Potential-decay method

The potential-decay method can be included in this group. Either a current is passed through the electrode for a certain period of time or the electrode is simply immersed in the solution and the dependence of the electrode potential on time is recorded in the currentless state. At a given electrolyte composition, various cathodic and anodic processes (e.g. anodic dissolution of the electrode) can proceed at the electrode simultaneously. The sum of their partial currents plus the charging current is equal to zero. As concentration changes thus occur in the electrolyte, the rates of the partial electrode reactions change along with the value of the electrode potential. The electrode potential has the character of a mixed potential (see Section 5.8.4). 

Li Q, Gang X, Hjuler HA, Bjerrum NJ (1995) Oxygen reduction on gas-diffusion electrodes for phosphoric acid fuel cells by a potential decay method. J Electrochem Soc 142 3250-3256... 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

RT/aF. One would expect such a result if the double layer is behaving as an ideally polarizable double layer ( Section 6.4.3) and there is, e.g., no adsorbed intermediate to slow down the decay. Figure 8.8 shows this case. When there is an adsorbed intermediate radical of significant concentration, the decay method can be used to determine the potential dependence of the fraction of the surface covered by the radical (Conway and Tilak, 1982). 

With potential step methods, the capacitive current is a transient, decaying with a time constant equal to RvCrii The usual procedure is to wait several of these time constants before making the current measurement, by which time the capacitive current has declined to a negligible value. It is therefore not a serious problem with potential step experiments. 

Potential step methods have emerged as valuable electrochemical methods due to the highly sensitive nature of the technique. The waveform employed in potential step methods, also referred to as pulsed methods, have some advantages over potential sweep methods. The main advantage is that the steplike waveform can discriminate and separate the capacitive current versus the faradaic current, the current due to the reduction or oxidation undergone by the analyte, increasing signal to noise. Capacitive versus faradaic current discrimination is the basis for all of the pulsed techniques. The rate of decay of the capacitive current and the faradaic current is not the same. The capacitive current has an exponential decay whereas the faradaic current decays as a function of t Since the rate of decay of the capacitive current is much... 

Suzuki et al. [22] demonstrated that this method may be used to measure the plating rate of electroless nickel and copper plating processes. An example of a potential decay curve is shown in Fig. 14. The authors also demonstrated the validity of Eq. (25) and determined the K values of various plating systems. 

In the treatment which follows, we assume that discharge of the doublelayer capacitance drives the reaction, and therefore use C = in Eq. (41). The effects of changes in coverage of the adsorbed intermediate are then taken into account by combining Eq. (41) with the kinetic equations for steps in the mechanism. In this method, no assumptions need then be made about the equivalent circuit or the nature of the pseudocapacitance, and the transient current during potential decay is not assumed to be equal to the steady-state current. The results then enable all three definitions of [Eqs. (46)-(48)] to be evaluated and compared, as illustrated in Fig. 10. 

Conway and Gileadi carried out electrochemical kinetic studies at the nickel oxide (Ni"-Ni ") electrode. A polarisation decay method was employed in which the reversible potential was approached both from the anodic and the cathodic directions. 

Linear scaling methods are dependent on vdiat Kohn (51) has termed the nearsightedness of equilibrium systems, which may be defined as, for large systems, the property diat the effects of an external potential decay rapidly (for systems without long-range electric fields) (50). Recendy, Goedecker published... 

The time variation of the oxygen overpotential at platinum has been examined by Busing and Kauzmann using the method of potential decay. They concluded that most of the time variation could be understood if a nonuniform electrode surface with the electrode process occurring at relatively few active centers was assumed. The number of active centers was considered potential dependent and the time variation was attributed to poisoning of these active sites by solution impurity or by intermediates. 

Conditions (118) and (119) and are not satisfied by conventional (LDA, GGA) where the XC potential decays exponentially. In Fig. 2 we report the XC potential for the benzene molecule, along one line in the molecular-plane, using the exchange-only OEP method (OEPx, namely LHF, see section 3.7.1) and the LDA functional. The two potentials differ in three main features ... 

TOF-RTOF instruments. An alternative approach to reacceleration uses a reflec-tron in MS2 to disperse the product-ion arrival times in the same manner as used for the post-source decay method. Additionally, the reflectron can be used in combination with reacceleration after the collision chamber. In the TOF-RTOF instrument reported by Cooks et al. (Figure 9.6), ion activation is provided by surface-induced dissociation (SID), and product ions are dispersed and focused by a reflectron. The target surface is raised to a potential above the drift region, so that product ions are dispersed by reacceleration as well. The effect of this combination is to... 

Chen G, Xu Z, Zhang L (2007) Measurement of the surface potential decay of corona-charged polymer films using the pulsed electroacoustic method. Meas Sci Technol 18 1453-1458... 

A first step towards a systematic improvement over DFT in a local region is the method of Aberenkov et al [189]. who calculated a correlated wavefiinction embedded in a DFT host. However, this is achieved using an analytic embedding potential fiinction fitted to DFT results on an indented crystal. One must be cautious using a bare indented crystal to represent the surroundings, since the density at the surface of the indented crystal will have inappropriate Friedel oscillations inside and decay behaviour at the indented surface not present in the real crystal. 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Agarwal et al. 1978), the quantification of these specific enzymes may indicate that exposure to endosulfan has occurred. Blood tests, such as decay curves for aminopyrine in plasma, which are semiquantitative indices of liver enzyme induction, have been used successfully in the past to demonstrate enzyme induction in pesticide-exposed workers. Because numerous chemicals found at hazardous waste sites also induce these hepatic enzymes, these measurements are not specific for endosulfan exposure. However, measurements of enzyme activity, together with the detection of the parent compound or its metabolites in tissue or excreta, can be useful indicators of exposure. All of these potential biomarkers require further verification in epidemiological studies. Further studies with focus on the development of methods to separate and measure the estrogenicity of endosulfan in in vitro assays would be valuable since these assays are more sensitive and discriminative than other conventional biomarkers. Preliminary results have been presented by Sonnenschein et al. (1995). 

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