Chloride equilibrium potential

Figure 11.5 Chloride distribution and the GABAa response. The change in membrane voltage (Fm) that results from an increase in chloride conductance following activation of GABAa receptors is determined by the resting membrane potential and the chloride equilibrium potential (Fci)- (a) Immature neurons accumulate CF via NKCC, while mature neurons possess a Cl -extruding transporter (KCC2). (b) In immature neurons GABAa receptor activation leads to CF exit and membrane depolarisation while in mature neurons the principal response is CF entry and h5q)erpolarisation. This is the classic inhibitory postsynaptic potential (IPSP)...

Cakium-aaivated chloride channels (CaCC), (Iciicai) like the equivalent K -current - is activated when there is Ca -mobilization within the cell, for instance, on activation of receptors coupled to the InsP3/DAG systems. This chloride channel provides the current commonly measured in oocytes as a means of detecting G-protein activation, on activation of experimentally expressed receptors. In smooth muscle, where the chloride equilibrium potential is typically more positive than the membrane potential this current can cause depolarization that follows a rise in [Ca],. 

At the dendritic level, GABA response is depolarizing because the chloride equilibrium potential is more positive than the membrane potential. Chloride ions and sodium or calcium ions may be involved in the response. The associated conductance increase is so large that the net result is inhibition because the membrane potential is clamped below the spike threshold. ... 

It should be emphasised that potential-pH diagrams can also be constructed from experimental E -I curves, where E is the polarised potential and / the current. These diagrams, which are of more direct practical significance than the equilibrium potential-pH equilibrium diagrams constructed from thermodynamic data, show how a metal in a natural environment (e.g. iron in water of given chloride ion concentration) may give rise... 

The pitting potential (point D in Figure 1.42) is sometimes treated as a type of equilibrium potential. There are empirical reasons for this. The pitting potential is often observed to decrease linearly with log (Cl ) (the chloride activity or concentration), giving an apparently Nernstian form . 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

Equilibrium potentials calculated at 37°C from the Nernst equation. Calculated assuming a —90 mV resting potential for the muscle membrane and assuming that chloride ions are at equilibrium at rest. 

In nimen-listulaled experimental animals, it has been observed that for certain distribution ratios of chloride in ihc ruminal fluids and hlood plasma, the calculated equilibrium potential Tor chloride is relatively the same as that measured directly with KCI-agar bridges and calomel electrodes However, in many circumstances, the calculated and measured values have been found lo be significantly different, an observation that could only be accounted for by the presence of an active transport mechanism responsible for the movement of chloride out of its equilibrium distribution. 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibility for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibility depends on pH. The susceptibility temperature increases parabolically from 65°C as pH is increased from zero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equilibrium potential into the passive region where a protective film is formed and maintained. 

The Tafel expressions for both the anodic and the cathodic reaction can be directly incorporated into a mixed potential model. In modeling terms, a Tafel relationship can be defined in terms of the Tafel slope (b), the equilibrium potential for the specific half-reaction ( e), and the exchange current density (70), where the latter can be easily expressed as a rate constant, k. An attempt to illustrate this is shown in Fig. 10 using the corrosion of Cu in neutral aerated chloride solutions as an example. The equilibrium potential is calculated from the Nernst equation e.g., for the 02 reduction reaction,... 

This equation is similar to the Nemst equation except that it simultaneously takes into account the contributions of all three permeant ions. It indicates that the membrane potential is governed by tw o factors (1) the ionic concentrations, which determine the equilibrium potentials for the ions, and, (2) their relative permeabilities, which determine the relative importance of a particular ion in governing where lies. For many cells, including most neurons and immune cells, this equation can be simplified the chloride term can be dropped altogether because the contribution of chloride to the resting membrane potential is insignificant. In this case, the Goldman equation becomes ... 

The resting membrane potential of the muscle and nerve is known to be close to the Cl" equilibrium potential. The potassium equilibrium potential is also close to the resting membrane potential. When the muscles are exposed to the K+ free solution, the resting membrane potential is known to hyperpolarize below the chloride ion equilibrium potential (28). When the avermectin-treated muscles were exposed to the K free saline solution containing 3 C1, the 3 C1 influx was suppressed to 60% of the control. 

Most often, the tabulated values of formal potential are given with respect to the normal hydrogen electrode (NHE), which has the defined potential 0 V. However, in practise, a silver/silver chloride (Ag/AgCl) electrode or a plain metal surface (e.g. Au or Pt) is commonly used as a RE. An Ag/AgCl RE, having an internal electrolyte of saturated KCl, has a characteristic potential of 197 mV with respect to the NHE. A plain metal surface, on the other hand, does not have a characteristic potential that can be expressed in terms of NHE. Instead, its potential depends on the prevailing conditions, affected by the deposited species and the electrolyte. E.g., if a Au surface is used as an RE to adjust the potential of, for instance, another Au surface (WE), both the RE and WE are affected by the same conditions. The equilibrium potential between such electrodes is ideally 0 V and a poised potential is directly an overpotential with respect to the equilibrium potential. 

Magnesium is often produced by electrolysis of a molten mixed chloride electrolyte containing MgCl2, at 750°C. Molten magnesium is released around the cast steel cathodes. Use an Ellingham diagram to determine the equilibrium potential necessary for decomposition. 

In the case of palladium, the measured equilibrium potential as a function of the chloride ion concentration (hydrochloric acid) at constant ionic strength (perchloric acid) together with the calculated concentrations of the various complexes using stability constant data leads to the standard poten-... 

As in the calomel electrode, the potential is governed by the chloride ion activity. Useful silver-chloride electrodes can be prepared by simply anodizing a silver wire in chloride media, but the apparent equilibrium potential of these electrodes may differ by several millivolts from one electrode to another. More care is necessary for the preparation of highly stable and reproducible electrodes [4]. 

The Tl(Hg)/TU reference electrode (Thalamid ) is said to be superior to either calomel or silver-chloride electrodes when measurements are made over a range of temperatures, because it attains its equilibrium potential very rapidly after changes in temperature. The half-cell can be written as... 

E° is the equilibrium potential for the degradation reaction, for example Eqs. (1) and (2), under standard conditions. Unfortunately, sufficient data (that is more than two complete data sets containing rate constant, reactant and product concentration, and Hj concentration) were available only for the redox couple ICE and DCE (Eq. (2)). The standard potential was converted from Gibbs free energy provided by Dolfing (2000) to be 0.72 V pH values were available only for one of the sites (pH 5.3), the unknown values were set to a value of seven under the assumption that such a value is a best guess for anoxic and reducing conditions. In case of chloride, concentrations were available for two of the studies (between 1 and 2 mmol L ), the unknown value was set to 1 mmol L . ... 

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