Electrolytic potential

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Reference electrode Me/Me" system Electrolyte Potential at 25°C (V) Temperature dependence (mV/°C) Application... 

Table 21-1 Structure/electrolyte potentials in a Kaplan turbine as in Fig. 21-3 before and after sivitching on the cathodic protection system.

Reference electrodes The generally accepted criterion for the effectiveness of a cathodic-protection system is the structure/electrolyte potential (Section 10.1). In order to determine this potential it is necessary to make a contact on the structure itself and a contact with the electrolyte (soil or water). The problem of connection to the structure normally presents no difficulties, but contact with the electrolyte must be made with a reference electrode. (If for example an ordinary steel prol e were used as a reference electrode, then inaccuracies would result for two main reasons first, electrochemical action between the probe and the soil, and second, polarisatibn of the probe owing to current flow through the measuring circuit.)... 

To measure structure/electrolyte potentials with electrolyte resistivities in excess of 2 kQ cm, a high-resistance potentiometer unit as shown in Fig. 10.43 or a potentiometric voltmeter as illustrated in Fig. 10.44 may be used. 

The technique adopted in measuring structure/electrolyte potential is illustrated in Fig. 10.45. While it is not truly within the scope of this chapter. 

As their name suggests, these instruments are capable of carrying out a variety of measurements, e.g. structure/electrolyte potentials, current, resistivity and voltage. Most instruments of this type contain two meters in one case, one being a low-resistance millivolt/voltmeter and milliamp/ammeter, and the second a high-resistance voltmeter. 

Fig. 4.12 Dependence of concentrations of negative charge carriers (ne) and positive charge carriers (np) on distance from the interface between the semiconductor (sc) and the electrolyte solution (1) in an w-type semiconductor. These concentration distributions markedly differ if the semiconductor/electrolyte potential difference A cp is (A) smaller than the flat-band potential AF

Besides being of obvious synthetic utility, these results remind us that changes in experimental parameters, particularly solvent, electrolyte, potential, and temperature, may cause substantial changes in the composition of the double layer and therefore may provoke changes in the course of electrochemical processes occurring at the electrode surface. 

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

Solid-solid contact (inc. solid breakup) Metal to metal Metal to semiconductor Semiconductor to semiconductor Volta potential (equalization of Fermi levels) Electrolytic potential (where adsorbed water films may be present)... 

Solvent Supporting electrolyte Potential region Electrode... 

At equilibrium there is a zero free-energy change, AG=0, that takes place between compartments separated by a membrane, with the free-energy change being dependent on the difference in concentration of various ions and the electrical potential difference that exists across the membrane. The relationships among sodium, potassium, and chloride ions, pH, and electrolytic potential have become known as Donnan equilibria. The concentrations and electrolytic potentials are related by the following equation ... 

The structure-to-electrolyte potential of corrosion protection systems used to protect aboveground tank bottoms and connecting underground pipes must be inspected annually. 

Can One Meaningfully Analyze an Electrode-Electrolyte Potential Difference ... 

Compounds Solvent Supporting electrolyte Potential Reference Potential... 

Vanadium predpitates the metal from solutions of salts of gold, silver, platinum, and iridium, and reduces solutions of mercuric chloride, cupric chloride and ferric chloride to mercurous chloride, cuprous chloride, and ferrous chloride, respectively. In these reactions the vanadium passes into solution as the tetravalent ion. No precipitation or reduction ensues, however, when vanadium is added to solutions of divalent salts of zinc, cadmium, nickel, and lead. From these reactions it has been estimated that the electrolytic potential of the change, vanadium (metal)—>-tetravalent ions, is about —0 3 to —0 4 volt, which is approximately equal to the electrolytic solution pressure of copper. This figure is a little uncertain through the difficulty of securing pure vanadium.5... 

Use of a reference electrode to measure the electrode—electrolyte potential difference also introduces a new reference—solution interface, but this is designed so that the potential gradient at the new interface is constant regardless of whatever electrode process occurs isothermally at the working electrode. Changes in electrode potential are thus proportional to changes in the interfacial potential difference... 

As for any chemical reaction, an electrode reaction is driven by a gradient in tree energy. Since charged particles such as electrons and/or ions are involved in electrode reactions at the electrode—solution interface, the electrode—electrolyte potential difference has a linear effect on the position of the free energy surfaces as depicted in Fig. 4. The effect of the high electrical field (i.e. 107 V cm-1) at the interface on uncharged species will be neglected for simplicity. 

In previous sections, apparent rate coefficients of electrode reactions have been described as a function of the electrode—electrolyte potential difference. As in other chemical processes, their dependence on temperature can be expressed by the Arrhenius equation... 

Electrode Electrolyte Potential Limit vs. Positive SCE, Va Negative... 

R. Abegg and C. Bodlander suggest that the stability of a complex depends upon the so-called electro-affinity of the ions, i.e, on the affinity of the radicles for electric charges or electrons. This can be approximately measured in terms of the electrolytic potential, on assumption that the unknown cone, of the free atoms in sat. soln. are the same for all elements. In a general way, the smaller the numerical value of the electrolytic potential (positive or negative) of a salt, the greater the tendency to form complex ions. In the further... 

For these situations, IR 0 and Ametal-electrolyte potential difference at electron-source and -sink areas. In general, however, the sink-to-source distance is on the order of microns or less, in which case the conducting path in the solution and therefore IR becomes negligible. Thus, the A(j>so is virtually equal to A(f>u, and any negligible difference that exists occurs over distances too small to be resolved by the probe used to measure the potential difference between the metal and the solution (Fig. 12.14). 

The oxidation rate depends not only on the gas composition and the temperature parameter, but also on the electric potential difference between the electronically conductive part of the anode electrode and the ionically conductive electrolyte. Defining the electric potential of the solid part of the anode electrode as zero potential, the reaction rate depends on the electric potential in the electrolyte, other hand, the reduction reaction rate depends on the electric potential difference at the cathode electrode, which is the difference between the given cell voltage, Uceii, and the electrolyte potential, equilibrium constants are determined by the... 

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