Chronocoulometry

Instead of studying the variation of current with time, we can integrate the current and study the variation of charge with time this is chronocoulometry1. Advantages are  

For a large potential step at a planar electrode and considering a reduction, we use the Cottrell equation, and by integration arrive at 

Therefore, a plot of Qi vs. t1/2 is linear with zero intercept. In practice, intercepts are non-zero and correspond to a capacitive charge, Qc, and even to reduction of adsorbed species (Fig. 10.3). 

The double potential step is very powerful in identifying adsorption phenomena by chronocoulometry. From (10.34), 

Double step chronocoulometry also gives information on the kinetics of coupled homogeneous reactions8. For example, any deviation, under diffusion control, from (10.35) and (10.36) implies a chemical complication, which can be compared with the responses for the various possible mechanisms. 

In 1834 Faraday suggested two fundamental laws of electrolysis. According to Faraday, the amount of material deposited or evolved (m) during electrolysis is directly proportional to the current (/) and the time (t), i.e., on the quantity of electricity (Q) that passes through the solution (first law). The amount of the product depends on the equivalent mass of the substance electrolyzed (second law). (In fact, Faraday s laws are based on two fundamental laws, i.e., on the conservation of matter and the conservation of charge.) Accordingly, 

If the current efficiency is 100%, i.e., the total charge is consumed only by a well-defined electrode reaction, the measurement of charge provides an excellent tool for both qualitative and quantitative analyses. For instance, knowing m and Q, Min can be obtained, which is characteristic to a given substance and its electrode reaction. 

By knowing M and n the amount of the substance in the solution can be determined. This method is known as coulometry. It is also possible to generate a reactant by electrolysis in a well-defined amount and then it will enter a reaetion with a component of the solution. It is used in coulometric titration where the end-point is detected in a usual way, e.g., by using an indicator. 

Chronocoulometry belongs to the family of step techniques. The essential features of chronoamperometry have already been discussed in Seet. 1.3.2. Instead of following the variation of current with time after application of a potential step perturbation, it is possible to deteet the amount of the charge passed as a function of 

Department of Physical Chemistiy, Eotvos Lor d University, 1518 Budapest 112, Hungary e-mail inzeltgy chem.elte.hu 

In this technique, introduced by Christie et al. [46] and developed extensively by Anson [47], essentially the current flowing after a potential step is integrated electronically to give the charge Q(t) transported between t = 0 and t. The function q(t) = Q(t)/A vs. tin has the shape depicted in Fig. 12 and is the sum of the contributions of double-layer charging and the faradaic process 

The chronocoulometric method offers some advantages over the chronoamperometric method, (a) The measured signal grows with time and hence the later part of the transient, that corresponds better with theory, suffers less from instrumental inaccuracy, (b) The act of integration smoothes random noise in the current signal and hence also improves the accuracy, (c) Even the long-time approximation, eqn. (37b), contains 

In order to estimate this term with sufficient accuracy, its magnitude has to be at least 10% of the total value of q(t) at the shortest accessible time, say t 100 ps. This leads to the condition l ca. 900 s 1/2, which corresponds to k values between 0.1 and lcms-1. However, the time-independent double-layer contribution, qc, has to be subtracted from q(t) first and this may reduce the accuracy of the kinetic information. Therefore, an upper limit for kt similar to that of the chronoampero-metric technique seems more realistic. The value of qc has to be determined separately from a chronocoulogram recorded in the absence of the electroactive species. 

In spite of its advantages and the simplicity of its performance, chrono-coulometry is seldom used for studying charge transfer kinetics. In fact, the method is much more popular because of its suitability for the study of reactant adsorption at the initial potential, which is manifested as an extra time-independent amount of faradaic charge involved in the potential step. This will be considered in more detail later. 

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Chronocoulometry, 62 Clark electrode, 190 Coated wire electrodes, 160 Cobalt, 82, 85 Cobalt phthalocyanine, 121 Collection efficiency, 113, 135 Collection experiments, 113 Combination electrode, 148 Compact layer, 19 Composite electrodes, 47, 114, 133 Computer control, 80, 106 Concentration profile, 7, 9, 11, 29, 36, 87, 132... 

According to experimental data,208,209 the SNIFTIR technique can be used to probe the electrical properties of the electrical double layer even in more concentrated solutions where cyclic voltammetry (cv), impedance, chronocoulometry, and other techniques are not applicable. Iwasita and Xia210 have used FTIR reflection-adsorption spectra to identify the potential at which the orientation of water molecules changes from hydrogen down to oxygen down. 

Guidelli and co-workers336-338 measured the potential of zero charge by chronocoulometry. They found that the pzc was independent of the electrolyte concentration in both NaC104 and Na2S04. However, Ea=0 in the presence of sulfates was ca. 40 mV more negative. These authors have explained this apparent discrepancy in terms of the perturbation of the solvent structure at the interface by the ions at the electrode surface, which are, however, nonspecifically adsorbed. 

Met. = Method. C = chronocoulometry, CC = corrosion current method, EC = Electrocapillary method, EL = Ellipsometric method, IR = Infrared spectroscopic method, CV = cychc voltammetry, RT = Radiotracer method, T = Tensammetry, TF = Thin film resistance, WL = weight loss method. When a combination of methods was used aU methods are listed separated by slashes. See also list of symbols. 

For the in situ characterization of modified electrodes, the method of choice is electrochemical analysis by cyclic voltammetry, ac voltammetry, chronoamperometry or chronocoulometry, or rotating disk voltametry. Cyclic voltammograms are easy to interpret from a qualitative point of view (Fig, 1). The other methods are less direct but they can yield quantitative data more readily. 

Appropriate electroanalytical procedures to verify the one or other case have been given in the references of this section. The main techniques are cyclic voltammetry, chronoamperometry, chronocoulometry, and rotating disk voltammetry. The last one appears to be best suited since constant mass transport in the film is a very important feature as outlined aixive Table 2 gives examples for... 

Having defined our near electrode region, we turn now to consider the various techniques that can be employed in the in situ investigation of the reactions that occur within it. The various methods that can be employed will each provide different types of information on the processes occurring there. As has already been discussed, cyclic voltammetry is the most common technique first employed in the investigation of a new electrochemical system. However, in contrast to the LSV and CV of adsorbed species, the voltammetry of electroactivc species in solution is complicated by the presence of an additional factor in the rate, the mass transport of species to the electrode. Thus, it may be more useful to consider first the conceptually more simple chronoamperometry and chronocoulometry techniques, in order to gain an initial picture of the role of mass transport. 

Thus, cyclic or linear sweep voltammetry can be used to indicate whether a reaction occurs, at what potential and may indicate, for reversible processes, the number of electrons taking part overall. In addition, for an irreversible reaction, the kinetic parameters na and (i can be obtained. However, LSV and CV are dynamic techniques and cannot give any information about the kinetics of a typical static electrochemical reaction at a given potential. This is possible in chronoamperometry and chronocoulometry over short periods by applying the Butler Volmer equations, i.e. while the reaction is still under diffusion control. However, after a very short time such factors as thermal... 

The work of Kunimatsu and Kita (1987) is very powerful evidence in favour of linearly adsorbed CO being the catalytic poison for methanol oxidation at a smooth platinum electrode in acid solution and has resulted in this hypothesis being generally accepted. However, there is some conflict between the IR results and those obtained by Vielstich and colleagues using chronocoulometry, ECTDMS and DEMS. 

At high methanol concentration, such as those employed in an actual fuel cell, the chronocoulometric and ECTDMS results agree with the IR data in that the predominant adsorbed species is C-O. However, the IR duta were obtained at methanol concentrations ranging from 10 2 M to 1 M. At methanol concentrations of 10 2 M the predominant surface species detected by the chronocoulometry and ECTDMS experiments is COH and it is evident that the energy difference between COH,ds and C=Oads must be slight. 

The third class of redox species are couples located near the conduction band of WSe2- The only outer-sphere example found, which is suitable for use in aqueous electrolytes, is Ru(NH3)e3+. Its reduction is characterized by an immediate onset upon accumulation in the semiconductor and a tafel slope of 130 mV/decade. The reduction mechanism appears to be direct reduction of the Ru(NH3)e3+ by electrons from the accumulation layer. The only member of the forth class of redox species is triiodide ion. It is characterized by adsorption onto the semiconductor surface as was demonstrated by the first application of chronocoulometry to a semiconductor electrode (another demonstration of the reproducibility and low background currents on... 

Chronic toxicology, pyridine, 21 118 Chronoamperometry, 9 568, 575-577 Chronocoulometry, 9 568 Chronological materials standards,... 

R.W. Murray, Chronoamperometry, Chronocoulometry and Chronopoten-tiometry, Physical Methods of Chemistry. Electrochemical Methods. A. Weissberger and B.W. Rossiter eds, Vol. 2. Wiley Interscience, New York, 1986. 

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. 

Fig. 4 Chronocoulometry (a) typical charge response (b) Anson plot for a double-step chronocoulometric experiment.

Because of its integral nature, chronocoulometry is less susceptible to noise... 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

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. 

Determinations of rate constants for the catalytic process were carried out by double potential step chronocoulometry... 

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]. 

The adsorption of reactant in the system Cd(II) +1 M KBr/Cd(Hg) was investigated by using chronocoulometry [41]. The values of the reactant surface excess were calculated. The authors proposed the adsorption isotherm, which allows the adsorption of the reactant and ligand to... 

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. 

Sottomayor etal. [273] have investigated adsorption of n-hexanol on Au(lll) electrode from 0.2 M NaCl04 aqueous solution, applying chronocoulometry to measure dynamic capacitance and capacitive charge. They have found that in this case, adsorption may be described by the Frumkin isotherm and have determined the corresponding parameters. The obtained results were discussed and... 

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. 

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