Zero current condition

Potentiometric measurements are usually performed under zero-current conditions in a galvanic cell of the type ... 

Potentiometry deals with the electromotive force (EMF) generated in a galvanic cell where a spontaneous chemical reaction is taking place. In practice, potentiometry employs the EMF response of a galvanostatic cell that is based on the measurement of an electrochemical cell potential under zero-current conditions to determine the concentration of analytes in measuring samples. Because an electrode potential generated on the metal electrode surface,... 

Thus, when the electrode, depending upon the concentrations of Ox and Red, assumes an equilibrium potential, Eeq, (keep in mind that E° is the particular equilibrium potential set up when the concentrations of Ox and Red at the electrode surface are identical) it will reach the zero current condition (i = 0), so that ... 

Voltammetric techniques involve perturbing the initial zero-current condition of an electrochemical cell by imposing a change in potential to the working electrode and observing the fate of the generated current as... 

In order to understand the meaning of such a sentence one must consider that the starting potential for a cyclic voltammetric scan must be selected on the basis of the zero-current condition, or to start from a potential value at which no electron transfer occurs at the working electrode. Even if one was wrong in the choice of the starting potential, i.e. one could have selected a potential at which an electrode process is taking place, it is very unlikely that one can realize it, in that the... 

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

Although the kinetic variable in electrode reactions in the current density, extensive use of the overpotential concept has been made in the electrochemical literature to indicate the departure from equilibrium [7]. Depending on the particular rate-determining process, in the overall electrode kinetics ohmic, charge transfer, reaction, concentration or mass transport, and crystallization overpotentials are described in the literature. Vetter [7] distinguished the concept of overpotential from that of polarization in the case of mixed potentials when the zero current condition does not correspond to an equilibrium potential as will be discussed in Sect. 8. 

Equilibrium experiments are carried out under zero-current conditions. Yielding no kinetic information, such experiments need not detain us. 

The two-electrode system is just fine under the zero-current condition, but this does not allow very much chemistry to be accomplished. Figure 6.2b represents the finite-current case, where A( )w is the same as in Figure 6.2a. If the... 

Since the values of Rb R2, and R3 are known values, the only unkown is Rx. The value of Rx can be calulated for the bridge during an ammeter zero current condition. Knowing this resistance value provides a baseline point for calibration of the instrument attached to the bridge circuit. The unknown resistance, Rx, is given by Equation 1-2. 

Wagner enhancement factor — describes usually the relationships between the classical - diffusion coefficient (- self-diffusion coefficient) of charged species i and the ambipolar - diffusion coefficient. The latter quantity is the proportionality coefficient between the - concentration gradient and the - steady-state flux of these species under zero-current conditions, when the - charge transfer is compensated by the fluxes of other species (- electrons or other sort(s) of -> ions). The enhancement factors show an increasing diffusion rate with respect to that expected from a mechanistic use of -> Ficks laws, due to an internal -> electrical field accelerating transfer of less mobile species [i, ii]. 

If m moles of AI were transferred from cell I to cell II until Donnan equilibrium was achieved, it would mean that 3m moles of H" " would be transferred from cell 11 to cell I, as the number of equivalents will be preserved in both the cells to maintain zero current conditions. This transfer would satisfy the conditions of Equation 34.5. Substituting for the valence term in this equation, m can be calculated. It would lead to the foUowing values at equilibrium ... 

If the zero current condition arises through a balemcing of different reactions, equilibrium is not achieved because the net rate for each reaction is not equal to zero. For example, if the corrosion of iron... 

A similar linear regime can be identified when the zero-current condition arises from the balancing of different anodic and cathodic reactions. 

Electrochemical transductors transform the effect of the electrochemical interaction between an analyte and the electrode into a primary signal. Such effects may be stimulated electrically or may result from a spontaneous interaction at the zero-current condition. The following subgroups may be distinguished ... 

The z-dependent potential F(z), in zero current conditions, is related to streaming potential Vp. In the case of ceramic membranes (])d and Vp are the two quantities which influence both retention of charged solutes and volume flux. 

Potentiometric sensors are based on the measurement of the voltage of a cell under equilibrium-like conditions, the measured voltage being a known function of the concentration of the analyte. Potentiometric measurements involve, in general, Nernstian responses under zero-current conditions that is, the measurement of the electromotive force of the electrochemical cell. 

In contrast to potentiometry that operates under zero current conditions, other electroanalytical methods impose an external source of electricity to the sample solution, to induce an electrochemical reaction that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions or organic compounds that can either be reduced or oxidized electrochemically. 

Construct a cell using a standard hydrogen electrode and an electrode designed around the redox couple of interest. The cel potential E is measured with a high impedance voltmeter under zero current conditions. When using SHE as a reference electrode, E is the desired half-reaction potential [7.13], Should the redox couple have one or more electroaclive species (i) that are solvated with concentration b, E must be measured over a range of b values. 

The first principles model [75] was used by Popov et al. to study the pulverization and corrosion of bare and cobalt encapsulated metal hydride electrodes [76]. EIS under zero current conditions enables the determination of the oxidation resistance and the evaluation of alloy corrosion without interference of any of the ohmic resistances. The total... 

Potentiometric sensors operate at thermodynamic equihbrium conditions. Thus, in practical potentiometric sensing, the potential measurement needs to be made under zero-current conditions. Consequently, a high-input impedance electrometer is often used for measurements. Also, the response time for a potentiometric sensor to reach equilibrium conditions in order to obtain a meaningful reading can be quite long. These considerations are essential in the design and selection of potentiometric sensors for biomedical apphcations. 

Electrochemical sensors can use amperometry, potentiometry, or conductometry as transduction principle [32], Potentiometric sensors make use of the development of an electrical potential at an electrode surface in contact with ions that exchange with the surface. The potential is measured under zero-current conditions against a reference electrode and is proportional to the logarithm of the analyte activity in the sample. Potentiometric sensors are limited to the measurement of charged species or of gases that dissociate to yield charged species in an electrolyte. Ion-selective electrodes are one example of this sensor type. 

The noble metals in saline solution are highly polarizable, and the DC voltage wiU be unstable and poorly defined unless under strict zero current conditions. Metal aUoy inhomogeneity will create noise as a function of local current flow between different parts of the metal surface. The noise generation of stainless steel electrodes is very dependent on the alloy composition (see Section 7.3). 

Under zero current condition, many metal electrodes are of interest in biological work. 

A pick-up electrode record tissue potential under zero current condition and are therefore not polarized. PU electrodes measure electric potential difference. 

It is not so easy to describe a monopolar potential reading electrode. Such a PU electrode is functioning under zero current conditions. The measured potential is then independent of the skin contact area. It is monopolar in the meaning that only one of the electrodes is in contact with an active tissue area. The other electrode must either be at a passive skin area so that it is indifferent, or the potential can come from the average potential of two or more electrodes like the Wilson terminal. 

In a potentiostatic system, the current flows between the auxiliary and the working electrodes while the third, or reference, electrode operates under essentially zero current conditions and controls the potential of the working electrode through the feedback arrangement of the operational amplifier circuit. 

In fact, with modern high-impedance (up to 10 Q) volimeters. cell potentials can be measured under essentially zero-current conditions, so the distinction between theoretical and practical cell potentials becomes negligible. 

To understand this phenomenon, imagine the situation that would exist at the junction if both solution phases had the same electric potential. An ion species with different chemical potentials in the two solutions would spontaneously diffuse across the junction in the direction of lower chemical potential. Different ions would diffuse at different rates, resulting in a net charge transfer across the junction and an electric potential difference. It is this electric potential difference in the equilibrium state that prevents further net charge transfer under zero-current conditions. 

The subscript "o being used to refer to quantitites evaluated in deep water in zero current conditions. 

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