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The anomalous behavior of liquid water is apparent from the temperature dependence of many experimental properties. Among these are the abnormal high limiting ionic conductivities λ(0) of protons and hydroxyl ions, which in a previous paper we found to depend linearly on the square root of the absolute temperature T(1/2). Here we describe a further study of these conductivities in both light and heavy water, together with the remarkable transition temperature T(0) [242.7 K in H(2)O and 250.5 K in D(2)O], where the supercooled liquid becomes inert and, for example, restricted water molecule rotation is arrested. From T(0) the rotation barrier heights for the two solvents are determined. The conductivity data enable to obtain experimentally the zero point energies of H(+) and D(+) in liquid water, with results in the right order of magnitude as compared to values estimated along quantum mechanical routes. Contrary to general opinion, the isotope effect λ(0)(H(+))/λ(0)(D(+)) is temperature dependent, its value being close to 2(1/2) only near 20 °C. The isotope effect λ(0)(OH(-))/λ(0)(OD(-)) also takes temperature dependent values, substantially higher than 2(1/2). Still, the linear relationships with T(1/2) sustain a model based on water rotation control for the conductivity mechanism. For a quantitative analysis, the rotation frequency ω is expressed by a simple linear function of T(1/2) in terms of the moment of inertia I and the quantity T(0)(1/2). This "ab initio" calculation is found to be in perfect agreement with theoretical and experimental data in the literature, when ω is identified with the reciprocal of the so-called single molecule relaxation time τ(s). The data analysis eventually leads to the conclusion that the hydrogen ion transfer between water molecules proceeds via two parallel pathways. Next to the rotation controlled hopping mechanism there exists a temperature independent transfer controlled by tunnelling, occurring at all relative orientations of the participating water molecules. The applicability of the excess conductivity concept for hydrogen and hydroxyl ions is critically discussed.

The anomalous behaviour of the Tl+/Tl(Hg) electrode in 1 KNO3 + 0.1 M KCl solution has been studied experimentally applying the theory described in parts IV and VI of this series. It is shown that this anomaly can be explained assuming that the double layer capacity increases on adding thallous ions. The increase Δ C appears to be proportional to the TI+ concentration. The phase angle Φ of the faradaic impedance is equal to π/4.

A comparison is made of studies of the Hg2++/Hg electrode by means of complex plane analysis 1, 2 and double pulse galvanostatic step 4. As a possible explanation for the inconsistent results, interference by the double layer capacity in the latter method is suggested. It is shown that the results obtained for the same electrode by Imai and Delahay3 with faradaic rectification do not satisfy the conditions required for the approximations introduced. The exchange current densities are likely to be larger than reported. Another approximation procedure is proposed, which applies for very fast electrode processes (k0a > 3.10−3√ω.

A new and simple derivation of the diffusion polarization as a function of time is presented for the case wherein a square wave current passes an electrode-solution interface. The moment of zero diffusion polarization, which was already known to be 0.09, is calculated more precisely to be 0.094 of the whole period of the square wave signal. The consequences as regards the cyclic current step method for the determination of rate constants of electrode reactions, are discussed.

The sources of error pertaining to the oscilloscopic determination of rate constants of electrode reactions are studied and the upper and lower limits of the methods are given. The former is mainly determined by the accuracy with which the moment of zero diffusion polarization can be adjusted. It is shown that the extrapolation of cell responses to zero frequency does not eliminate the contribution of the double layer capacitance completely. The remaining contribution is a negative resistance,-2 σ2 Cd, irrespective of waveform.

The views developed in parts III and IV of this series are applied to alternating current polarography in the impedance plane. It is shown that from such polarograms a quantity q can be determined which is inversely proportional to concentration. The method covers a concentration range of 5.10−9/n2

From impedance measurements it is shown that the hydrogen ion reduction on mercury behaves as a case of pure activation polarization between -1.1 and -1.3 V vs S.C.E. with 1-α = 0.56 and ksh = 10−15 cm · sec−1. The value of the double layer capacity in that potential range is reported. No influence of hydrogen on the double layer capacity could be observed.

The values of the double layer capacity of the mercury-1 M HCIO4 solution interface at potentials where the dissolution of Hg contributes to the cell impedance, have been determined by the method previously described in parts VI1 and VII2.

The theory developed in part IV1 and part VI2 of this series has been verified by experiments. The results are

The accuracy of the double layer capacity measurements at high potentials described in part IX 1 is critically examined, especially with a view to the correct determination of the ohmic resistance of the cell. A limit for the exchange current density of the redox couple, which can be detected by the method of frequency variation, is determined. The systematic error in the calculated double layer capacity values, if this limit is just reached, is estimated. From impedance measurements in 0.1 M-HClO4 solution it appears that the capacity-potential curve is identical with that in 1 M-HClO4 solution.

The literature values of the limiting ionic conductivities of H(+), OH(-), K(+), Cl(-), Ag(+), and Na(+) in water between 0 and 156 degrees C are analyzed as for the two possible mechanisms of conduction, i.e., controlled by an activation process or by the rotation of the water molecules. Plots of the data versus T(1/2) give straight lines for H(+) and OH(-), which supports the rotation control mechanism for these ions. The other ions give curved plots and therefore are investigated in terms of the activation control mechanism. A remarkable phenomenon is discovered, namely, that except for H(+), the graphs for the other ions on extrapolation to lower temperatures have a common intersection point, T(0)(1/2), with the abscissa corresponding to T(0) = 243.4 K, i.e., -30 degrees C. This seems to indicate the presence of a virtual phase transition at about -30 degrees C, foreboding itself at higher temperatures. Below this temperature the supercooled water does not allow ions to migrate. Also, diffusion of solutes is found to cease, and dissociation constants drop to zero. The values of many physical properties of water appear also to approach zero at -30 degrees C, viz. the self-diffusion coefficient, reciprocal dielectric relaxation time, and solubilities of sparingly soluble salts. From data on the fluidity (reciprocal viscosity) and self-ionization constant it follows that the transition temperature of supercooled D(2)O is 9 degrees higher than of H(2)O. From the nearly quadratic shape of the several temperature dependencies it is inferred that the phase transition in question possibly is of some higher order. Implications for the transport number of protons to be expected in supercooled water are finally discussed.

The Zn(II)/Zn(Tl+Ga) electrode reaction was studied at the dropping Tl+Ga alloy electrode with a bulk composition of 0.06 mol% thallium in acidified aqueous 1 M NaClO4 solutions. By means of sampled dc voltammetry a strongly irreversible Zn(II)/Zn(Tl+Ga) wave was obtained, which occurred around E°f=−0.989 V versus SSCE and was separated into a distinct anodic and cathodic branch. The kinetic parameters of the reaction were determined by analysis of the sampled dc wave and of the ac impedance. It was found that the overall Zn(II) reduction process proceeds via the two-step electron transfer mechanism, the first step of which is rate determining in the entire accessible potential region. At the Tl+Ga electrode the rate constant of the first step A1 is three orders of magnitude smaller than that observed at pure gallium. This was discussed in terms of concomitant changes of the properties of the electrode surface and its interface with the electrolyte solution. These results are hard to reconcile with current theories on the kinetics of electrochemical electron transfer reactions.

The surface charge densities of liquid In+Ga alloy electrodes with indium bulk contents ranging from 0 to 16.5 mol% were obtained as functions of the electrode potential by means of dropping microelectrode charging current and ac impedance measurements in aqueous 1 M NaClO4 solutions of varied pH at 32°C. Using surface tension literature data of liquid In+Ga in vacuum and quasi-chemical multilayer model calculations they were integrated to electrocapillary curves. From the latter, the dependences of the indium surface excess on alloy composition, electrode potential and charge density were derived. A strong indium adsorption was found in the entire accessible range of bulk compositions. It extends over several atomic surface layers and can be accounted for by the surface tension difference between pure liquid gallium and indium, combined with the positive heat of mixing of the liquid In+Ga alloy system. It is shown that the indium surface excess decreases if the electrode potential or surface charge density is made more negative. A qualitative explanation in terms of pure component work function differences is given.

Combining the results of three techniques the rate constant kF of Cd(II) reduction from aqueous 0.5 M Na2SO4 is obtained in a wide potential range. Just as has been found earlier with other base electrolytes the In kf vs. potential plot is strongly curved, indicating the mechanism of the electrode reaction to consist of several steps. Mathematical analysis of this curve compels us to the conclusion that the mechanism is CEeC. An attempt is made to reconcile the different values of the standard rate constant ks and transfer coefficient α published over many years, taking into account that experimental data were obtained in different regimes of the electrode potential and often unduly extrapolated to the standard potential.

An earlier study on the properties of supercooled water and some solutes obtained by simple graphical extrapolation of data available in the literature, determined above the freezing point, led to the hypothesis that water could have a temperature of structural arrest at t0 =-30 °C for H2 O and -22 °C for D2 O. From the present investigation of many more properties of a widely varied nature, this discovery is confirmed throughout. The method is compared with the classical logarithmic Arrhenius and van't Hoff methods that predict t0 always to occur at 0 K, and with the more recently used power law dependence method. With the latter, extrapolated plots intersect the temperature axis at or near to a value of ts =-45 °C, that is, systematically more negative than t0 . It is argued that this ts value, which is the so-called singularity temperature, is likely to be an artefact. It is suggested that, instead of using modified van 't Hoff or Arrhenius relationships to describe temperature dependences in the full temperature range, polynomials in (t-t0 ) should be used.

The reduction of Zn(II) from aqueous l M NaClO 4 solution in the presence of 1 mM HClO 4 was studied at dropping gallium and mercury electrodes. The dropping microelectrode technique was succesfully extended to the use of gallium as the electrode material. Net Zn(II)/Zn(Ga) waves were obtained after subtracting the H 3 O + reduction current recorded in the absence of the Zn(II)/Zn(Ga) couple from the experimental voltammograms. The waves obtained at the conventional dropping gallium electrode proved to be reversible and occurred around E f 0 (Zn(II)/Zn(Ga)) = ∮.989 V vs. SSCE. On the short timescale of the dropping gallium and mercury microelectrodes the Zn(II) reduction appeared quasi-reversible, which made it possible to determine the kinetic parameters of the reaction. The Zn(II) reduction at liquid gallium was found to proceed via the same two-step electron transfer mechanism as that established earlier at mercury. Within the entire accessible potential range the reduction rate constant at gallium was found to be about four times higher than that at mercury.

The faradaic admittance for a consecutive E-C-E mechanism with two stable intermediates is derived with the assumption that the chemical reaction is unidirectional. Both cases where this reaction is homogeneous or heterogeneous are treated. The admittance derived assuming that the chemical step is slow in comparison with the frequency of the a.c. perturbation has the same frequency dependence as that derived by Armstrong and Firman for a consecutive E-E mechanism with a stable intermediate. The presence of the chemical step in the E-C-E mechanism appears in the altered potential dependence of the admittance data. Four cases are distinguished in which the general admittance expression reduces to the simpler Randles expression (pseudo-Randles cases). The theory is applied to the reduction of nitromethane in aqueous 1 M NaCl solutions of pH 2.4. Several methods of analysis are proposed. It is found that a simultaneous analysis of the frequency and potential dependence of the admittance data is to be preferred compared with the habitual procedure of frequency analysis at successive fixed potentials. The final reaction parameters obtained from the analysis are discussed in comparison with previous data obtained in basic media, which conform well to an E-Ce mechanism with only one stable intermediate.

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.

Starting from the electrocapillary equation for the ideally reversible electrode a relation has been derived between the parameters q*-qM+nFΛ0 (qM=electrode charge density, Λ0=surface excess of oxidant) and the derivatives of the free energies of adsorption of the Ox and Red component with respect to potential. In this way a model is obtained for the explicit interpretation of the low frequency capacity CLF and the high frequency capacity CHF, occurring as parameters in the expression for the electrode admittance in the case of reactant and/or product adsorption. The experimental results obtained by Timmer et al. for the Pb(II) reduction in 1 M KCl appear to fit to this model if a linear isotherm with a potential-dependent adsorption coefficient is assumed. It is concluded that most probably the neutral PbCl2 species is adsorbed.

The theoretical expressions for the faradaic admittance and the faradaic demodulation voltage are rewritten, introducing the thermodynamic restrictions proposed by Reinmuth in 1972 and without any specification of the mechanism of the electrode reaction. The result is applied to general first-order kinetics of the type CECEC…etc. A treatment is also given in the case that one or more of the steps are of a higher order. Although the overall rate equation becomes extremely complex in such a case, it is shown that the expressions describing the faradaic admittance and the faradaic demodulation voltage are relative simple. From some numerical examples it is made clear that, in general, by these methods discrimination between possible mechanisms can be realized on the basis of the potential dependences of the overall rate constant, operational transfer coefficient, irreversibility quotient and demodulation voltage. This work is believed to provide a framework for future quantitative studies of electrode reactions. Some ideas about its possible further elaboration are given.

The two-step reduction of Zn(II) ions at the dropping mercury electrode in 1 M NaClO4/NaI mixtures is studied by the impedance method. Both steps are found to be catalysed, the increase in the logarithm of the rate constant being proportional to the surface excess of I−. Attempts are made to interpret the results quantitatively with existing models and the applicability of these models to this electrode reaction is discussed.

The parameters describing the electrode admittance in the case of a reversible electrode reaction with reactant and/or product adsorption, are expressed as functions of the d.c. potential, assuming, that the adsorption obeys the Langmuir isotherm. On the basis of these expressions theoretical a.c. polarograms are predicted. It is shown that for relatively weak adsorption the polarographic peaks increase and shift away from the reversible half-wave potential. When the adsorption is stronger, the peaks split. When only one component is adsorbed, the observed behaviour is mainly due to the low-frequency capacity CLF. When both components are equally adsorbed, the high frequency capacity CHF is the most important.

A comprehensive treatment is given of the relationships concerning the interfacial admittance of an electrode at which a reversible electrode reaction with reactant adsorption proceeds. It is shown that these relationships permit the determination of the parameters of a postulated isotherm, as well as the potential dependence of the free energy of adsorption, the latter irrespective of the isotherm chosen. Procedures to analyse the admittance spectra are developed and the attainable accuracy and reliability of such analyses are critically discussed. It is shown that the information to be obtained is highly detailed, provided that internal checks are carefully applied.

A general formalism is developed for the theory of the second order contribution to the non-linear behaviour of an electrochemical cell. The derivations result into a set of linear relationships for both the faradaic process and the double-layer charging process, which can be combined to deduce the ultimate cell response under various experimental conditions. The treatment is applicable to any accepted input waveform and gives a clear insight in the close connection between the so-called potentiostatic and galvanostatic approach. As an example the second harmonic distortion and the rectification effect arising from a single harmonic perturbation are briefly discussed. The implications of the treatment for the study of charge transfer kinetics are indicated in a general way, i.e. without specifying an a priori model, but in terms of operational parameters.