Passivity indicated ions

Fig. 10.6. The effect of respiration and membrane potential (Ai )) on Cl permeation in brown adipose tissue mitochondria. When brown fat mitochondria were incubated in KCl in the presence of the ionophore, nigericin, they swelled (A, B). If a respiratory substrate (here G-3-P glycerol-3-phosphate) was added to the expanded mitochondria, they contracted, and this contraction ceased immediately and swelling was reintroduced if azide (NaNj) and an uncoupler (FCCP) were added (Fig. A). The passive halide ion permeability can be inhibited by GDP (cf.. Fig. 10.5), but respiration-driven contraction in KCl-expanded mitochondria was only partially inhibited by the presence of GDP (Fig. B) if again azide and uncoupler were added during the contraction, the mitochondria did not swell, indicating that the thermogenin channel was closed by GDP. This behaviour can partly be explained by the fact that the Cl permeation is driven by the membrane potential. Indeed, when, under similar conditions, the rate of contraction was plotted as a function of the membrane potential, it was seen that the rate was membrane potential dependent. It should, however, he noted that at low membrane potentials GDP nearly totally abolished the Cl permeation but when the membrane potential was increased above 30 mV, the inhibitory effect of GDP was apparently partially lost. The basis for this phenomenon is not understood it is not even known if there is a lower affinity of thermogenin for GDP in the energized membrane, as measurements of GDP affinities always refer to the non-energized situation. (Adapted from Nicholls et al. [27] (A, B) and Nicholls [94] (C).)...

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Pure aluminum is used in the electrolysis protection process, which does not passivate in the presence of chloride and sulfate ions. In water very low in salt with a conductivity of x < 40 yUS cm" the polarization can increase greatly, so that the necessary protection current density can no longer be reached. Further limits to its application exist at pH values < 6.0 and >8.5 because there the solubility of Al(OH)3 becomes too high and its film-forming action is lost [19]. The aluminum anodes are designed for a life of 2 to 3 years. After that they must be renewed. The protection currents are indicated by means of an ammeter and/or a current-operated light diode. In addition to the normal monitoring by service personnel, a qualified firm should inspect the rectifier equipment annually. 

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... 

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

More details of other factors that affect the critical pitting potential have been discussed by Uhlig and his co-workers" . They indicated that for stainless steel the critical pitting potential decreased with increasing concentration of chloride ion. At a fixed chloride level, passivating ions in solution, such as sulphate and nitrate, etc., cause the pitting potential to become more positive at a sufficient concentration these ions totally inhibited pitting, as shown in Fig. 19.40 for SO and CIO . 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

Figure 19. Effects of chloride ion and proton on anodic dissolution current of passive metal.20 pCl - og[CT], pH -log[H+]. p.s. pit and a.s. pit indicate polishing-state pit and active-state pit, respectively.

Sonication of 0.05 M Hg2(N03)2 solution for 10,20 and 30 min and the simultaneous measurements of conductivity, temperature change and turbidity (Table 9.2) indicated a rise in the turbidity due to the formation of an insoluble precipitate. This could probably be due to the formation of Hg2(OH)2, as a consequence of hydrolysis, along with Hg free radical and Hg° particles which could be responsible for increase in the turbidity after sonication. The turbidity increased further with time. Mobility of NO3 ions was more or less restricted due to resonance in this ion, which helped, in the smooth and uniform distribution of charge density over NO3 ion surface. Hence the contribution of NOJ ion towards the electrical conductance was perhaps much too less than the conduction of cationic species with which it was associated in the molecular (compound) form. Since in case of Hg2(N03)2, Hg2(OH)2 species were being formed which also destroyed the cationic nature of Hg22+, therefore a decrease in the electrical conductance of solution could be predicted. The simultaneous passivity of its anionic part did not increase the conductivity due to rise in temperature as anticipated and could be seen through the Table 9.2. These observations could now be summarized in reaction steps as under ... 

Most human cells are exposed to less than 2 mM Li+, and in most tissues the intracellular Li+ concentration is lower than the extracellular concentration. The level inside cells is generally below that expected for the passive diffusion of the Li+ ion across the cell membrane, indicating that Li+ is actively transported out of cells. For instance, the concentration of Li+ inside the erythrocytes from people taking lithium salts is low with a typical ratio of intra- to extracellular Li+ of 0.5 [53]. 

Another naturally occurring ribozyme which catalyzes phosphodiester transfer reactions is the hairpin ribozyme. The hairpin ribozyme has been the subject of a number of excellent review articles [24,25]. Several independent studies performed recently have indicated that the hairpin ribozyme has an interesting feature which distinguishes it from the aforementioned ribozymes mechanistically While the HHR, the group I intron, the HDV ribozyme and many other ribozymes that we are going to meet in this review are metalloenzymes and require divalent metal ions in their active sites for functional group activation, divalent metals ions only play a passive role (they are mainly required for cor-... 

A parameter (usually symbolized by P, and often containing a subscript to indicate the specific ion) that is a measure of the ease with which an ion can cross a unit area of membrane by simple (or passive) diffusion through a membrane experiencing a 1.0 M concentration gradient. For a particular biological membrane, the permeabilities are dependent on the concentration and activity of various channel or transporter proteins. In an electrically active cell (e.g., a neuron), increasing the permeability of K+ or CF will usually result in hyperpolarization of the membrane. Increasing will cause depolarization. 

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

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