated Pt and Ni-based thin mercury film rotating disk electrodes was studied in sodium carbonate solutions by cyclic voltammetry. The stability of TMF electrodes and reproducibility of voltammetric curves was tested with respect to different amounts of mercury deposited, and mechanical resistivity against film rupture. It was shown that the kinetic parameters obtained using TMF electrodes are independent of the base metal nature, and that these can be used for hydrodynamic conditions up to Re = 6000. Results indicate that data on k s for irreversible reactions, overlapping with reduction of the ion of the supporting electrolyte, can be assessed only to an ultimate precision of ± 30%.

Adsorption was studied of small monofunctional organic molecules on the surface of controlled-pore glass using flow microcalorimetry. Adsorption isotherms were obtained and monolayer coverage and heats of adsorption estimated. The hypothesis is put forward that the adsorbent-adsorbate interaction is predominantly pair specific, and that the relative basicity of the adsorbate correlates best with the observed heats of adsorption per unit surface area. A simple schematic model is advanced to help in the interpretation of the results.

Measurements of electrical conductance and of the thermoelectric force are reported for oriented solid Na-DNA samples with different amounts of adsorbed H2O. It is shown that H2O has a profound influence on the electrical transport mechanism, influencing both the absolute values of conductance, its energy of activation, and the sign of charge carriers. The semiconductor band model, with the adsorbed H2O molecules acting either as electron donors or acceptors, or proton donors, has been used in interpretation of results, and shown to be, at least, in qualitative agreement.

Electrokinetic potentials of several glasses of well defined bulk composition in aqueous electrolyte solutions were computed from streaming current measurements. The composition of the bulk is reflected in the magnitude of the potential and in the shape of the curves relating the electrokinetic potential and the pH of the solution. No differences have been found between the crystalline and the vitreous state of silica. Addition of Al 2 O 3 to the bulk results not in doping effects but rather in surface neutralization making the surface less acidic. Glasses with an appreciable amount of B 2 O 3 have higher negative potentials at the same pH, and the derivatives of electrokinetic potential vs pH show them to belong to a different type of surface than vitreous or crystalline silica. Studies involving controlled pore glasses of various pore sizes have shown the importance of below-the-surface ionic penetration and/or adsorption in the pores in establishing the solid-movable liquid interfacial electrical equilibrium.

The reduction of the nickel(II) cyanide complex in aqueous solutions, with and without an excess of free cyanide, was studied using the techniques of cyclic chronopotentiometry aided with computer simulation. The mechanism of reduction is an e.c.e. sequence, the chemical rate determining step being a dissociation reaction. Superimposed onto this reaction sequence is a fast equilibrium disproportionation reaction. The rate coefficient of the chemical reaction was determined by fitting working curves, generated by numerical simulation of the reaction sequence, to experimental data. In solutions containing an excess of free cyanide (0.1 mol l −1 KCN +0.1 mol l −1 KCl) the rate coefficient of the chemical reaction is κ c =0.34 s −1 P2 for any equilibrium constant of the disproportionation reaction, K , between 10 and 100. Corresponding values for 0.1 mol l −1 KCl solutions, in absence of free cyanide, are: κ c =0.03 s −1 , and K =10 −3 . P2

Flow microcalorimetry was used to study the adsorption of butanol on controlled pore glass (CPG) surfaces. Heats of adsorption and some thermodynamic data are reported for the adsorption process at the original, hydroxylated and hydrated surfaces and for those heat treated at temperatures of 450. 650 and 900°C. It was found that the molar free energy of adsorption is the same, 17 kJ mole −1 for all the surfaces studied. The molar enthalpy and entropy of adsorption are indicative of steric effects caused by water molecules. For surfaces heat treated to 650°C monolayer coverage of butanol is close to 5 molecules per nm 2 , the same figure as reported for the total number of vicinal and isolated silanol groups on the surface of silica.

Surface film formation studies using interfacial impedance are described for electrode surfaces of a Ni+Cr+Fe alloy (INCONEL-600) used extensively in various technological applications, particularly heat exchangers. Its corrosion resistivity and thermal conductivity are known to be strongly dependent on the state of the surface. Studies involved samples m the form of a rotating disk electrode, cyclic voltammetric identification of the active/passive transition, comparison between electrochemically and thermally modified surfaces, and interfacial impedance measurements. It is shown that the surface layer, prepared under increased temperature and pressure (480 kPa and 150 ° C), is passive and resembles the electrochemically grown passive layer obtained at potentials of polarization more positive than −0.2 V vs. SCE using a single, relatively dilute electrolyte, 1 × 10 −2 mol/dm 3 H 2 SO 4 . From the analysis of impedance parameters, it is concluded that an initial faradaic reaction runs in parallel to an adsorption process, both being inhibited successively by the formation of an electrochemically inactive surface film. The analysis of the potential, time and frequency dependence of the complex impedance data is used to indicate the relative contributions of faradaic and adsorption processes to the overall impedance in the active/passive transition of this material.

Summary The reduction-reoxidation mechanism for uranium(VI) in acidic perchlorate and nitrate solutions has been investigated by cyclic chronopotentiometry. By analysis of data for the second and third transition times it has been shown that possibly no single mechanism of either the reduction-disproportionation sequence or the e.c.e. mechanism is solely operative. The low order pH dependence, the negative dependence on ionic strength of the formal rate coefficient for the chemical reaction following electron transfer, and the consistently shorter third transition time with respect to the second, suggest interpretation based on the involvement of polynuclear complexes, on overlapping of reoxidation waves, and on the existence of an e.c.e. mechanism only in the reverse (reoxidation) step.

Batch and flow microcalorimetry were used to study adsorption of aliphatic amines on silica gel, controlled pore glass (CPG), high alumina glass, and surface-derivatized controlled pore glass. The adsorption of basic aliphatic amines is indicative of the acidity of the surface. Silica gel and controlled pore glass show the same average heat of adsorption for butylamine, 35 ± 2 kJ mole −1, but the CPG, in contrast to the silica gel has nearly homoenergetic surface. High alumina glass (17% Al2O3) has a less acidic surface, and the relevant value for the heat of adsorption of butylamine is only 4.7 kJ mole−1. Surface derivatization with a long-chain aliphatic silane (octadecylsilane) produces a highly hydrophobic surface, when water molecules are used as probes. Interaction with butylamine remains strong, as in the original state of CPG. Derivatization with aminopropylsilane yields a basic surface and the interaction with butylamine is weak, showing a heat of adsorption of only 3–4 kJ mole−1. The paper indicates that surface acidity of silica and glasses can be understood only as an interactive process between the surface and the adsorbate.

Electrical conductivity measurements were done on samples of solid bovine methaemoglobin of various hydration states. The energies of activation calculated from these data, are discussed with regard to Rosenberg's theory. The lowest value of the activation energy (observed at 35% hydration) corresponds to that of pure water. Using spectrophotometric techniques the interaction of “chromohaemoglobin” (“anhydrohaemoglobin” after Haurowitz) with oxygen was followed in situ. The importance of the shell of hydration for the reversible oxygenation is discussed and a new hypothesis advanced. An festen Methamoglobinen verschiedenen Hydratationsgrades wurden Messungen der elektrischen Leitfahigkeit vorgenommen. Die aus den Mesergebnissen berechneten Aktivierungsenergien wurden im Zusammenhang mit einer Theorie von Rosenberg diskutiert. Der niedrigste Wert fur die Aktivierungsenergie (beobachtet bei 35% Hydratation) entspricht der des reinen Wassers. Unter Verwendung spektrophotometrischer Methoden wurde die Wechselwirkung zwischen Chromohamoglobin (Anhydrohamoglobin nach Haurowitz) und Sauerstoff untersucht. Die Bedeutung der Hydratationssphare fur die reversible Oxygenierung wird untersucht und eine neue Hypothese aufgestellt.

An electronic device is described for the automatic reversal of the current flowing through an electrochemical cell at a predetermined potential. Provision is made for accurate selection and reproducibility of the potential for current reversal with the accuracy of ± 5 mV. In employing fast mercury-wetted contact relays the device has a switching time of 5 msec.

The chronopotentiometric technique has been used in studying the reduction of uranyl on mercury electrodes in sodium carbonate solutions At 1 M of free carbonate no kinetic complications were observed at experimental conditions pertinent to galvanostatic techniques at quiescent solutions. At low free carbonate concentrations (0.10 M ) the electrochemical reduction is followed by a second order chemical reaction the rate of which is dependent on the concentration of the free carbonate ion. The potential-time relation, is a suitable means of studying the kinetics as dependent on the free ligand concentration and on the concentration of the electroactive uranyl ion. The data indicate that the uranyl tricarbonate complex undergoes reduction without dissociation, while the coupled chemical reaction (disproportionation) involves rearrangement in the transition of uranium(V) into the uranium(IV) and uranium(VI) carbonate complex forms.

Summary The investigation of the mechanism of the electrochemical oxidation of uranium(IV) in sodium bicarbonate solutions is described. The investigation of the electrochemical reaction was carried out using the chronopotentiometric technique and is confined to concentrations of uranium and bicarbonate, at which the former is soluble and stable (1–4 mM of U4+ and 0.4–1.0 M NaHCO3). The electrochemical oxidation is an irreversible (β=0.7±0.02), two-electron transfer reaction with an Eτ/4=−0.17±0.01 V vs. SCE for 1 mM uranium(IV) in 1 M sodium bicarbonate. Evidence is given for chemical kinetic steps, preceding the electrochemical reaction, dependent both on the concentration of uranium and of carbonate. A hypothesis of a “deactivation” mechanism, involving competing association-dissociation reactions, is advanced.

The electrokinetic properties of nickel hydroxide, prepared by slow hydrolysis from NiCl 2 solutions, were studied using microelectrophoresis and streaming potential techniques. The particles of a stable Ni(OH) 2 colloidal dispersion were found to be negatively charged throughout the pH range from 12 to 3 where electrophoretic mobility measurements were made. The isoelectric points (IEP) were determined by extrapolation of the mobility -pH curves. They were: 1.50 in HCl (variable ionic strength), 1.79 with NaCl, and 2.77 with CaCl 2 as background electrolytes (constant ionic strength). Experiments in the liotropic series of alkali chlorides showed decreasing mobilities at constant pH, and increasing values of the IEPs of 1.64, 1.77, 2.04, and 2.49 at constant, 10 −3 M/l of LiCl, NaCl, KCl, and CsCl, respectively. The IEP of precipitated, washed and dried (100°C in vacuo) Ni(OH) 2 was found shifted to alkaline region: 9.7 in NaCl and 9.4 in CaCl solutions. NiO, produced by thermal decomposition (250°C in vacuo) of these precipitates was found to have the IEP at pH 11.0 in 10 −3 M/l NaCl solutions. The interpretation suggests that the acidic surface OH groups are structural elements of Ni(OH) 2 . Successive dehydration results both in the aggregation of particles and in the appearance of hydrophobic parts of the surface.

The initial stages of surface oxidation of the Ni-Cr-Fe alloy INCONEL-600 are investigated using cyclic voltammetric and impedance measurements.