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It is shown that the reduction of Cd(II) can be studied using the dropping mercury microelectrode (DMμE) so that the preceding chemical step is kinetically fully rate-determining. Consequently its rate constant can be obtained with good accuracy, which allows us to show that its weak potential dependence can be explained quantitatively using simple double-layer theory. The DMμE voltammogram is shown to vary with base electrolyte concentration in accordance with these ideas.

A full mathematical description of impedance voltammetry and demodulation voltammetry is given for the case where the electrode reaction is activation controlled and is also complicated by reactant adsorption. Several limiting cases are treated. The method is compared with a method proposed earlier based on an intuitive hybrid equivalent circuit comprising the separate cases of either slow electron transfer or reactant adsorption is made and the limits of its validity are assessed. Application of the complete theory to Tl + reduction showed that it proceeded by a CeC mechanism, complicated by both Tl and Tl + adsorption. The necessity of combining first- and second-order experimental data is stressed. The main reason for this is that the effects of adsorption and slow electron transfer tend to compensate each other. However, the presence of reactant adsorption raises the possibility of measuring unusually high electron transfer rate constants.

From the voltammogram of a dropping zinc amalgam microelectrode extending for over 1 V, the kinetic parameters found earlier with impedance voltammetry were confirmed. However, with the microelectrode technique the kinetic parameters of the second electron transfer could be obtained with much better accuracy. From the data at the more positive potentials the existence of one more step was discovered, the rate of which depends only weakly on potential.

The relationship between the experimental zero charge potential, Epzc, of indium and thallium amalgams and their electronic work function, Φ, is studied and discussed. For indium amalgams, the linear relationship, Epzc = Φ −const., is found to be obeyed in the surprisingly wide composition range of 0.02 < xIn < 0.6. The data for pure mercury do not fit on the line, presumably because of its low electronic density of states. Potentials at constant negative charge density, Epσc, depend on the work function in a characteristic manner, which is interpreted by taking into account the desorption of indium from the metal vb solution interface and the degree of water orientation in the compact double layer. Also, the experimental data are used to discuss the concept of “electrochemical work functions” determined as the shift between two parallel charge densities vs. potential curves. Using electrochemical work functions determined for the thallium amalgams, their behaviour is found to be quite similar to that of the indium amalgams. It appears that these amalgams constitute a most useful continuous series of metal phases to study the metal dependency of interfacial properties.

The rate equations for a two-electron reduction, also involving heterogeneous protonation steps, in which several pathways are followed in parallel have been derived using two procedures. The first is based on the “scheme of squares” to which the kinetic laws are applied. The second is a more intuitive method formulated in terms of equivalent resistances which allows the possible mechanisms to be studied easily in a more systematic way. The charge transfer resistance versus potential curves for pyrimidine reduction in the pH range 6.5–7.7 are analysed in terms of the rate equations deduced for several branched mechanisms. The influence of the potential and pH allows the pathways preferred by the system to be determined.

Using the exact theoretical equations derived to describe the response in single-step chronocoulometry (SSC) and a reported method for correction of the double layer charge, the chronocoulometric response of the Cd(II) + KBr(l M)/Cd(Hg) system is analyzed. The applicability of our theoretical equations and the quantitative use of the chronocoulometric technique are demonstrated. The potential step starts in the pre-faradaic potential range and ends at some potential inside the d.c. polarographic wave. From the charge response obtained with steps ending at the limiting current plateau, the value of the reactant surface excess corrected for double layer charging, was calculated. A detailed analysis of the dependence on the electrical variable σ m of the reactant surface excess led us to propose a plausible adsorption isotherm to which both the Cd(II)-complex species and the bromide anion adsorbed on the electrode contribute.

It is shown that, within a restricted potential region around the potential of zero charge at a dropping mercury microelectrode, the reduction of small particles of HgCl (±0.3 μm), dispersed in a 0.1 M KCl + 0.9 M KF solution, can be observed in the form of current spikes. The potential dependence of the frequency of their occurrence is related to the structure of the electrical double layer at the mercury surface. Their distribution in time appears to be close to random. From the charge connected with one single spike, the size of the particle can be calculated, so that the method allows the size distribution of the colloid to be obtained. It is argued that, outside their faradaic region, the particles cannot be reduced, as a result of electrostatic repulsion.

From the double layer charge densities, obtained with the dropping mercury microelectrode technique, analysed for aqueous 1 M KF, 1 M KCl, and their mixtures, it is found that the thiourea (TU) adsorption is congruent if described at constant electrode potential and not at constant charge density. Application of some familiar isotherms is attempted and the consequences are discussed. The nature of the anion does not influence TU adsorption. On the contrary, Cl − adsorption is changed considerably by the presence of TU. A phase transition in the adsorbed phase seems to exist also at a relatively small surface excess of TU.

It is explained why and how in particular cases additional information from complementary methods should be used to validate and to improve the information obtained from impedance measurements. As an example we present the direct determination of charge density vs potential curves with a newly developed dropping mercury micro electrode, and its application to prove the specific adsorption of the alkali ions at the mercury / aqueous solution interface. As a second example the study of electrode reactions with a finite rate of charge transfer and complicated by adsorption of the reactant is discussed. Here the combination of impedance measurements with high frequency demodulation measurements is indispensable. This is illustrated by some results for the system (TI (I) / Tl (Hg) in 1 M HCI O4.

It is shown that, although the mass transfer is complex, the average faradaic current for a dropping mercury microelectrode (DMμE) behaves very similarly to that of a normal DME, so that the usual analysis procedures can be applied to the polarogram. Owing to the fast mass transport quite large rate constants, up to 0.5 cm s −1 , can be obtained in a potential range more negative than usual. The small drop time allows polarograms to be recorded in far less than 1 s, and polarographic maxima will be absent.

Explicit expressions are derived for the distribution of stationary nucleation rates around a stable cluster of a new phase growing under (i) joint diffusion and charge transfer, (ii) pure diffusion and (iii) joint diffusion and migration limitations. Experimental results are reported for the deposition of mercury droplets on platinum in the presence and the absence of a large excess of supporting electrolyte. The expressions derived for the distribution of the nucleation rates around growing nuclei have been used in computer simulations of progressive nucleation. The simulated and experimental spatial distributions of the nuclei were analysed and compared with the theoretical distribution functions of the distances between the ηth neighbours. Deviations of the simulated spatial distributions from the random distributions were found to be in qualitative agreement with the experimental findings. Light scattering results indicated that in the absence of supporting electrolyte the nuclei were distributed on the surface in a structured pattern.

The diffusion-controlled growth at constant potential of an assembly of mercury spheres on a platinum surface is studied by means of the light scattering technique. It is shown that deceleration of the growth rate, as a result of diffusional interaction between neighbouring particles, can be directly measured using this technique. Two models for the description of the growth and concomitant current transient are compared and their validity is verified. A horizontal position of the working electrode was found to be essential in order to prevent dislodging and coalescence of the growing nuclei.

The absorption spectra of a metal-free phthalocyanine octasubstituted with branched alkoxy chains, 2,3,9,10,16,17,23,24-octakis(3,7-dimethyloctoxy)phthalocyanine (PcH 2 (8,2)), in dodecane were analyzed in terms of monomer-dimer-trimer equilibria. The pure PcH 2 (8,2) derivative is known to have a columnar discotic mesophase at room temperature and forms a one-dimensional, self-assembled structure. In dodecane solution the same type of stacked aggregation is shown to exist with a high dimerization constant K 1 =(1.50±0.04) X 10 6 M -1 and a trimerization constant K 2 =(5.2±0.4) X 10 4 M -1

This paper presents the results of a stochastic analysis of the distances between clusters randomly distributed on a flat surface. The probability distribution functions of the distances between first, second, third and nth neighbour clusters are derived. The theoretical formulae obtained are compared with experimental distributions of the distances between mercury droplets electrochemically deposited on a platinum electrode.

Mathematical expressions are derived and discussed with which the second-order demodulation method, combined with the first-order impedance method, can be used to unravel the effects due to diffusion, charge transfer and reactant adsorption simultaneously controlling an electrode reaction. By assuming that the adsorption is relatively weak, approximations are introduced in order to keep the mathematics tractable. The validity of the treatment is verified by a study of the Tl + /Tl(Hg) system in 1 M HClO 4 supporting electrolyte. The Tl + adsorption is described by the Langmuir adsorption isotherm, with the standard adsorption energy linearly dependent on the electrode potential. The charge-transfer process follows simple Butler-Volmer kinetics, with the transfer coefficient α close to 0.5 and the standard rate constant k s = 1.2 cm s −1 just above the detection limit of the impedance method. All parameters determined are independent of the Tl + concentration, which confirms the internal consistency of the treatment.

The absorption spectra of a metal-free phthalocyanine octasubstituted with branched alkoxy chains, 2,3,9,10,16,17,23,24-octakis(3,7-dimethyloctoxy)phthalocyanine (PcH 2 (8,2)), in dodecane were analyzed in terms of monomer-dimer-trimer equilibria. The pure PcH 2 (8,2) derivative is known to have a columnar discotic mesophase at room temperature and forms a one-dimensional, self-assembled structure. In dodecane solution the same type of stacked aggregation is shown to exist with a high dimerization constant K 1 =(1.50±0.04) X 10 6 M -1 and a trimerization constant K 2 =(5.2±0.4) X 10 4 M -1

The specific adsorption of Cs + ions demonstrated previously by Parsons and Stockton and by Damaskin and coworkers via capacitance measurements is confirmed from charge density vs. potential data measured using a mercury dropping microelectrode. Surprisingly also, Rb + , K + and Na + ions were found to adsorb from their mixed 1 M chloride solutions with 1 M LiCl. It is shown that data analysis should not neglect changes in the potential reference scale due to changes in the liquid junction potential and chloride ion activity.

The analysis of admittance and impedance data in the case of a particular ECE mechanism in which a stable intermediate is assumed to undergo a unidirectional chemical reaction followed by an infinitely fast electron transfer is evaluated critically. It is shown that situations can prevail which might be misinterpreted as “pseudo-Randles” behaviour with frequency. To avoid this risk some procedures are proposed for obtaining the kinetic parameters from the frequency dependence of the impedance data, using the complete equation deduced previously for this mechanism. These procedures are applied to the reduction of nitromethane on mercury at two extreme pH values: at pH 9 where a “pseudo-Randles” circuit holds and at pH 1 where the “non-Randles” behaviour requires the application of the new procedures. In the latter conditions the more complex potential dependence of the impedance parameters contradicts the mechanism proposed previously on the basis of polarographic data.

The electrocrystallization of lead chloride and lead sulphate on a lead amalgam electrode has been studied. The instant at which the nuclei start to grow shows up most clearly in the light scattering signal, while in potential sweep and potential step experiments that instant is less identifiable or almost undetectable in the current transient. The light scattering patterns also show the changes in surface roughness during the process.

It is shown that the steps in the earlier established CEE mechanism for the charge transfer process of Cd(II) reduction all are slowed down upon addition of sucrose. An attempt is made to correlate the individual standard rate constants with (i) the surface coverage by sucrose, and (ii) the viscosity of the solution layer adjacent to the electrode surface. No clear relationships are obtained, thus leaving the physical nature of the effects unexplained.

A new application of the light scattering technique is presented, in which an electrode surface is illuminated perpendicularly and the light scattered during the electrochemical process is measured at a fixed angle. The method is applied to the study of the passivation of nickel and to its pitting corrosion, as examples of electrochemical surface roughening processes. It is concluded that the method can be a valuable tool in the study of such processes, although as yet no theory is available with which the observed light scattering patterns can be interpreted.

A family plans a ski vacation in Utah, and their 10-year-old daughter browses the World Wide Web (WWW or Web) to find an inexpensive package tour. A college student plugs into the Internet jack in his residence hall room to communicate daily with friends and family via electronic mail (email). A health professions student subscribes to an electronic mail list and gains instant access to a lively dialog among her future colleagues on hot topics in their field. A man, worried about a tick bite a friend got on a Sunday hike, connects to the Centers for Disease Control in Atlanta to read the latest information about Lyme disease and to download color images of its characteristic skin rash. These are just a few examples that I know of the amazing ways the Internet and the WWW have changed our way of life. The Internet originated in December 1969 as a primitive link-up of 4 computers located at the Los Angeles and Santa Barbara campuses of the University of California, the Stanford Research Institute, and the University of Utah. It is doubtful that anyone then could have imagined just how fast their tiny computer network would grow. At the time of this writing, approximately 1/4 million registered local computer networks comprise more than 15 million host computers that serve more than 50 million users worldwide. These local networks are in turn joined into a single, vast, unified network, spanning more than 170 countries and linking computers of all different sizes, types, and operating systems with people who speak many different languages. One commonality among all these computers is their use of the same procedures for transmitting and receiving information over the net. These standards are embodied in the Internet Protocol (IP), which determines the conventions for addressing and routing information, and the Transmission Control Protocol (TCP), which governs how electronic messages are broken up into uniform-sized "packets" of data for transmission across the network and subsequent reassem

A model description is presented of the conductance of H + + D + in mixed H 2 O + D 2 O solution. The result of a mathematical derivation agrees very well with the non-linear dependence of the conductance on the isotopic mixing ratio, known since 1935, and that has never been explained satisfactorily. The experimental diffusion coefficient varies similarly.

It has been suggested that synchronous activation of cortical loci in the two cerebral hemispheres during development leads to the stabilization of juvenile callosal connections in some areas of the visual cortex. One way in which loci in opposite hemispheres can be synchronously activated is if they receive signals generated by the same stimulus viewed through different eyes. These ideas lead to the prediction that shifts in the cortical representation of the visual field caused by misalignment of the visual axes (strabismus) should change the width of the callosal zone in the striate cortex. We tested this prediction by using quantitative techniques to compare the tangential distribution of callosal neurons in the striate cortex of strabismic cats to that in normally reared cats. Animals were rendered strabismic surgically at 8-10 days of age and were allowed to survive a minimum of 18 weeks, at which time multiple intracortical injections of the tracer horseradish peroxidase (HRP) were used to reveal the distribution of callosally projecting cells in the contralateral striate cortex. HRP-labeled cells were counted in coronal sections, and data from four animals with divergent strabismus (exotropia) and four with convergent strabismus (esotropia) were compared to those from four normally reared animals. Although our data from strabismic cats do not differ markedly from those reported previously, we find that the distribution of callosal cells in the striate cortex of these cats does not differ significantly from that in our normally reared control cats. These results do not bear out the prediction that surgically shifting the visual axes leads to stabilization of juvenile callosal axons in anomalous places within the striate cortex.

Cytochrome-oxidase blobs are central to two of the most influential ideas in contemporary visual neuroscience--cortical modularity and parallel processing pathways. In particular, the regular 2D array of cytochrome-oxidase-rich blobs in primate visual cortex is arguably the most compelling evidence for cortical modularity and has been hypothesized to mark a separate processing stream through the visual cortex. Although previously a variety of mammals have been studied, blobs have only been demonstrated in the visual cortex of primates, which has led to the conclusion that blobs represent a primate-specific feature of visual cortical organization. Here we demonstrate the presence of cytochrome-oxidase blobs in a nonprimate species. Throughout the full tangential extent of layers II-III in cat visual cortex the cytochrome-oxidase staining pattern is distinctly patchy, with the darkly stained blobs forming a regular 2D array. In addition, the blobs in cat visual cortex are functionally related to the underlying ocular dominance columns. The presence of cytochrome-oxidase blobs in the cat clearly demonstrates that they no longer can be considered a primate-specific feature of visual cortical organization.

This paper summarizes the work currently underway on three major rail tunnel projects ni The Netherlands: (1) Rotterdam Railway Tunnel; (2) the doubling of the railway tunnel underneath Schiphol Airport; (3) the Rijswijk underground railway station; and (4) reconstruction of the rail station at Leiden.

Possible explanations for the occurrence of instabilities and oscillations in electrocatalytic reactions are studied for three different kinds of external control of the electrochemical cell: potentiostatic control, fixed-potential control in the case where account must be taken of the cell resistance or an external resistor and galvanostatic control. The relation of our models to existing models and experiments is discussed. In this paper we have investigated simple models of electrocatalytic reactions with the aim of determining the possible origins of instabilities and oscillations under three fundamentally different ways of external control of the electrical state of the electrochemical cell. The conclusions can be summarized as follows. Under truly potentiostatic conditions, i.e. when the interfacial potential can be considered constant irrespective of the current flowing through the cell, the origin of instabilities and oscillations should be purely chemical, i.e. autocatalytic surface chemistry, deviations from ideal Langmuir adsorption, (surface) phase transitions etc. Oscillations cannot be explained by simply referring to the negative slope in the current-voltage characteristic of the reaction. Under potentiostatic conditions a simple negative slope in the J-E curve, owing to passivation for example, cannot give rise to oscillations. Only if a suitable ohmic series resistance, be it only the cell's electrolyte resistance or in addition to this an external series resistor, is present can a simple negative slope give rise to spontaneous oscillations. However, not every negative slope will give rise to oscillations in this way. A negative slope in J-E curve due to competitive adsorption between the electroactive species and a poison will not give oscillations, whatever the value of the external resistance. Potential oscillations under galvanostatic conditions cannot be explained solely by a negative polarization resistance. Potential oscillations can arise if some coupled nonelectrochemical process in the system is assumed to be a potential dependent. We have shown this for a simple model in which a potential-dependent adsorption precedes the electron transfer reaction. This potential-dependent adsorption is capable of hiding the negative slope, but the instability is retained and oscillations occur within a potential interval in the J-E curve with positive slope. This property of the model, and its bifurcation structure in general, are in qualitative agreement with several reports of galvanostatic potential oscillations found experimentally. Because of the simplicity of the models presented in this paper, particularly as these are often reduced to two variables, they can only model simple monoperiodic oscillations. Of course, far more complex oscillatory reponses have been observed experimentally. In principle, one can try to model these by introducing a more complex chemistry or by removing some of the steady-state assumptions that we have introduced. However, these extensions will not significantly change our conclusions about the origin of the oscillations. In this respect, our assumption about the uniform current distribution is a much more serious obstacle. It is not intuitively clear how a nonuniform current distribution (whatever its origin) influences the steady-state stability of an electrochemical cell. It seems that this point needs further eleboration in the future. Finally, the reader will have noticed that we have relied heavily on the mathematical conditions for obtaining oscillations. We have deliberately avoided expressing the different states within a single oscillation into chemical or physical terms. Describing the different events taking place sequentially during a spontaneous oscillation is a difficult and rather tricky business, and it is our experience that such a description is either rather untransparent or, when translated into mathematical terms, will not pass the mathematical test for oscillatory behavior.