Controlled current voltammetry

Controlled-current voltammetry generally involves either a sinusoidal waveform (see Chap. 4) or a constant current [Fig. 18(a)]. Constant-current voltammetry, or chronopotentiometry [64, 65], generates a potential—time signal, as in Fig. 18(b), characterized by a transition time r. 

The largest division of interfacial electrochemical methods is the group of dynamic methods, in which current flows and concentrations change as the result of a redox reaction. Dynamic methods are further subdivided by whether we choose to control the current or the potential. In controlled-current coulometry, which is covered in Section IIC, we completely oxidize or reduce the analyte by passing a fixed current through the analytical solution. Controlled-potential methods are subdivided further into controlled-potential coulometry and amperometry, in which a constant potential is applied during the analysis, and voltammetry, in which the potential is systematically varied. Controlled-potential coulometry is discussed in Section IIC, and amperometry and voltammetry are discussed in Section IID. 

The term chronoamperometry means the measurement of currents as a function of time, and can be thought of as a kind of controlled-potential voltammetry or controlled-potential microelectrolysis (in unstirred solution) . 

Controlled-current techniques in stationary solution saw extensive development and application in the 1960s. However, they were largely supplanted by con-trolled-potential techniques, especially cyclic voltammetry, in the 1970s. Today, controlled-current techniques in stationary solutions are used occasionally. 

A complete comprehension of Single Pulse electrochemical techniques is fundamental for the study of more complex techniques that will be analyzed in the following chapters. Hence, the concept of half-wave potential, for example, will be defined here and then characterized in all electrochemical techniques [1, 3, 8]. Moreover, when very small electrodes are used, a stationary current-potential response is reached. This is independent of the conditions of the system prior to each potential step and even of the way the current-potential was obtained (i.e., by means of a controlled potential technique or a controlled current one) [9, 10]. So, the stationary solutions deduced in this chapter for the current-potential curves for single potential step techniques are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. Moreover, many of the functional dependences shown in this chapter for different diffusion fields are maintained in the following chapters when multipulse techniques are described if the superposition principle can be applied. 

In this section, we will show that the stationary responses obtained at microelectrodes are independent of whether the electrochemical technique employed was under controlled potential conditions or under controlled current conditions, and therefore, they show a universal behavior. In other words, the time independence of the I/E curves yields unique responses independently of whether they were obtained from a voltammetric experiment (by applying any variable on time potential), or from chronopotentiometry (by applying any variable on time current). Hence, the equations presented in this section are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. 

CA 54, 9551 (I960) (Voltammetry at controlled current) 29)W.H.Reinmuth, Anal Chem 32, 1514-17 (I960) (Chronopotentiometric potential-time curves and their interpretation) 30)P.Delahay, "Advances in Electrochemistry and Electrochemical Engineering , Interscience, NY (1961)... 

Current step— The excitation signal used in controlled current techniques in which the potential is measured at a designated time [i]. See also - chronopotentiometry, -> cyclic chronopotentiometry, - staircase voltammetry. Ref. [i] Heineman WR, Kissinger PT (1984) In Kissinger PT, Heine-man WR (eds) Laboratory techniques in electroanalytical chemistry. Marcel Dekker, New York, pp 129-142... 

The study of current-potential relations in an electrolysis cell where the current is determined solely by the rate of diffusion of an electroactive species is called voltammetry. To obtain diffusion-controlled currents, the solution must be unstirred and the temperature of the cell thermostatically controlled so as to eliminate mechanical and thermal convection. In addition,... 

The plateau current of a simple reversible wave is controlled by mass transfer and can be used to determine any single system parameter that affects the limiting flux of electroreactant at the electrode surface. For waves based on either the sampling of early transients or steady-state currents, the accessible parameters are the fi-value of the electrode reaction, the area of the electrode, and the diffusion coefficient and bulk concentration of the electroactive species. Certainly the most common application is to employ wave heights to determine concentrations, typically either by calibration or standard addition. The analytical application of sampled-current voltammetry is discussed more fully in Sections 7.1.3 and 7.3.6. 

At UMEs, the picture is quite different, because the currents are extremely small consequently, the error in potential control in a voltammetric experiment is often much smaller than in the same experiment with an electrode of conventional size. Consider, for example, a disk UME with radius tq at which we desire to carry out sampled-current voltammetry. What are the conditions that will allow the recording of a voltammogram in which the half-wave potential is shifted less than 5 mV by the effect of uncompensated resistance ... 

Several coulometric and pulse techniques are used in electroanalytical chemistry. Rather low detection limits can be achieved, and kinetic and transport parameters can be deduced with the help of these fast and reliable techniques. Since nowadays the pulse sequences are controlled and the data are collected and analyzed using computers, different pulse programs can easily be realized. Details of a wide variety of coulometric and pulse techniques, instrumentation and applications can be found in the following literature controlled current coulometry [6], techniques, apparatus and analytical applications of controlled potential coulometry [7], coulostatic pulse techniques [8], normal pulse voltammetry [9], differential pulse voltammetry [9], and square-wave voltammetry [10]. 

Sampled current voltammetry (SC V) involves applying a potential-step wave form of increasing amplitude, covering a potential window where initially no redox reaction occurs, to one where the current response is diffusion-controlled. The current decay from the exciting pulses is then recorded instantaneously at several different times. In the work under discussion, plots of /nm versus were linear in accordance with the Cottrell equation. Charge transport rates evaluated using this technique were essentially the same, within experimental error, as those evaluated using chronamperometry. 

Polarography is voltammetry, preferably with the dripping mercury electrode and with a diffusion-controlled current in a monopolar system. 

The greater sensitivity of differential-pulse voltammetry can be attributed to two sources. The first is an enhancement of the faradaic current, and the second is a decrease in the nonfaradaic charging current. To account for the enhancement, let us consider the events that must occur in the surface layer around an electrode as the potential is suddenly increased by 50 mV. If an electroactive species is present in this layer, there wilt be a surge of current that lowers the reactant concentration to that demanded by the new potential (see Figure 25-9b). As the equilibrium concentration for that potential is approached, however, the current decays to a level just sufficient to counteract diffusion that is, to the diffusion-controlled current. In classical voltammetry, the initial surge of current is not observed because the time scale of the measurement is long relative to the lifetime of the momentary current. On the other hand, in pulse voltammetry, the current measurement is made... 

The CV peaks for 20 pm Mb-DDAB films (Figure 3) have a characteristic diffusional tail consistent with the influence of diffusion. Peak separations were about 100 mV at 0.1 V s , and peak current Ip was proportional to the square root of scan rate (v) consistent [21,22] with diffusion-controlled film voltammetry. Analysis of the linear Ip vs Vi plot gave a charge transport diffusion coeffi-... 

Case 3 If diifusion layers overlap, neighbouring electrodes will shield each other s diffusion zones. Therefore, the current at each electrode will be less than predicted by the steady-state equation for Case 2. Additionally, the voltammetry is slightly peak-shaped, since depletion at the boundaries between diffiisional zones associated with each electrode will limit diffusion-controlled current. No simple analytical expressions exist for this regime, and therefore numerical simulation is essential. 

The cathodic reduction of vanadium in VCl3-NaCl-KCl melts is a two-step process a one-electron reduction and a two-electron V process. Under a constant applied current both stages are diffusion-controlled. During voltammetry measurements the mechanism of the electrode reactions remains unchanged at polarization rates below about 200mV/s. The diffusion coefficients of V(II) and V(III) ions were determined from the results of cyclic voltammetry measurements. It was also found that tungsten reacts with V(in) ions and thus cannot be used as electrode material for studying electrochemical... 

In the 1950s, the alternative to voltammetry started appearing among electrochemical methods the controlled current was applied and the ensuing electrode potential was measured—it was called chromato-polarography [415] or poten-tiometric polarography-controlled current scanning [416]. In these methods, the... 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

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