Diffusion chloride ions

Due to both carbonization and penetration of chloride ions, steel will pass from a passive to an active condition and (consequently) may corrode. If the mortar is completely surrounded by water, oxygen diffusion in wet mortar is extremely low so that the situation is corrosion resistant because the cathodic partial reaction according to Eq. (2-17) scarcely occurs. For this reason the mortar lining of waste pipes remains protective against corrosion even if it is completely carbonated or if it is penetrated by chloride ions. 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

Kittelberger and Elm measured the rate of diffusion of sodium chloride through a number of paint films. Calculations based on their results showed clearly that the rate of diffusion of ions was very much smaller than the rate of diffusion of either water or oxygen. Furthermore, they found that there was a linear relationship between the rate of diffusion and the reciprocal of the resistance of the film. This relationship suggests that the sodium chloride diffused through the membrane as ions and not as ion pairs, since the diffusion through the film of un-ionised material would not affect the resistance, because if a current is to flow, either ions of similar charge... 

Little work has been carried out on the mechanism of inhibition of the corrosion. of copper in neutral solutions by anions. Inhibition occurs in solutions containing chromate , benzoate or nitrite ions. Chloride ions and sulphide ions act aggressively. There is evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies which permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

Molecular dispersion < 1.0 nm Particles invisible by electron microscopy pass through semipermeable membranes generally rapid diffusion Oxygen molecules, potassium and chloride ions dissolved in water... 

Meanwhile, computational methods have reached a level of sophistication that makes them an important complement to experimental work. These methods take into account the inhomogeneities of the bilayer, and present molecular details contrary to the continuum models like the classical solubility-diffusion model. The first solutes for which permeation through (polymeric) membranes was described using MD simulations were small molecules like methane and helium [128]. Soon after this, the passage of biologically more interesting molecules like water and protons [129,130] and sodium and chloride ions [131] over lipid membranes was considered. We will come back to this later in this section. 

The transport of chloride ions into the cell as a result of outwards diffusion of bicarbonate in order to maintain electrical neutrality. 

Passive diffusion is considered to be the major pathway by which xenobiotics cross the placenta. Paracellular diffusion was shown to be the predominant pathway for transfer of hydrophilic solutes, such as chloride ions across perfused placental lobes and opioid peptides and dextrans across BeWo cells [11-13], It has been proposed that denudations in the syncytiotrophoblasts-containing fibrinoid deposits provide a possible paracellular route across the placenta [14], Transtrophoblast channels in the syncytiotrophoblasts could also be responsible for this mode of diffusion [15], For more lipophilic solutes, the transplacental route appears to be the preferred mode of passage... 

In the first cycle, methanol oxidation peaks are seen in both the anodic and cathodic sweeps around 0.7 V. As mentioned earlier, P -OH formation on Ptdll) does not occur to any substantial extent until 1.2 V. Therefore this current decrease over 0.7 V is not due to deactivation of platinum by the svuface Pt-OH formation. The cxirrent increase on the reversed sweep indicated that this current is not limited by methanol diffusion or active accumulated intermediates, either. It simply seems that platinum loses its catalytic activity over 0.7 V regardless whether platinvim is oxidized or not. Anion effects is not likely the reason because the same phenomena are found in percloric add also. Trace amount of impurities, such as chloride ions, may play some roles. 

NHE (25 °C) or [Hg/HgO/ln NaOH in H2O] E = +140 mV versus NHE (25 °C) are, for example, interesting if chloride ions are not allowed. A problem of all mercury containing systems is the toxicity. A nontoxic system is [Ag/AgCl/sat. KCl in H2O] E = +197 mV versus NHE (25 °C). It is nearly temperature independent and insensitive against current flow. It can be easily prepared by anodic oxidation of a silver wire in KCl solution (coating with an AgCl layer, subsequently an excess of solid KCl is added to the electrolyte round the silver wire). But, diffusion of silver ions out of the partially soluble AgCl can be a problem. 

The junction potentials for cells A and B can be assumed to be very similar in magnitude because their liquid junction potentials will be dominated by chloride ions diffusing out from the sinter at the bottom of the SCE (see Figure 3.4), rather than by silver diffusing into the SCE. Cell C has no sinter but a salt bridge. If we therefore consider cells A and B ... 

Chemical/Physical. Matheson and Tratnyek (1994) studied the reaction of fine-grained iron metal in an anaerobic aqueous solution (15 °C) containing chloroform (107 pM). Initially, chloroform underwent rapid dehydrochlorination forming methylene chloride and chloride ions. As the concentration of methylene chloride increased, the rate of reaction appeared to decrease. After 140 h, no additional products were identified. The authors reported that reductive dehalogenation of chloroform and other chlorinated hydrocarbons used in this study appears to take place in conjunction with the oxidative dissolution or corrosion of the iron metal through a diffusion-limited surface reaction. 

In sum, the natural tendency will be for sodium, calcium, and chloride ions to flow into the neuron and for potassium ions to flow out, and in so doing to reduce the membrane potential to zero. In reality, this is not so easy. The plasma membrane of the neuron is not very permeable to these ions. If it were, it would be impossible to sustain concentration gradients across it. The rate of passive diffusion of these ions across this membrane is very slow, though not zero, and different for each ion. So how do ions get across the neuronal plasma membrane rapidly There are two ways gated channels and active transport by pumps. 

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