Chronocoulometry study

Even refined electrochemical methods cannot alone provide full information about the molecular structure of the metal/ solution interface. Hence, many nonelectrochemical techniques have been developed in the past few decades to study the double layer. They include spectroscopic, microscopic, radiochemical, microgravimetric, and other methods. A combination of electrochemical (chronovoltammetry, chronocoulometry, impedance spectroscopy, etc.) and nonelectrochemical methods is often used in studying mechanisms of the electrode process. 

Jarbawi and Heineman have used differential pulse voltammetry and chronocoulometry to study the effect of immersing a wax-impregnated graphite electrode in an aqueous solution of chlorpromazine [157]. Extraction of substance into the electrode and absorption at the interface were both found to occur. 

The double-layer structure at the electro-chemically polished and chemically treated Cd(OOOl), Cd(lOlO), Cd(1120), Cd(lOh), and Cd(1121) surface electrodes was studied using cyclic voltammetry, impedance spectroscopy, and chronocoulometry [9, 10]. The limits of ideal polarizahility, Epzc, and capacity of the inner layer were established in the aqueous surface inactive solutions. The values of iipzc decrease, and the capacity of the inner layer increases, if the superficial density of atoms decreases. The capacity of metal was established using various theoretical approximations. The effective thickness of the thin metal layer increases in the sequence of planes Cd(1120) < Cd(lOiO) < Cd(OOOl). It was also found that the surface activity of C104 was higher than that of F anions [10]. 

Hamelin [47] has shown that specific adsorption of OH ions increases in the following order Au(lll) < Au(lOO) < Au(311). Chen and Lipkowski [48] have applied chronocoulometry and subtractively normalized interfacial Fourier transform infrared spectroscopy to study adsorption of hydroxide ions on Au(lll) electrode. This process proceeded in three steps. Bonding of OH with gold atoms that is quite polar at negatively charged surface becomes less polar at positively... 

Subtractively normalized interfacial FTIR has been employed [242] to study the changes in the surface coordination of pyridine molecules on Au(lll). It has been deduced from the experiments that pyridine molecule is positioned upright at positive potentials and its plane rotates somewhat with respect to the electrode surface. In situ FTIR has also been used [243] to investigate adsorption of pyridine on Au(lll), Au(lOO), andAu(llO) electrodes. For the low-index electrodes, the behavior of band intensity located at 1309 cm and corresponding to the total adsorbed pyridine, agreed with the surface excess results obtained earlier from chronocoulometry. 

Chronocoulometry and photon polarization modulation infrared reflec-tion/absorption spectroscopy have been employed [311] to study the fusion of dimyristoylphosphatidylcholine vesicles onto an Au(lll) electrode. The fusion was controlled either by the electrode potential, or charge. Film characteristics was also potential dependent. After removing the film from the electrode surface (negative potential), phospholipid molecules remained in its close proximity, in the ad-vesicle state. Several electrochemical and nonelec-trochemical methods have been applied [312, 313] to investigate the spreading of small unilamellar vesicles onto Au(lll) electrode. Vesicles fused onto the surface at > —0.5 V (versus SSCE), to form defected bilayers in contact with the metal surface. At more negative potentials, the film was removed from the electrode surface, but it still remained in its close proximity. 

Reconstruction of Au(lll) is observed in STM images as double rows separated from each other by 6.3 nm [335]. Some model calculations have been performed [362] to show that the energy difference of the reconstructed and unreconstructed Au(lll) is small. The effect of Triton X-100 on the reconstruction process of Au(lll) surface has been studied in chloride media [363] applying CV and double potential-step chronocoulometry. It has been found that adsorption of Triton X-100 stabilizes the reconstructed face of Au(lll). Hobara etal. [364] have used in situ STM to study reconstruction of Au(lll), following reductive desorption of 2-mercaptoethanesulfonic acid SAMs. 

Thallium UPD on Au(lll) has been studied, applying potential-step chronocoulometry and quartz crystal microbalance [482]. The UPD surface coverage increased with the increasing cathodic potential. At low coverage, the sublayer was not completely discharged, as it appeared from electrosorption valency. 

Recently, Alvarez et al. [136] have studied interfacial and electrochemical behavior of diphenylselenide on HMDE in DMF-water mixture (3 7, v v). Applying ac voltammetry and chronocoulometry, it has been shown that a multilayer film of chemisorbed diselenide of the progressively increasing thickness is formed. 

Indicators and Dyes Abdel-Hamid [154] has studied adsorption of phe-nolphthalein at a HMDE in aqueous buffer solutions containing 10% v/v ethanol, applying cychc voltammetry and double potential-step chronocoulometry. At pH =... 

Duan etal. [187] have studied adsorptive and electrochemical behavior of estradiol valerate at a static mercury electrode using cyclic voltammetry and chronocoulometry. A sensitive and... 

In contrast to the chronocoulometric technique, the information obtained from the impedance technique concerns the derivatives with respect to potential of To, In /30 and d(ln (30)/dE. It is therefore needed to postulate an explicit isotherm in order to fit experimental data to the theoretical equations. However, the results, e.g. for (dT0/dE, can be integrated to give T0 as a function of potential within the faradaic region. As chronocoulometry is, by definition, applicable to potentials outside the faradaic region, the two methods may be considered as complementary tools to study adsorption phenomena that are hard to detect in the classical way, i.e. from double-layer data (e.g. the capacity Cd) that are influenced by the presence of electro-inactive absorbates. 

Adsorption phenomena have been studied by means of virtually every electrochemical technique, including recently developed spectroelectrochemical methods. Electrocapillary methods and measurements of double-layer capacitance have played a central role in the understanding of adsorption. AC studies have also been very useful and are very sensitive to adsorption effects. More recently, chronocoulometry (Chap. 3, Sec. II.C) has been applied effectively to the measurement of quantities of adsorbed electroactive species. The interested reader is referred to the sections that deal with these techniques for more detailed information. 

Chronocoulometry can also be used for the study of homogeneous chemical reactions that are coupled to the electrode reaction. The technique has not yet seen extensive application to this end, although the theory is quite highly developed [18]. 

Under ideal conditions, charge consumed by the double-layer capacitance and adsorbed reactants will follow the same time course as discussed earlier for ordinary chronocoulometry. Since the ratio of electrode area to solution volume is larger for thin-layer experiments, charge thus accounted for may represent a much greater proportion of the total. This fact points to an advantage of restricted diffusion experiments for studying some surface phenomena. 

To a large extent, the discovery and application of adsorption phenomena for the modification of electrode surfaces has been an empirical process with few highly systematic or fundamental studies being employed until recent years. For example, successful efforts to quantitate the adsorption phenomena at electrodes have recently been published [1-3]. These efforts utilized both double potential step chronocoulometry and thin-layer spectroelectrochemistry to characterize the deposition of the product of an electrochemical reaction. For redox systems in which there is product deposition, the mathematical treatment described permits the calculation of various thermodynamic and transport properties. Of more recent origin is the approach whereby modifiers are selected on the basis of known and desired properties and deliberately immobilized on an electrode surface to convert the properties of the surface from those of the electrode material to those of the immobilized substance. 

The chronocoulometry and chronoamperometry methods are most useful for the study of adsorption phenomena associated with electroactive species. Although less popular than cyclic voltammetry for the study of chemical reactions that are coupled with electrode reactions, these chrono- methods have merit for some situations. In all cases each step (diffusion, electron transfer, and chemical reactions) must be considered. For the simplification of the data analysis, conditions are chosen such that the electron-transfer process is controlled by the diffusion of an electroactive species. However, to obtain the kinetic parameters of chemical reactions, a reasonable mechanism must be available (often ascertained from cyclic voltammetry). A series of recent monographs provides details of useful applications for these methods.13,37,57... 

A number of studies have been made of the reduction of model enones in buffered aqueous or buffered ethanolic solutions in order to elucidate the sequence of electron transfer, proton transfer, and coupling steps as a function of pH [39-42,91-95]. The experimental methods applied include polarography, CV, LSV, and chronocoulometry. 

The bromide-induced adsorption of thallium complex on pc-Ag electrode has been studied using CV and chronocoulometry [125]. 

Lobacz et al. [52] have described partial adsorption of Tl -cryptand (2,2,2) complex on mercury electrode. From voltocoulom-etry, cyclic voltammetry, and chronocoulometry, it has been deduced that electroreduction of this complex proceeds via two parallel pathways from the solution and from the adsorbed states, which are energetically close. Also, Damaskin and coworkers [53] have studied adsorption of the complexes of alkali metal cations with cryptand (2,2,2) using differential capacity measurements and a stationary drop electrode. It has been found that these complexes exhibit strong adsorption properties. Novotny etal. [54] have studied interfacial activity and adsorptive accumulation of U 2 -cupferron and UO2 + -chloranilic acid complexes on mercury electrodes at various potentials in 0.1 M acetate buffer of pH 4.6 and 0.1 M NaClO4, respectively. 

At solid polarizable electrodes, the charge density on the electrode can be measured directly using chronocoulometry [11]. This has been applied successfully to the study of molecular adsorption at single crystal gold electrodes. As discussed above, a reference potential E is chosen at which molecular adsorption is absent. The potential of the polarizable electrode is then stepped to a new value f ,- and the current transient which results is recorded. By integrating this transient, one obtains the change in charge density between the two potentials ... 

The area of the electrode, A, in Equation 3 was measured in this study by means of chronocoulometry (g) in an aqueous ferricyanide solutions. 

Surface excesses of electroactive species are often examined by methods sensitive to the faradaic reactions of the adsorbed species. Cyclic voltammetry, chronocoulometry, polarography, and thin layer methods are all useful in this regard. Discussions of their application to this type of problem are provided in Section 14.3. In addition to these electrochemical methods for studying the solid electrode/electrolyte interface, there has been intense activity in the utilization of spectroscopic and microscopic methods (e.g., surface enhanced Raman spectroscopy, infrared spectroscopy, scanning tunneling microscopy) as probes of the electrode surface region these are discussed in Chapters 16 and 17. 

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