Electrodes concentration dependence

In acidic electrolytes only lead, because it forms passive layers on the active surfaces, has proven sufficiently chemically stable to produce durable storage batteries. In contrast, in alkaline medium there are several substances basically suitable as electrode materials nickel hydroxide, silver oxide, and manganese dioxide as positive active materials may be combined with zinc, cadmium, iron, or metal hydrides. In each case potassium hydroxide is the electrolyte, at a concentration — depending on battery systems and application — in the range of 1.15 - 1,45 gem"3. Several elec-... 

The use of a catalyst with oxidase enzyme is an example of the use of a combined enzyme system, which illustrates the wide potential offered by multi-enzyme electrode systems. Various enzymes can be arranged to work sequentially to transform quite complex substances and eventually produce a measurable concentration-dependent change, which is detected by the output signal and recorded for analysis. 

An electrochemically oxidized boron-doped diamond electrode was used. The observed rates were concentration-dependent. 

The specific rate of an electrode reaction depends not only on electrode polarization but also on tfie reactant concentrations. Changes in reactant concentrations affect not only reaction rates but also the values of equilibrium potentials. To differentiate both these influences, kinetic equations are generally used (especially at high values of polarization), relating the current density not with the value of polarization AE but with the potential of the electrode E ... 

E (A4>). This relation can be used to plot y (E ) from Fig. 5.7 as a function of the electrode potential, y [E (A(j))], for different electrolytes and concentrations, depending on which experimental capacity measurements have been used for the integration. Since these measurements were performed with an SCE, we have added a corresponding subscript to the electrode potential. 

It can be seen from the table that, in dilute solutions, the diffuse layer may extend some hundreds of angstroms out from the electrode. In contrast, in more concentrated solutions, i.e. 0.1 M, the diffuse layer thickness decreases to < 10 A not much more than the thickness of the Helmholtz layer. As CH has no concentration dependence it remains constant on changing the concentration however, from equations (2.22) and (2,23), CGC decreases as the concentration of the electrolyte increases. Thus, at low concentration ... 

Here, Ws is the work function of electrons in the semiconductor, q is the elementary charge (1.6 X 1CT19 C), Qt and Qss are charges located in the oxide and the surface and interface states, respectively, Ere is the potential of the reference electrode, and Xso is the surface-dipole potential of the solution. Because in expression (2) for the flat-band voltage of the EIS system all terms can be considered as constant except for tp (which is analyte concentration dependent), the response of the EIS structure with respect to the electrolyte composition depends on its flat-band voltage shift, which can be accurately determined from the C-V curves. 

Catalase was immobilized with gelatin by means of glutaraldehyde and fixed on a pretreated Teflon membrane served as enzyme electrode to determine hydrogen peroxide [248], The electrode response reached a maximum when 50mM phosphate buffer was used at pH 7.0 and at 35°C. Catalase enzyme electrode response depends linearly on hydrogen peroxide concentration between 1.0 X 10-5 and 3.0 X 10-3 M with response time 30 s. 

Substrate concentration dependence of response current of the gold-black electrode was compared with that of gold disk electrode. The ferrocene-modified glucose oxidase which was used in this measurement had 11 ferrocenes per glucose oxidase. The electrode potential was controlled at 0.4 V vs. Ag/AgCl. The response current was recorded when the output reached at a steady state. The response current was enhanced when ferrocene-modified glucose oxidase was self-assembled on a porous gold-black electrode. 

Fig.21 Glucose concentration dependence of response current on ferrocene-modified glucose oxidase self-assembledon the gold black ( ) and plain gold (0) electrodes...

Figure 2a shows the result of these calculations. The values obtained are dependent upon the assumptions made, in particular the concentration of the solution for which the calculations are carried out, and the coverage on the electrode. The situation is worse (maximum interference from the solution) when the coverage is small and the concentration of the solution high, and then 60% of the information can come from the solution. In more usual circumstances, however, when the solution concentration is medium or low, and the electrode concentration is medium or high, more than 90% of the information does come from the electrode surface itself (Figure 2b). The details are given elsewhere (11).

Inert electrodes comprise of chemically inert conductors, for instance Au, Pt and C which do not necessarily take part either directly or indirectly in the various redox processes. However, the potential developed at an inert electrode solely depends upon both the nature as well as the prevailing concentration of the different redox-reagents present in the solution. 

Little is known about the mechanisms that cause the three other current extrema ]2 to J4. The kinetic and diffusional contributions of the characteristic currents Ji to J4 show a different concentration dependence. While the diffusion current is found to be roughly proportional to Cp, the kinetic current shows an exponent of 2 < <2.5 [Ha3]. No dependence of the characteristic currents to ]4 on doping kind and density is observed. This indicates again that to ]4 depend on mass transport and reaction kinetics rather than on charge supply. For n-type electrodes, of course, strong illumination is necessary in order to generate a sufficient number of minority carriers to support the currents. 

An adsorption isotherm describes the way in which the concentration of the species adsorbed on the electrode, r(, depends on the activity, a(, of the species in solution. Langmuir s isotherm is the simplest equation for adsorption phenomena, but it must be kept in mind that it is generally valid only for non-ionic species. 

The electrode roughness factor can be determined by using the capacitance measurements and one of the models of the double layer. In the absence of specific adsorption of ions, the inner layer capacitance is independent of the electrolyte concentration, in contrast to the capacitance of the diffuse layer Q, which is concentration dependent. The real surface area can be obtained by measuring the total capacitance C and plotting C against Cj, calculated at pzc from the Gouy-Chapman theory for different electrolyte concentrations. Such plots, called Parsons-Zobel plots, were found to be linear at several charges of the mercury electrode. ... 

In addition, the time-dependence of these concentrations also contains (albeit in encoded form) the homogeneous parameters of the particular mechanism being considered. These latter techniques are termed convolutions. Convolution (and its reverse, i.e. deconvolution) are ideal for the electroanalyst because the theoretical calculation of current, and direct comparison with experimental data, is often not viable. This alternative of testing experimental currents via convolutions results in expressions for concentrations at the electrode which arise directly from the data rather than requiring iterations(s). The electrode concentrations thus estimated for a particular mechanism then allow for correlations to be drawn between potential and time, thereby assessing the fit between the data and the model. 

The activation overpotentials for both electrodes are high therefore, the electrochemical kinetics of the both electrodes can be approximated by Tafel kinetics. The concentration dependence of exchange current density was given by Costamagna and Honegger.The open-circuit potential of a SOFC is calculated via the Nernst equation.The conductivity of the electrolyte, i.e., YSZ, is a strong function of temperature and increases with temperature. The temperature dependence of the electrolyte conductivity is expressed by the Arrhenius equation. 

The simplest but also the least reliable calibration method is the use of a single standard solution. The electrode response is assumed to be Nernstian. The slope of the potential versus concentration dependence can also be determined experimentally, by using two standard solutions with different concentrations. To avoid large errors, the standard concentration should be as close as possible to the sample concentration in calibration with a single standard solution. 

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

Equation (5.9) is the general Nemst equation giving the concentration dependence of the equilibrium cell voltage. It will be used in Section 5.4 to derive the equilibrium electrode potential for metal/metal-ion and redox electrodes. 

Nernst Equation for Concentration Dependence of RedOx Potential. Equation (5.9) applied to the general RedOx electrode (5.16) yields... 

The overall rate of an electrochemical reaction is measured by the current flow through the cell. In order to make valid comparisons between different electrode systems, this current is expressed as cunent density,/, the current per unit area of electrode surface. Tire current density that can be achieved in an electrochemical cell is dependent on many factors. The rate constant of the initial electron transfer step depends on the working electrode potential, Tlie concentration of the substrate maintained at the electrode surface depends on the diffusion coefficient, which is temperature dependent, and the thickness of the diffusion layer, which depends on the stirring rate. Under experimental conditions, current density is dependent on substrate concentration, stirring rate, temperature and electrode potential. 

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